Semiconductors Group

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List of Publications

This list may be displayed with or without abstracts. View our publication list on Google Scholar, RESEARCHERID or ORCID.

Unraveling loss mechanisms arising from energy-level misalignment between metal halide perovskites and hole transport layers,
J. E. Lee, S. G. Motti, R. D. J. Oliver, S. Yan, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Advanced Functional Materials, 34 (2024), p. 2401052. [journal | article | SI ]

Metal halide perovskites are promising light absorbers for multijunction photovoltaic applications because of their remarkable bandgap tunability, achieved through compositional mixing on the halide site. However, poor energy-level alignment at the interface between wide-bandgap mixed-halide perovskites and charge-extraction layers still causes significant losses in solar-cell performance. Here, the origin of such losses is investigated, focusing on the energy-level misalignment between the valence band maximum and the highest occupied molecular orbital (HOMO) for a commonly employed combination, FA_0.83Cs_0.17Pb(I_1-xBr_x)₃ with bromide content x ranging from 0 to 1, and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine| (PTAA). A combination of time-resolved photoluminescence spectroscopy and numerical modeling of charge-carrier dynamics reveals that open-circuit voltage (VOC) losses associated with a rising energy-level misalignment derive from increasing accumulation of holes in the HOMO of PTAA, which then subsequently recombine non-radiatively across the interface via interfacial defects. Simulations assuming an ideal choice of hole-transport material to pair with FA_0.83Cs_0.17Pb(I_1-xBr_x)₃ show that such VOC losses originating from energy-level misalignment can be reduced by up to 70 mV. These findings highlight the urgent need for tailored charge-extraction materials exhibiting improved energy-level alignment with wide-bandgap mixed-halide perovskites to enable solar cells with improved power conversion efficiencies.

Charting the irreversible degradation modes of low bandgap Pb-Sn perovskite compositions for de-risking practical industrial development,
C. Kamaraki, M. T. Klug, V. J.-Y. Lim, N. Zibouche, L. M. Herz, M. S. Islam, C. Case, and L. M. Perez,
Advanced Energy Materials, 14 (2024), p. 2303313. [journal | article | SI ]

The commercialization of a solar technology necessitates the fulfillment of specific requirements both regarding efficiency and stability to enter and gain space in the photovoltaic market. These aims are heavily dependent on the selection of suitable materials, which is critical for suppressing any reliability risks arising from inherent instabilities. Focusing on the absorber material, herein the most suitable low bandgap lead-tin composition candidate for all-perovskite tandem applications is investigated by studying their degradation mechanisms with both widely available and advanced characterization techniques. Three irreversible degradation processes are identified in narrow bandgap Pb-Sn perovskite absorbers: 1) Tin (Sn) oxidation upon air exposure, 2) methylammonium (MA) loss upon heat exposure, and 3) formamidinium (FA) and cesium (Cs) segregation leading to impurity phase formation. From an industrial perspective, it is proposed to refocus attention on FASn_0.5Pb_0.5I₃ which minimizes all three effects while maintaining a suitable bandgap for a bottom cell and good performance. Moreover, a practical and highly sensitive characterization method is proposed to monitor the oxidation, which can be deployed both in laboratory and industrial environments and provide useful information for the technological development process, including, the effectiveness of encapsulation methods, and the acceptable time windows for air exposure.

Trace water in lead iodide affecting perovskite crystal nucleation limits the performance of perovskite solar cells,
R. Guo, Q. Xiong, A. Ulatowski, S. Li, Z. Ding, T. Xiao, S. Liang, J. E. Heger, T. Guan, X. Jiang, K. Sun, L. K. Reb, M. A. Reus, A. Chumakov, M. Schwartzkopf, M. Yuan, Y. Hou, S. V. Roth, L. M. Herz, P. Gao, and P. Müller‐Buschbaum,
Advanced Materials, 36 (2024), p. 2310237. [journal | article | SI ]

The experimental replicability of highly efficient perovskite solar cells (PSCs) is a persistent challenge faced by laboratories worldwide. Although trace impurities in raw materials can impact the experimental reproducibility of high-performance PSCs, the in situ study of how trace impurities affect perovskite film growth is never investigated. Here, light is shed on the impact of inevitable water contamination in lead iodide (PbI₂) on the replicability of device performance, mainly depending on the synthesis methods of PbI₂. Through synchrotron-based structure characterization, it is uncovered that even slight additions of water to PbI₂ accelerate the crystallization process in the perovskite layer during annealing. However, this accelerated crystallization also results in an imbalance of charge-carrier mobilities, leading to a degradation in device performance and reduced longevity of the solar cells. It is also found that anhydrous PbI₂ promotes a homogenous nucleation process and improves perovskite film growth. Finally, the PSCs achieve a remarkable certified power conversion efficiency of 24.3%. This breakthrough demonstrates the significance of understanding and precisely managing the water content in PbI₂ to ensure the experimental replicability of high-efficiency PSCs.

Compositional transformation and impurity-mediated optical transitions in co-evaporated Cu₂AgBiI₆ thin films for photovoltaic applications,
B. W. J. Putland, M. Righetto, H. Jin, M. Fischer, A. J. Ramadan, K.-A. Zaininger, L. M. Herz, H. C. Sansom, and H. J. Snaith,
Advanced Energy Materials, 14 (2024), p. 2303313. [journal | article | SI ]

Quaternary copper-silver-bismuth-iodide compounds represent a promising new class of wide-bandgap (2 eV) semiconductors for photovoltaic and photodetector applications. In this study, vapor phase co-evaporation is utilized to fabricate Cu₂AgBiI₆ thin films and photovoltaic devices. The findings show that the properties of vapor-deposited films are highly dependent upon processing temperature, exhibiting increased pinhole density and transforming into a mixture of quaternary, binary, and metallic phases depending on the post-deposition annealing temperature. This change in phase is accompanied by an enhancement in photoluminescence (PL) intensity and charge-carrier lifetime, along with the emergence of an additional absorption peak at high energy ( 3 eV). Generally, increased PL is a desirable property for a solar absorber material, but this change in PL is ascribed to the formation of CuI impurity domains, whose defect-mediated optical transition dominates the emission properties of the thin film. Via optical pump terahertz probe spectroscopy, it is revealed that CuI impurities hinder charge-carrier transport in Cu₂AgBiI₆ thin films. It is also revealed that the predominant performance limitation in Cu₂AgBiI₆ materials is the short electron-diffusion length. Overall, the findings pave the way for potential solutions to critical issues in copper-silver-bismuth-iodide materials and indicate strategies to develop environmentally compatible wide-bandgap semiconductors.

The role of the organic cation in developing efficient green perovskite leds based on quasi-2D perovskite heterostructures,
A. J. Ramadan, W. H. Jeong, R. D. J. Oliver, J. Jiang, A. Dasgupta, Z. Yuan, J. Smith, J. E. Lee, S. G. Motti, O. Gough, Z. Li, L. M. Herz, M. B. Johnston, H. Choi, J. Even, C. Katan, B. R. Lee, and H. J. Snaith,
Advanced Functional Materials, 34 (2024), p. 2309653. [journal | article | SI ]

Two dimensional/three-dimensional (2D/3D) metal halide perovskite heterostructures have attracted great interest in photovoltaic and light-emitting diode (LEDs) applications. In both, their implementation results in an improvement in device efficiency yet the understanding of these heterostructures remains incomplete. In this work the role of organic cations, essential for the formation of 2D perovskite structures is unraveled, in a range of metal halide perovskite heterostructures. These heterostructures are used to fabricate efficient green perovskite LEDs and a strong dependence between cation content and device performance is shown. The crystal structure, charge-carrier transport and dynamics, and the electronic structure of these heterostructures are studied and it is shown that the presence of crystalline 2D perovskite inhibits electron injection and ultimately lowers device performance. This work highlights the importance of optimizing the composition of these heterostructures in ensuring optimal device performance across all parameters and suggests that developing routes to inject charge-carriers directly into 2D perovskite structures will be important in ensuring the continued development of perovskite LEDs based on these heterostructures.

Alloying effects on charge-carrier transport in silver−bismuth double perovskites,
M. Righetto, S. Caicedo-Davila, M. T. Sirtl, V. J.-Y. Lim, J. B. Patel, D. A. Egger, T. Bein, and L. M. Herz,
J. Phys. Chem. Lett., 14 (2023), p. 10340–10347. [journal | article | SI ]

Alloying is widely adopted for tuning the properties of emergent semiconductors for optoelectronic and photovoltaic applications. So far, alloying strategies have primarily focused on engineering bandgaps rather than optimizing charge-carrier transport. Here, we demonstrate that alloying may severely limit charge-carrier transport in the presence of localized charge carriers (e.g., small polarons). By combining reflection–transmission and optical pump–terahertz probe spectroscopy with first-principles calculations, we investigate the interplay between alloying and charge-carrier localization in Cs₂AgSb_xBi_1–xBr₆ double perovskite thin films. We show that the charge-carrier transport regime strongly determines the impact of alloying on the transport properties. While initially delocalized charge carriers probe electronic bands formed upon alloying, subsequently self-localized charge carriers probe the energetic landscape more locally, thus turning an alloy’s low-energy sites (e.g., Sb sites) into traps, which dramatically deteriorates transport properties. These findings highlight the inherent limitations of alloying strategies and provide design tools for newly emerging and highly efficient semiconductors.

Cation‐disorder engineering promotes efficient charge‐carrier transport in AgBiS₂ nanocrystal films,
M. Righetto, Y. Wang, K. A. Elmestekawy, C. Q. Xia, M. B. Johnston, G. Konstantatos, and L. M. Herz,
Advanced Materials, 35 (2023), p. 2305009. [journal | article | SI ]

Efficient charge-carrier transport is critical to the success of emergent semiconductors in photovoltaic applications. So far, disorder has been considered detrimental for charge-carrier transport, lowering mobilities, and causing fast recombination. This work demonstrates that, when properly engineered, cation disorder in a multinary chalcogenide semiconductor can considerably enhance the charge-carrier mobility and extend the charge-carrier lifetime. Here, the properties of AgBiS₂ nanocrystals (NCs) are explored as a function of Ag and Bi cation-ordering, which can be modified via thermal-annealing. Local Ag-rich and Bi-rich domains formed during hot-injection synthesis are transformed to induce homogeneous disorder (random Ag-Bi distribution). Such cation-disorder engineering results in a sixfold increase in the charge-carrier mobility, reaching ˜2.7 cm² V⁻¹ s⁻¹ in AgBiS₂ NC thin films. It is further demonstrated that homogeneous cation disorder reduces charge-carrier localization, a hallmark of charge-carrier transport recently observed in silver-bismuth semiconductors. This work proposes that cation-disorder engineering flattens the disordered electronic landscape, removing tail states that would otherwise exacerbate Anderson localization of small polaronic states. Together, these findings unravel how cation-disorder engineering in multinary semiconductors can enhance the efficiency of renewable energy applications.

A templating approach to controlling the growth of coevaporated halide perovskites,
S. Yan, J. B. Patel, J. E. Lee, K. A. Elmestekawy, S. R. Ratnasingham, Q. Yuan, L. M. Herz, N. K. Noel, and M. B. Johnston,
ACS Energy Lett., 8 (2023), p. 4008–4015. [journal | article | SI ]

Metal halide perovskite semiconductors have shown significant potential for use in photovoltaic (PV) devices. While fabrication of perovskite thin films can be achieved through a variety of techniques, thermal vapor deposition is particularly promising, allowing for high-throughput fabrication. However, the ability to control the nucleation and growth of these materials, particularly at the charge-transport layer/perovskite interface, is critical to unlocking the full potential of vapor-deposited perovskite PV. In this study, we explore the use of a templating layer to control the growth of coevaporated perovskite films and find that such templating leads to highly oriented films with identical morphology, crystal structure, and optoelectronic properties independent of the underlying layers. Solar cells incorporating templated FA_0.9Cs_0.1PbI_3–xCl_x show marked improvements with steady-state power conversion efficiency over 19.8%. Our findings provide a straightforward and reproducible method of controlling the charge-transport layer/coevaporated perovskite interface, further clearing the path toward large-scale fabrication of efficient PV devices.

Contrasting charge-carrier dynamics across key metal-halide perovskite compositions through in situ simultaneous probes,
A. M. Ulatowski, K. A. Elmestekawy, J. B. Patel, N. K. Noel, S. Yan, H. Kraus, P. G. Huggard, M. B. Johnston, and L. M. Herz,
Advanced Functional Materials, 33 (2023), p. 2305283. [journal | article | SI ]

Metal-halide perovskites have proven to be a versatile group of semiconductors for optoelectronic applications, with ease of bandgap tuning and stability improvements enabled by halide and cation mixing. However, such compositional variations can be accompanied by significant changes in their charge-carrier transport and recombination regimes that are still not fully understood. Here, a novel combinatorial technique is presented to disentangle such dynamic processes over a wide range of temperatures, based on transient free-space, high-frequency microwave conductivity and photoluminescence measurements conducted simultaneously in situ. Such measurements are used to reveal and contrast the dominant charge-carrier recombination pathways for a range of key compositions: prototypical methylammonium lead iodide perovskite (MAPbI₃), the stable mixed formamidinium-caesium lead-halide perovskite FA_0.83Cs_0.17PbBr_0.6I_2.4 targeted for photovoltaic tandems with silicon, and fully inorganic wide-bandgap CsPbBr₃ aimed toward light sources and X-ray detector applications. The changes in charge-carrier dynamics in FA_0.83Cs_0.17PbBr_0.6I_2.4 across temperatures are shown to be dominated by radiative processes, while those in MAPbI₃ are governed by energetic disorder at low temperatures, low-bandgap minority-phase inclusions around the phase transition, and non-radiative processes at room temperature. In contrast, CsPbBr₃ exhibits significant charge-carrier trapping at low and high temperatures, highlighting the need for improvement of material processing techniques for wide-bandgap perovskites.

Chalcohalide antiperovskite thin films with visible light absorption and high charge-carrier mobility processed by solvent-free and low-temperature methods,
P. Sebastia-Luna, N. Rodkey, A. S. Mirza, S. Mertens, S. Lal, A. M. G. Carranza, J. Calbo, M. Righetto, M. Sessolo, L. M. Herz, K. Vandewal, E. Orti, M. Morales-Masis, H. J. Bolink, and F. Palazon,
Chem. Mater., 35 (2023), p. 6482–6490. [journal | article | SI ]

Silver chalcohalide antiperovskites represent a rather unexplored alternative to lead halide perovskites and other semiconductors based on toxic heavy metals. All synthetic approaches reported so far for Ag₃SI and Ag₃SBr require long synthesis times (typically days, weeks, or even months) and high temperatures. Herein, we report the synthesis of these materials using a fast and low-temperature method involving mechanochemistry. Structural and optical properties are examined experimentally and supported by first-principles calculations. Furthermore, we deposit Ag₃SI as thin films by pulsed laser deposition and characterize its optoelectronic properties using optical-pump-terahertz-probe measurements, revealing a high charge-carrier mobility of 49 cm² V^–1 s^–1. This work paves the way to the implementation of chalcohalide antiperovskites in various optoelectronic applications.

Bandlike transport and charge-carrier dynamics in BiOI films,
S. Lal, M. Righetto, A. M. Ulatowski, S. G. Motti, Z. Sun, J. L. MacManus-Driscoll, R. L. Z. Hoye, and L. M. Herz,
J. Phys. Chem. Lett., 14 (2023), p. 6620–6629. [journal | article | SI ]

Following the emergence of lead halide perovskites (LHPs) as materials for efficient solar cells, research has progressed to explore stable, abundant, and nontoxic alternatives. However, the performance of such lead-free perovskite-inspired materials (PIMs) still lags significantly behind that of their LHP counterparts. For bismuth-based PIMs, one significant reason is a frequently observed ultrafast charge-carrier localization (or self-trapping), which imposes a fundamental limit on long-range mobility. Here we report the terahertz (THz) photoconductivity dynamics in thin films of BiOI and demonstrate a lack of such self-trapping, with good charge-carrier mobility, reaching ∼3 cm² V^–1 s^–1 at 295 K and increasing gradually to ∼13 cm² V^–1 s^–1 at 5 K, indicative of prevailing bandlike transport. Using a combination of transient photoluminescence and THz- and microwave-conductivity spectroscopy, we further investigate charge-carrier recombination processes, revealing charge-specific trapping of electrons at defects in BiOI over nanoseconds and low bimolecular band-to-band recombination. Subject to the development of passivation protocols, BiOI thus emerges as a superior light-harvesting semiconductor among the family of bismuth-based semiconductors.

Atomistic understanding of the coherent interface between lead iodide perovskite and lead iodide,
M. U. Rothmann, K. B. Lohmann, J. Borchert, M. B. Johnston, K. P. McKenna, L. M. Herz, and P. D. Nellist,
Advanced Materials Interfaces, 10 (2023), p. 2300249. [journal | article | SI ]

Metal halide perovskite semiconductors have shown great performance in solar cells, and including an excess of lead iodide (PbI₂) in the thin films, either as mesoscopic particles or embedded domains, often leads to improved solar cell performance. Atomic resolution scanning transmission electron microscope micrographs of formamidinium lead iodide (FAPbI₃) perovskite films reveal the FAPbI₃:PbI₂ interface to be remarkably coherent. It is demonstrated that such interface coherence is achieved by the PbI₂ deviating from its common 2H hexagonal phase to form a trigonal 3R polytype through minor shifts in the stacking of the weakly van-der-Waals-bonded layers containing the near-octahedral units. The exact crystallographic interfacial relationship and lattice misfit are revealed. It is further shown that this 3R polytype of PbI₂ has similar X-ray diffraction (XRD) peaks to that of the perovskite, making XRD-based quantification of the presence of PbI₂ unreliable. Density functional theory demonstrates that this interface does not introduce additional electronic states in the bandgap, making it electronically benign. These findings explain why a slight PbI₂ excess during perovskite film growth can help template perovskite crystal growth and passivate interfacial defects, improving solar cell performance.

Temperature-dependent reversal of phase segregation in mixed-halide perovskites,
A. D. Wright, J. B. Patel, M. B. Johnston, and L. M. Herz,
Advanced Materials, 35 (2023), p. 2210834. [journal | article | SI ]

Understanding the mechanism of light-induced halide segregation in mixed-halide perovskites is essential for their application in multijunction solar cells. Here, photoluminescence spectroscopy is used to uncover how both increases in temperature and light intensity can counteract the halide segregation process. It is observed that, with increasing temperature, halide segregation in CH₃NH₃Pb(Br_0.4I_0.6)₃ first accelerates toward 290 K, before slowing down again toward higher temperatures. Such reversal is attributed to the trade-off between the temperature activation of segregation, for example through enhanced ionic migration, and its inhibition by entropic factors. High light intensities meanwhile can also reverse halide segregation; however, this is found to be only a transient process that abates on the time scale of minutes. Overall, these observations pave the way for a more complete model of halide segregation and aid the development of highly efficient and stable perovskite multijunction and concentrator photovoltaics.

Exciton formation dynamics and band-like free charge-carrier transport in 2D metal halide perovskite semiconductors,
S. G. Motti, M. Kober-Czerny, M. Righetto, P. Holzhey, J. Smith, H. Kraus, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Advanced Functional Materials, 33 (2023), p. 2300363. [journal | article | SI ]

Metal halide perovskite (MHP) semiconductors have driven a revolution in optoelectronic technologies over the last decade, in particular for high-efficiency photovoltaic applications. Low-dimensional MHPs presenting electronic confinement have promising additional prospects in light emission and quantum technologies. However, the optimisation of such applications requires a comprehensive understanding of the nature of charge carriers and their transport mechanisms. This study employs a combination of ultrafast optical and terahertz spectroscopy to investigate phonon energies, charge-carrier mobilities, and exciton formation in 2D (PEA)₂PbI₄ and (BA)₂PbI₄ (where PEA is phenylethylammonium and BA is butylammonium). Temperature-dependent measurements of free charge-carrier mobilities reveal band transport in these strongly confined semiconductors, with surprisingly high in-plane mobilities. Enhanced charge-phonon coupling is shown to reduce charge-carrier mobilities in (BA)₂PbI₄ with respect to (PEA)₂PbI₄. Exciton and free charge-carrier dynamics are disentangled by simultaneous monitoring of transient absorption and THz photoconductivity. A sustained free charge-carrier population is observed, surpassing the Saha equation predictions even at low temperature. These findings provide new insights into the temperature-dependent interplay of exciton and free-carrier populations in 2D MHPs. Furthermore, such sustained free charge-carrier population and high mobilities demonstrate the potential of these semiconductors for applications such as solar cells, transistors, and electrically driven light sources.

Photovoltaic performance of FAPbI₃ perovskite is hampered by intrinsic quantum confinement,
K. A. Elmestekawy, B. M. Gallant, A. D. Wright, P. Holzhey, N. K. Noel, M. B. Johnston, H. J. Snaith, and L. M. Herz,
ACS Energy Letters, 8 (2023), p. 2543–2551. [journal | article | SI ]

Formamidinium lead trioiodide (FAPbI₃) is a promising perovskite for single-junction solar cells. However, FAPbI₃ is metastable at room temperature and can cause intrinsic quantum confinement effects apparent through a series of above-bandgap absorption peaks. Here, we explore three common solution-based film-fabrication methods, neat N,N-dimethylformamide (DMF)–dimethyl sulfoxide (DMSO) solvent, DMF-DMSO with methylammonium chloride, and a sequential deposition approach. The latter two offer enhanced nucleation and crystallization control and suppress such quantum confinement effects. We show that elimination of these absorption features yields increased power conversion efficiencies (PCEs) and short-circuit currents, suggesting that quantum confinement hinders charge extraction. A meta-analysis of literature reports, covering 244 articles and 825 photovoltaic devices incorporating FAPbI₃ films corroborates our findings, indicating that PCEs rarely exceed a 20% threshold when such absorption features are present. Accordingly, ensuring the absence of these absorption features should be the first assessment when designing fabrication approaches for high-efficiency FAPbI₃ solar cells.

Thermally stable perovskite solar cells by all-vacuum deposition,
Q. Yuan, K. B. Lohmann, R. D. J. Oliver, A. J. Ramadan, S. Yan, J. M. Ball, M. G. Christoforo, N. K. Noel, H. J. Snaith, L. M. Herz, and M. B. Johnston,
ACS Appl. Mater. Interfaces, 15 (2023), p. 772−781. [journal | article | SI ]

Vacuum deposition is a solvent-free method suitable for growing thin films of metal halide perovskite (MHP) semiconductors. However, most reports of high-efficiency solar cells based on such vacuum-deposited MHP films incorporate solution-processed hole transport layers (HTLs), thereby complicating prospects of industrial upscaling and potentially affecting the overall device stability. In this work, we investigate organometallic copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) as alternative, low-cost, and durable HTLs in all-vacuum-deposited solvent-free formamidinium-cesium lead triodide (CH(NH₂)₂)_0.83Cs_0.17PbI₃ (FACsPbI₃) perovskite solar cells. We elucidate that the CuPc HTL, when employed in an “inverted” p–i–n solar cell configuration, attains a solar-to-electrical power conversion efficiency of up to 13.9%. Importantly, unencapsulated devices as large as 1 cm² exhibited excellent long-term stability, demonstrating no observable degradation in efficiency after more than 5000 h in storage and 3700 h under 85^oC thermal stressing in N₂ atmosphere.

Narrowband, angle-tunable, helicity-dependent terahertz emission from nanowires of the topological dirac semimetal Cd₃As₂,
J. L. Boland, D. A. Damry, C. Q. Xia, P. Schönherr, D. Prabhakaran, L. M. Herz, T. Hesjedal, and M. B. Johnston,
ACS Photonics, 10 (2023), p. 1473–1484. [journal | article | SI ]

All-optical control of terahertz pulses is essential for the development of optoelectronic devices for next-generation quantum technologies. Despite substantial research in THz generation methods, polarization control remains difficult. Here, we demonstrate that by exploiting band structure topology, both helicity-dependent and helicity-independent THz emission can be generated from nanowires of the topological Dirac semimetal Cd₃As₂. We show that narrowband THz pulses can be generated at oblique incidence by driving the system with optical (1.55 eV) pulses with circular polarization. Varying the incident angle also provides control of the peak emission frequency, with peak frequencies spanning 0.21–1.40 THz as the angle is tuned from 15 to 45^o. We therefore present Cd₃As₂ nanowires as a promising novel material platform for controllable terahertz emission.

Charge-carrier dynamics of solution-processed antimony- and bismuth-based chalcogenide thin films,
Z. Jia, M. Righetto, Y. Yang, C. Q. Xia, Y. Li, R. Li, Y. Li, B. Yu, Y. Liu, H. Huang, M. B. Johnston, L. M. Herz, and Q. Lin,
ACS Energy Letters, 8 (2023), p. 1485–1492. [journal | article | SI ]

Chalcogenide-based semiconductors have recently emerged as promising candidates for optoelectronic devices, benefiting from their low-cost, solution processability, excellent stability and tunable optoelectronic properties. However, the understanding of their fundamental optoelectronic properties is far behind the success of device performance and starts to limit their further development. To fill this gap, we conduct a comparative study of chalcogenide absorbers across a wide material space, in order to assess their suitability for different types of applications. We utilize optical-pump terahertz-probe spectroscopy and time-resolved microwave conductivity techniques to fully analyze their charge-carrier dynamics. We show that antimony-based chalcogenide thin films exhibit relatively low charge-carrier mobilities and short lifetimes, compared with bismuth-based chalcogenides. In particular, AgBiS₂ thin films possess the highest mobility, and Sb₂S₃ thin films have less energetic disorder, which are beneficial for photovoltaic devices. On the contrary, Bi₂S₃ showed ultralong carrier lifetime and high photoconductive gain, which is beneficial for designing photoconductors.

Chloride-based additive engineering for efficient and stable wide-bandgap perovskite solar cells,
X. Shen, B. M. Gallant, P. Holzhey, J. A. Smith, K. A. Elmestekawy, Z. Yuan, P. V. G. M. Rathnayake, S. Bernardi, A. Dasgupta, E. Kasparavicius, T. Malinauskas, O. Shargaieva, Y.-H. Lin, M. M. McCarthy, E. Unger, V. Getautis, A. Widmer-Cooper, L. M. Herz, and H. J. Snaith,
Advanced Materials, 35 (2023), p. 2211742. [journal | article | SI ]

Metal halide perovskite based tandem solar cells are promising to achieve power conversion efficiency beyond the theoretical limit of their single-junction counterparts. However, overcoming the significant open-circuit voltage deficit present in wide-bandgap perovskite solar cells remains a major hurdle for realizing efficient and stable perovskite tandem cells. Here, a holistic approach to overcoming challenges in 1.8 eV perovskite solar cells is reported by engineering the perovskite crystallization pathway by means of chloride additives. In conjunction with employing a self-assembled monolayer as the hole-transport layer, an open-circuit voltage of 1.25 V and a power conversion efficiency of 17.0% are achieved. The key role of methylammonium chloride addition is elucidated in facilitating the growth of a chloride-rich intermediate phase that directs crystallization of the desired cubic perovskite phase and induces more effective halide homogenization. The as-formed 1.8 eV perovskite demonstrates suppressed halide segregation and improved optoelectronic properties.

Interplay of structure, charge-carrier localization and dynamics in copper-silver-bismuth-halide semiconductors,
L. R. Buizza, H. C. Sansom, A. D. Wright, A. M. Ulatowski, M. B. Johnston, H. J. Snaith, and L. M. Herz,
Advanced Functional Materials, 32 (2022), p. 2108392. [journal | article | SI ]

Silver-bismuth based semiconductors represent a promising new class of materials for optoelectronic applications because of their high stability, all-inorganic composition, and advantageous optoelectronic properties. In this study, charge-carrier dynamics and transport properties are investigated across five compositions along the AgBiI₄–CuI solid solution line (stoichiometry Cu_4x(AgBi)_1−xI₄). The presence of a close-packed iodide sublattice is found to provide a good backbone for general semiconducting properties across all of these materials, whose optoelectronic properties are found to improve markedly with increasing copper content, which enhances photoluminescence intensity and charge-carrier transport. Photoluminescence and photoexcitation-energy-dependent terahertz photoconductivity measurements reveal that this enhanced charge-carrier transport derives from reduced cation disorder and improved electronic connectivity owing to the presence of Cu⁺. Further, increased Cu⁺ content enhances the band curvature around the valence band maximum, resulting in lower charge-carrier effective masses, reduced exciton binding energies, and higher mobilities. Finally, ultrafast charge-carrier localization is observed upon pulsed photoexcitation across all compositions investigated, lowering the charge-carrier mobility and leading to Langevin-like bimolecular recombination. This process is concluded to be intrinsically linked to the presence of silver and bismuth, and strategies to tailor or mitigate the effect are proposed and discussed.

Atomically resolved electrically active intragrain interfaces in perovskite semiconductors,
S. Cai, J. Dai, Z. Shao, M. U. Rothmann, Y. Jia, C. Gao, M. Hao, S. Pang, P. Wang, S. P. Lau, K. Zhu, J. J. Berry, L. M. Herz, X. C. Zeng, and Y. Zhou,
J. Am. Chem. Soc., 144 (2022), p. 1910–1920. [journal | article | SI ]

Deciphering the atomic and electronic structures of interfaces is key to developing state-of-the-art perovskite semiconductors. However, conventional characterization techniques have limited previous studies mainly to grain-boundary interfaces, whereas the intragrain-interface microstructures and their electronic properties have been much less revealed. Herein using scanning transmission electron microscopy, we resolved the atomic-scale structural information on three prototypical intragrain interfaces, unraveling intriguing features clearly different from those from previous observations based on standalone films or nanomaterial samples. These intragrain interfaces include composition boundaries formed by heterogeneous ion distribution, stacking faults resulted from wrongly stacked crystal planes, and symmetrical twinning boundaries. The atomic-scale imaging of these intragrain interfaces enables us to build unequivocal models for the ab initio calculation of electronic properties. Our results suggest that these structure interfaces are generally electronically benign, whereas their dynamic interaction with point defects can still evoke detrimental effects. This work paves the way toward a more complete fundamental understanding of the microscopic structure–property–performance relationship in metal halide perovskites.

Understanding and suppressing non-radiative losses in methylammonium-free wide-bandgap perovskite solar cells,
R. D. Oliver, P. Caprioglio, F. Pena-Camargo, L. Buizza, F. Zu, A. J. Ramadan, S. Motti, S. Mahesh, M. McCarthy, J. H. Warby, Y.-H. Lin, N. Koch, S. Albrecht, L. M. Herz, M. B. Johnston, D. Neher, M. Stolterfoht, and H. J. Snaith,
Energy Environ. Sci., 15 (2022), p. 714–726. [journal | article | SI ]

With power conversion efficiencies of perovskite-on-silicon and all-perovskite tandem solar cells increasing at rapid pace, wide bandgap (41.7 eV) metal-halide perovskites (MHPs) are becoming a major focus of academic and industrial photovoltaic research. Compared to their lower bandgap (<=1.6 eV) counterparts, these types of perovskites suffer from higher levels of non-radiative losses in both the bulk material and in device configurations, constraining their efficiencies far below their thermodynamic potential. In this work, we investigate the energy losses in methylammonium (MA) free high-Br-content wide bandgap perovskites by using a combination of THz spectroscopy, steady-state and time-resolved photoluminescence, coupled with drift-diffusion simulations. The investigation of this system allows us to study charge-carrier recombination in these materials and devices in the absence of halide segregation due to the photostabilty of formamidinium-cesium based lead halide perovskites. We find that these perovskites are characterised by large non-radiative recombination losses in the bulk material and that the interfaces with transport layers in solar cell devices strongly limit their opencircuit voltage. In particular, we discover that the interface with the hole transport layer performs particularly poorly, in contrast to 1.6 eV bandgap MHPs which are generally limited by the interface with the electron-transport layer. To overcome these losses, we incorporate and investigate the recombination mechanisms present with perovskites treated with the ionic additive 1-butyl-1- methylpipiderinium tetrafluoroborate. We find that this additive not only improves the radiative efficiency of the bulk perovskite, but also reduces the non-radiative recombination at both the hole and electron transport layer interfaces of full photovoltaic devices. In addition to unravelling the beneficial effect of this specific treatment, we further optimise our solar cells by introducing an additional LiF interface treatment at the electron transport layer interface. Together these treatments enable MA-free 1.79 eV bandgap perovskite solar cells with open-circuit voltages of 1.22 V and power conversion efficiencies approaching 17%, which is among the highest reported for this material system.

Solvent-free method for defect reduction and improved performance of p-i-n vapor-deposited perovskite solar cells,
K. B. Lohmann, S. G. Motti, R. D. J. Oliver, A. J. Ramadan, H. C. Sansom, Q. Yuan, K. A. Elmestekawy, J. B. Patel, J. M. Ball, L. M. Herz, H. J. Snaith, and M. B. Johnston,
ACS Energy Lett., 7 (2022), p. 1903–1911. [journal | article | SI ]

As perovskite-based photovoltaics near commercialization, it is imperative to develop industrial-scale defect-passivation techniques. Vapor deposition is a solvent-free fabrication technique that is widely implemented in industry and can be used to fabricate metal-halide perovskite thin films. We demonstrate markably improved growth and optoelectronic properties for vapor-deposited [CH(NH₂)₂|_0.83Cs_0.17PbI₃ perovskite solar cells by partially substituting PbI₂ for PbCl₂ as the inorganic precursor. We find the partial substitution of PbI₂ for PbCl₂ enhances photoluminescence lifetimes from 5.6 ns to over 100 ns, photoluminescence quantum yields by more than an order of magnitude, and charge-carrier mobility from 46 cm²/(V s) to 56 cm²/(V s). This results in improved solar-cell power conversion efficiency, from 16.4% to 19.3% for the devices employing perovskite films deposited with 20% substitution of PbI₂ for PbCl₂. Our method presents a scalable, dry, and solvent-free route to reducing nonradiative recombination centers and hence improving the performance of vapor-deposited metal-halide perovskite solar cells.

Optoelectronic properties of mixed iodide-bromide perovskites from first principles computational modeling and experiment,
Y. Chen, S. G. Motti, R. D. J. Oliver, A. D. Wright, H. J. Snaith, M. B. Johnston, L. M. Herz, and M. R. Filip,
J. Phys. Chem. Lett., 13 (2022), p. 4184–4192. [journal | article | SI ]

Halogen mixing in lead-halide perovskites is an effective route for tuning the band gap in light emission and multijunction solar cell applications. Here we report the effect of halogen mixing on the optoelectronic properties of lead-halide perovskites from theory and experiment. We applied the virtual crystal approximation within density functional theory, the GW approximation, and the Bethe–Salpeter equation to calculate structural, vibrational, and optoelectronic properties for a series of mixed halide perovskites. We separately perform spectroscopic measurements of these properties and analyze the impact of halogen mixing on quasiparticle band gaps, effective masses, absorption coefficients, charge-carrier mobilities, and exciton binding energies. Our joint theoretical–experimental study demonstrates that iodide–bromide mixed-halide perovskites can be modeled as homovalent alloys, and local structural distortions do not play a significant role for the properties of these mixed species. Our study outlines a general theoretical–experimental framework for future investigations of novel chemically mixed systems.

Air-degradation mechanisms in mixed lead-tin halide perovskites for solar cells,
V. J.-Y. Lim, A. M. Ulatowski, C. Kamaraki, M. T. Klug, L. Miranda Perez, M. B. Johnston, and L. M. Herz,
Advanced Energy Materials, 12 (2022), p. 2200847. [journal | article | SI ]

Owing to the bandgap-bowing effect, mixed lead-tin halide perovskites provide ideal bandgaps for the bottom subcell of all-perovskite tandem photovoltaic devices that offer fundamentally elevated power-conversion efficiencies. However, these materials suffer from degradation in ambient air, which worsens their optoelectronic properties and hinders their usability for photovoltaic applications. Such degradation pathways are not yet fully understood, especially for the perovskites in the middle of the APb_xSn_1-xI₃ solid solution line, which offer the narrowest bandgaps across the range. This study unravels the degradation mechanisms of APb_xSn_1-xI₃ perovskites, reporting clear differences between mixed lead-tin (x = 0.5) and tin-only (x = 0) perovskites. The dynamic optoelectronic properties, electronic structure, crystal structure, and decomposition products of the perovskite thin films are examined in situ during air exposure. Both perovskite compositions suffer from the formation of defects over the timescale of hours, as indicated by a significant reduction in their charge-carrier diffusion lengths. For tin-only perovskite, degradation predominantly causes the formation of energetically shallow tin vacancies and hole doping. However, for mixed lead-tin perovskite, deep trap states are formed that significantly accelerate charge-carrier recombination, yet leave mobilities relatively unaffected. These findings highlight the need for passivation strategies tailored specifically to mixed lead-tin iodide perovskites.

Controlling intrinsic quantum confinement in formamidinium lead triiodide perovskite through Cs substitution,
K. A. Elmestekawy, A. D. Wright, K. B. Lohmann, J. Borchert, M. B. Johnston, and L. M. Herz,
ACS Nano, 16 (2022), p. 9640–9650. [journal | article | SI ]

Lead halide perovskites are leading candidates for photovoltaic and light-emitting devices, owing to their excellent and widely tunable optoelectronic properties. Nanostructure control has been central to their development, allowing for improvements in efficiency and stability, and changes in electronic dimensionality. Recently, formamidinium lead triiodide (FAPbI₃) has been shown to exhibit intrinsic quantum confinement effects in nominally bulk thin films, apparent through above-bandgap absorption peaks. Here, we show that such nanoscale electronic effects can be controlled through partial replacement of the FA cation with Cs. We find that Cs-cation exchange causes a weakening of quantum confinement in the perovskite, arising from changes in the bandstructure, the length scale of confinement, or the presence of δ_H-phase electronic barriers. We further observe photon emission from quantum-confined regions, highlighting their potential usefulness to light-emitting devices and single-photon sources. Overall, controlling this intriguing quantum phenomenon will allow for its suppression or enhancement according to need.

Silver-bismuth based 2D double perovskites (4FPEA)₄AgBiX₈ (X = Cl, Br, I): Highly oriented thin films with large domain sizes and ultrafast charge-carrier localization,
R. Hooijer, A. Weis, A. Biewald, M. T. Sirtl, J. Malburg, R. Holfeuer, S. Thamm, A. A. Y. Amin, M. Righetto, A. Hartschuh, L. M. Herz, and T. Bein,
Advanced Optical Materials, 10 (2022), p. 2200354. [journal | article | SI ]

Two-dimensional (2D) hybrid double perovskites are a promising emerging class of materials featuring superior intrinsic and extrinsic stability over their 3D parent structures, while enabling additional structural diversity and tunability. Here, we expand the Ag–Bi-based double perovskite system, comparing structures obtained with the halides chloride, bromide, and iodide and the organic spacer cation 4-fluorophenethylammonium (4FPEA) to form the n = 1 Ruddlesden–Popper (RP) phases (4FPEA)₄AgBiX₈ (X = Cl, Br, I). We demonstrate access to the iodide RP-phase through a simple organic spacer, analyze the different properties as a result of halide substitution and incorporate the materials into photodetectors. Highly oriented thin films with very large domain sizes are fabricated and investigated with grazing incidence wide angle X-ray scattering, revealing a strong dependence of morphology on substrate choice and synthesis parameters. First-principles calculations confirm a direct band gap and show type Ib and IIb band alignment between organic and inorganic quantum wells. Optical characterization, temperature-dependent photoluminescence, and optical-pump terahertz-probe spectroscopy give insights into the absorption and emissive behavior of the materials as well as their charge-carrier dynamics. Overall, we further elucidate the possible reasons for the electronic and emissive properties of these intriguing materials, dominated by phonon-coupled and defect-mediated polaronic states.

Excellent long-range charge-carrier mobility in 2D perovskites,
M. Kober-Czerny, S. G. Motti, P. Holzhey, B. Wenger, J. Lim, L. M. Herz, and H. J. Snaith,
Advanced Functional Materials, 32 (2022), p. 2203064. [journal | article | SI ]

The use of layered, 2D perovskites can improve the stability of metal halide perovskite thin films and devices. However, the charge carrier transport properties in layered perovskites are still not fully understood. Here, the sum of the electron and hole mobilities (Σμ) in thin films of the 2D perovskite PEA₂PbI₄, through transient electronically contacted nanosecond-to-millisecond photoconductivity measurements, which are sensitive to long-time, long-range (micrometer length scale) transport processes is investigated. After careful analysis, accounting for both early-time recombination and the evolution of the exciton-to-free-carrier population, a long-range mobility of 8.0 0.6 cm² (V s)^–1, which is ten times greater than the long-range mobility of a comparable 3D material FA_0.9Cs_0.1PbI₃ is determined. These values are compared to ultra-fast transient time-resolved THz photoconductivity measurements, which are sensitive to early-time, shorter-range (tens of nm length scale) mobilities. Mobilities of 8 and 45 cm² (V s)^–1 in the case of the PEA₂PbI₄ and FA_0.9Cs_0.1PbI₃, respectively, are obtained. This previously unreported concurrence between the long-range and short-range mobility in a 2D material indicates that the polycrystalline thin films already have single-crystal-like qualities. Hence, their fundamental charge carrier transport properties should aid device performance.

Strong absorption and ultrafast localisation in NaBiS₂ nanocrystals with microsecond charge-carrier lifetimes,
Y.-T. Huang, S. R. Kavanagh, M. Righetto, M. Rusu, T. Unold, A. J. Sneyd, S. J. Zelewski, K. Zhang, L. Dai, A. J. Britton, J. J. J. Ye, M. P. Napari, Z. Zhang, J. Xiao, M. Laitinen, L. Torrente-Murciano, S. D. Stranks, A. Rao, L. M. Herz, D. O. Scanlon, A. Walsh, and R. L. Z. Hoye,
Nature Communications, 13 (2022), p. 4960. [journal | article | SI ]

I-V-VI₂ ternary chalcogenides are gaining attention as earth-abundant, nontoxic, and air-stable absorbers for photovoltaic applications. However, the semiconductors explored thus far have slowly-rising absorption onsets, and their charge-carrier transport is not well understood yet. Herein, we investigate cation-disordered NaBiS₂ nanocrystals, which have a steep absorption onset, with absorption coefficients reaching >10⁵ cm⁻¹ just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-time scale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS₂ nanocrystals. These unusual features arise because of the localised, non-bonding S p character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes. This work reveals the critical role of cation disorder in these systems on both absorption characteristics and charge-carrier kinetics.

Advances and challenges in understanding the microscopic structure-property-performance relationship in perovskite solar cells,
Y. Zhou, L. M. Herz, A. K.-Y. Jen, and M. Saliba,
Nature Energy, 7 (2022), p. 794–807. [journal | article | SI ]

The emergence of perovskite photovoltaic technology is transforming the landscape of solar energy. Its rapid development has been driven by the advances in our understanding of the thin-film microstructures of metal halide perovskites and their intriguing correlations with optoelectronic properties, device efficiency and long-term stability. Here we discuss the morphological characteristics of three key microstructure types encountered in perovskites, which include grain boundaries, intragrain defects and surfaces. To reveal detailed structural information of these microstructure types via tailored characterizations is crucial to probe their detrimental, neutral or beneficial effects on optoelectronic properties. We further elaborate the impacts of these microstructures on the degradation modes of perovskites. Representative examples are also presented, which have translated fundamental understandings to achieve state-of-the-art perovskite solar cells. Finally, we call for more attention in probing hidden microstructures and developing high-spatiotemporal-resolution characterizations, as well as harnessing the potential merits of microstructural imperfections, towards an elevated understanding of microstructure–property–performance relationships for the next solar cell advances.

Impact of hole-transport layer and interface passivation on halide segregation in mixed-halide perovskites,
V. J.-Y. Lim, A. J. Knight, R. D. J. Oliver, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Advanced Functional Materials, 32 (2022), p. 2204825. [journal | article | SI ]

Mixed-halide perovskites offer ideal bandgaps for tandem solar cells, but photoinduced halide segregation compromises photovoltaic device performance. This study explores the influence of a hole-transport layer, necessary for a full device, by monitoring halide segregation through in situ, concurrent X-ray diffraction and photoluminescence measurements to disentangle compositional and optoelectronic changes. This work demonstrates that top coating FA_0.83Cs_0.17Pb(Br_0.4I_0.6)₃ perovskite films with a poly(triaryl)amine (PTAA) hole-extraction layer surprisingly leads to suppression of halide segregation because photogenerated charge carriers are rapidly trapped at interfacial defects that do not drive halide segregation. However, the generation of iodide-enriched regions near the perovskite/PTAA interface enhances hole back-transfer from the PTAA layer through improved energy level offsets, increasing radiative recombination losses. It is further found that while passivation with a piperidinium salt slows halide segregation in perovskite films, the addition of a PTAA top-coating accelerates such effects, elucidating the specific nature of trap types that are able to drive the halide segregation process. This work highlights the importance of selective passivation techniques for achieving efficient and stable wide-bandgap perovskite photovoltaic devices.

Bending a photonic wire into a ring,
H. Gotfredsen, J.-R. Deng, J. V. Raden, M. Righetto, J. Hergenhahn, M. Clarke, A. Bellamy-Carter, J. Hart, J. O’Shea, T. D. W. Claridge, A. S. F. Duarte, L. M. Herz, and H. L. Anderson,
Nature Chemistry, 14 (2022), p. 1436–1442. [journal | article | SI ]

Natural light-harvesting systems absorb sunlight and transfer its energy to the reaction centre, where it is used for photosynthesis. Synthetic chromophore arrays provide useful models for understanding energy migration in these systems. Research has focused on mimicking rings of chlorophyll molecules found in purple bacteria, known as `light-harvesting system 2'. Linear meso–meso linked porphyrin chains mediate rapid energy migration, but until now it has not been possible to bend them into rings. Here we show that oligo-pyridyl templates can be used to bend these rod-like photonic wires to create covalent nanorings that consist of 24 porphyrin units and a single butadiyne link. Their elliptical conformations have been probed by scanning tunnelling microscopy. This system exhibits two excited state energy transfer processes: one from a bound template to the peripheral porphyrins and one, in the template-free ring, from the exciton-coupled porphyrin array to the π-conjugated butadiyne-linked porphyrin dimer segment.

Nanotechnology for catalysis and solar energy conversion,
U. Banin, N. Waiskopf, L. Hammarström, G. Boschloo, M. Freitag, E. M. J. Johansson, J. Sa, H. Tian, M. B. Johnston, L. M. Herz, R. L. Milot, M. G. Kanatzidis, I. S. W Ke, K. L. Kohlstedt, G. C. Schatz, N. Lewis, T. Meyer, A. J. Nozik, M. C. Beard, V. S. Batista, G. W. Brudvig, C. A. Schmuttenmaer, F. Armstrong, and C. F. Megarity,
Nanotechnology, 32 (2021), p. 042003. [journal | article | SI ]

This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: `high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing' to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as described by Spanopoulos et al. `Next generation' solar cells using multiple exciton generation (MEG) from hot carriers, described in the article by Nozik and Beard, could lead to remarkable improvement in photovoltaic efficiency by using quantization effects in semiconductor nanostructures (quantum dots, wires or wells). These challenges will not be met without simultaneous improvement in nanoscale characterization methods. Terahertz spectroscopy, discussed in the article by Milot et al. is one example of a method that is overcoming the difficulties associated with nanoscale materials characterization by avoiding electrical contacts to nanoparticles, allowing characterization during device operation, and enabling characterization of a single nanoparticle. Besides experimental advances, computational science is also meeting the challenges of nanomaterials synthesis. The article by Kohlstedt and Schatz discusses the computational frameworks being used to predict structure–property relationships in materials and devices, including machine learning methods, with an emphasis on organic photovoltaics. In addition, biohybrid approaches can take advantage of efficient and specific enzyme catalysts. The contribution by Megarity and Armstrong presents the `electrochemical leaf' for improvements in electrochemistry and beyond. These articles present the nanoscience and technology at the forefront of renewable energy development that will have significant benefits to society.

Polarons and charge localisation in metal-halide semiconductors for photovoltaic and light-emitting devices,
L. R. V. Buizza and L. M. Herz,
Advanced Materials, 33 (2021), p. 2007057. [journal | article | SI ]

Metal-halide semiconductors have shown excellent performance in optoelectronic applications such as solar cells, light-emitting diodes and detectors. In this review, we summarise the role of charge-lattice interactions and polaron formation in a wide range of these promising materials, including three-dimensional perovskites and double perovskites, Ruddlesden-Popper layered perovskites, nanocrystals, vacancy-ordered systems and other novel structures. We discuss the formation of Fröhlich-type `large' polarons in archetypal bulk metal-halide ABX₃ perovskites and its dependence on A-cation, B-metal and X-halide composition, which is now relatively well understood. We find that for nanostructured and novel metal-halide materials, a much larger variation in the strengths of polaronic effects is reported across the literature, potentially deriving from variations in potential barriers and the presence of interfaces at which lattice relaxation may be enhanced. We further discuss such findings in the context of different experimental approaches used to explore polaronic effects, cautioning that firm conclusions are often hampered by the presence of alternate processes and interactions giving rise to similar experimental signatures. Overall, a complete understanding of such polaronic effects will prove essential given their direct influence on optoelectronic properties such as charge-carrier mobilities and emission spectra, which are critical to the performance of energy and optoelectronic applications.

Roadmap on organic-inorganic hybrid perovskite semiconductors and devices,
L. Schmidt-Mende, V. Dyakonov, S. Olthof, F. Ünlü, K. M. T. Le, S. Mathur, A. D. Karabanov, D. C. Lupascu, L. M. Herz, A. Hinderhofer, F. Schreiber, A. Chernikov, D. A. Egger, O. Shargaieva, C. Cocchi, E. Unger, M. Saliba, M. M. Byranvand, M. Kroll, F. Nehm, K. Leo, A. Redinger, J. Höcker, T. Kirchartz, J. Warby, E. Gutierrez-Partida, D. Neher, M. Stolterfoht, U. Würfel, M. Unmüssig, J. Herterich, C. Baretzky, J. Mohanraj, M. Thelakkat, C. Maheu, W. Jaegermann, T. Mayer, J. Rieger, T. Fauster, D. Niesner, F. Yang, S. Albrecht, T. Riedl, A. Fakharuddin, M. Vasilopoulou, Y. Vaynzof, D. Moia, J. Maier, M. Franckevicius, V. Gulbinas, R. A. Kerner, L. Zhao, B. P. Rand, N. Glück, T. Bein, F. Matteocci, L. A. Castriotta, A. D. Carlo, C. Draxl, and M. Scheffler,
APL Materials, 9 (2021), p. 109202. [journal | article | SI ]

Metal halide perovskites are the first solution processed semiconductors that can compete in their functionality with conventional semiconductors, such as silicon. Over the past several years, perovskite semiconductors have reported breakthroughs in various optoelectronic devices, such as solar cells, photodetectors, light emitting and memory devices, and so on. Until now, perovskite semiconductors face challenges regarding their stability, reproducibility, and toxicity. In this Roadmap, we combine the expertise of chemistry, physics, and device engineering from leading experts in the perovskite research community to focus on the fundamental material properties, the fabrication methods, characterization and photophysical properties, perovskite devices, and current challenges in this field. We develop a comprehensive overview of the current state-of-the-art and offer readers an informed perspective of where this field is heading and what challenges we have to overcome to get to successful commercialization.

Highly absorbing lead-free semiconductor Cu₂AgBiI₆ for photovoltaic applications from the quaternary CuI-AgI-BiI₃ phase space,
H. C. Sansom, G. Longo, A. D. Wright, L. R. V. Buizza, S. Mahesh, B. Wenger, M. Zanella, M. Abdi-Jalebi, M. J. Pitcher, M. S. Dyer, T. D. Manning, R. H. Friend, L. M. Herz, H. J. Snaith, J. B. Claridge, and M. J. Rosseinsky,
JACS, 143 (2021), p. 3983–3992. [journal | article | SI ]

Since the emergence of lead halide perovskites for photovoltaic research, there has been mounting effort in the search for alternative compounds with improved or complementary physical, chemical, or optoelectronic properties. Here, we report the discovery of Cu₂AgBiI₆: a stable, inorganic, lead-free wide-band-gap semiconductor, well suited for use in lead-free tandem photovoltaics. We measure a very high absorption coefficient of 1.0 × 10⁵ cm^–1 near the absorption onset, several times that of CH₃NH₃PbI₃. Solution-processed Cu₂AgBiI₆ thin films show a direct band gap of 2.06(1) eV, an exciton binding energy of 25 meV, a substantial charge-carrier mobility (1.7 cm² V^–1 s^–1), a long photoluminescence lifetime (33 ns), and a relatively small Stokes shift between absorption and emission. Crucially, we solve the structure of the first quaternary compound in the phase space among CuI, AgI and BiI₃. The structure includes both tetrahedral and octahedral species which are open to compositional tuning and chemical substitution to further enhance properties. Since the proposed double-perovskite Cs₂AgBiI₆ thin films have not been synthesized to date, Cu₂AgBiI₆ is a valuable example of a stable Ag⁺/Bi³⁺ octahedral motif in a close-packed iodide sublattice that is accessed via the enhanced chemical diversity of the quaternary phase space.

Halide segregation in mixed-halide perovskites: Influence of A-site cations,
A. J. Knight, J. Borchert, R. D. J. Oliver, J. B. Patel, P. G. Radaelli, H. J. Snaith, M. B. Johnston, and L. M. Herz,
ACS Energy Letters, 6 (2021), p. 799–808. [journal | article | SI ]

Mixed-halide perovskites offer bandgap tunability essential for multijunction solar cells; however, a detrimental halide segregation under light is often observed. Here we combine simultaneous in situ photoluminescence and X-ray diffraction measurements to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br_0.5I_0.5)₃ and FA_0.83Cs_0.17Pb(Br_0.4I_0.6)₃ films. We report evidence for low-barrier ionic pathways in MAPb(Br_0.5I_0.5)₃, which allow for the rearrangement of halide ions in localized volumes of perovskite without significant compositional changes to the bulk material. In contrast, FA_0.83Cs_0.17Pb(Br_0.4I_0.6)₃ lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation. Our work elucidates links between composition, ionic pathways, and halide segregation, and it facilitates the future engineering of phase-stable mixed-halide perovskites.

Crystallization of CsPbBr₃ single crystals in water for X-ray detection,
J. Peng, C. Q. Xia, Y. Xu, R. Li, L. Cui, J. K. Clegg, L. M. Herz, M. B. Johnston, and Q. Lin,
Nature Communications, 12 (2021), p. 1531. [journal | article | SI ]

Metal halide perovskites have fascinated the research community over the past decade, and demonstrated unprecedented success in optoelectronics. In particular, perovskite single crystals have emerged as promising candidates for ionization radiation detection, due to the excellent opto-electronic properties. However, most of the reported crystals are grown in organic solvents and require high temperature. In this work, we develop a low-temperature crystallization strategy to grow CsPbBr₃ perovskite single crystals in water. Then, we carefully investigate the structure and optoelectronic properties of the crystals obtained, and compare them with CsPbBr₃ crystals grown in dimethyl sulfoxide. Interestingly, the water grown crystals exhibit a distinct crystal habit, superior charge transport properties and better stability in air. We also fabricate X-ray detectors based on the CsPbBr₃ crystals, and systematically characterize their device performance. The crystals grown in water demonstrate great potential for X-ray imaging with enhanced performance metrics.

Limits to electrical mobility in lead-halide perovskite semiconductors,
C. Q. Xia, J. Peng, S. Ponce, J. B. Patel, A. D. Wright, T. W. Crothers, M. U. Rothmann, J. Borchert, R. L. Milot, H. Kraus, Q. Lin, F. Giustino, L. M. Herz, and M. B. Johnston,
J. Phys. Chem. Lett., 12 (2021), p. 3607–3617. [journal | article | SI ]

Semiconducting polycrystalline thin films are cheap to produce and can be deposited on flexible substrates, yet high-performance electronic devices usually utilize single-crystal semiconductors, owing to their superior charge-carrier mobilities and longer diffusion lengths. Here we show that the electrical performance of polycrystalline films of metal-halide perovskites (MHPs) approaches that of single crystals at room temperature. Combining temperature-dependent terahertz conductivity measurements and ab initio calculations we uncover a complete picture of the origins of charge-carrier scattering in single crystals and polycrystalline films of CH₃NH₃PbI₃. We show that Fröhlich scattering of charge carriers with multiple phonon modes is the dominant mechanism limiting mobility, with grain-boundary scattering further reducing mobility in polycrystalline films. We reconcile the large discrepancy in charge-carrier diffusion lengths between single crystals and films by considering photon reabsorption. Thus, polycrystalline films of MHPs offer great promise for devices beyond solar cells, including light-emitting diodes and modulators.

Ultrafast excited-state localization in Cs₂AgBiBr₆ double perovskite,
A. D. Wright, L. R. V. Buizza, K. J. Savill, G. Longo, H. J. Snaith, M. B. Johnston, and L. M. Herz,
J. Phys. Chem. Lett., 12 (2021), p. 3352–3360. [journal | article | SI ]

Cs₂AgBiBr₆ is a promising metal halide double perovskite offering the possibility of efficient photovoltaic devices based on lead-free materials. Here, we report on the evolution of photoexcited charge carriers in Cs₂AgBiBr₆ using a combination of temperature-dependent photoluminescence, absorption and optical pump−terahertz probe spectroscopy. We observe rapid decays in terahertz photoconductivity transients that reveal an ultrafast, barrier-free localization of free carriers on the time scale of 1.0 ps to an intrinsic small polaronic state. While the initially photogenerated delocalized charge carriers show bandlike transport, the self-trapped, small polaronic state exhibits temperature-activated mobilities, allowing the mobilities of both to still exceed 1 cm²V⁻¹s⁻¹ at room temperature. Self-trapped charge carriers subsequently diffuse to color centers, causing broad emission that is strongly red-shifted from a direct band edge whose band gap and associated exciton binding energy shrink with increasing temperature in a correlated manner. Overall, our observations suggest that strong electron−phonon coupling in this material induces rapid charge-carrier localization.

Charge-carrier mobility and localization in semiconducting Cu₂AgBiI₆ for photovoltaic applications,
L. R. V. Buizza, A. D. Wright, G. Longo, H. C. Sansom, C. Q. Xia, M. J. Rosseinsky, M. B. Johnston, H. J. Snaith, and L. M. Herz,
ACS Energy Letters, 6 (2021), p. 1729–1739. [journal | article | SI ]

Lead-free silver–bismuth semiconductors have become increasingly popular materials for optoelectronic applications, building upon the success of lead halide perovskites. In these materials, charge-lattice couplings fundamentally determine charge transport, critically affecting device performance. In this study, we investigate the optoelectronic properties of the recently discovered lead-free semiconductor Cu₂AgBiI₆ using temperature-dependent photoluminescence, absorption, and optical-pump terahertz-probe spectroscopy. We report ultrafast charge-carrier localization effects, evident from sharp THz photoconductivity decays occurring within a few picoseconds after excitation and a rise in intensity with decreasing temperature of long-lived, highly Stokes-shifted photoluminescence. We conclude that charge carriers in Cu₂AgBiI₆ are subject to strong charge-lattice coupling. However, such small polarons still exhibit mobilities in excess of 1 cm² V^–1 s^–1 at room temperature because of low energetic barriers to formation and transport. Together with a low exciton binding energy of ~29 meV and a direct band gap near 2.1 eV, these findings highlight Cu₂AgBiI₆ as an attractive lead-free material for photovoltaic applications.

Hot electron cooling in InSb probed by ultrafast time-resolved terahertz cyclotron resonance,
C. Q. Xia, M. Monti, J. L. Boland, L. M. Herz, J. Lloyd-Hughes, M. R. Filip, and M. B. Johnston,
Phys. Rev. B, 103 (2021), p. 245205. [journal | article | SI ]

Measuring terahertz (THz) conductivity on an ultrafast timescale is an excellent way to observe charge-carrier dynamics in semiconductors as a function of time after photoexcitation. However, a conductivity measurement alone cannot separate the effects of charge-carrier recombination from effective mass changes as charges cool and experience different regions of the electronic band structure. Here we present a form of time-resolved magneto-THz spectroscopy that allows us to measure cyclotron effective mass on a picosecond timescale. We demonstrate this technique by observing electron cooling in the technologically significant narrow-bandgap semiconductor indium antimonide. A significant reduction of electron effective mass from 0.032 to 0.017 m_e is observed in the first 200 ps after injecting hot electrons. The measured electron effective mass in InSb as a function of photoinjected electron density agrees well with conduction band nonparabolicity predictions from ab initio calculations of the quasiparticle band structure.

Ultrafast photo-induced phonon hardening in MAPbI₃ single-crystal and polycrystalline perovskites,
C. Q. Xia, S. Ponce, J. Peng, A. M. Ulatowski, J. B. Patel, A. D. Wright, R. Milot, H. Kraus, Q. Lin, L. Herz, F. Giustino, and M. B. Johnston,
J. Phys. Mater., 4 (2021), p. 044017. [journal | article | SI ]

Metal-halide perovskite semiconductors have attracted intensive interest in the last decade, particularly for applications in photovoltaics. Low-energy optical phonons combined with significant crystal anharmonicity play an important role in charge-carrier cooling and scattering in these materials, strongly affecting their optoelectronic properties. We have observed optical phonons associated with Pb-I stretching in both single crystals and polycrystalline thin films of methylammonium lead triiodide (MAPbI₃) as a function of temperature by measuring their terahertz (THz) conductivity spectra with and without photoexcitation. An anomalous bond hardening was observed under above-bandgap illumination for both single-crystal and polycrystalline MAPbI₃. First-principles calculations reproduced this photo-induced bond hardening and identified a related lattice contraction (photostriction), with the mechanism revealed as Pauli blocking. For single-crystal MAPbI₃, phonon lifetimes were significantly longer and phonon frequencies shifted less with temperature, compared with polycrystalline MAPbI₃. We attribute these differences to increased crystalline disorder, associated with grain boundaries and strain in the polycrystalline MAPbI₃. Thus we provide fundamental insight into the photoexcitation and electron--phonon coupling in MAPbI₃.

Optoelectronic properties of tin−lead halide perovskites,
K. J. Savill, A. M. Ulatowski, and L. M. Herz,
ACS Energy Letters, 6 (2021), p. 2413–2426. [journal | article | SI ]

Mixed tin−lead halide perovskites have recently emerged as highly promising materials for efficient single- and multi-junction photovoltaic devices. This Focus Review discusses the optoelectronic properties that underpin this performance, clearly differentiating between intrinsic and defect-mediated mechanisms. We show that from a fundamental perspective, increasing tin fraction may cause increases in attainable charge-carrier mobilities, decreases in exciton binding energies, and potentially a slowing of charge-carrier cooling, all beneficial for photovoltaic applications. We discuss the mechanisms leading to significant bandgap bowing along the tin−lead series, which enables attractive near-infrared bandgaps at intermediate tin content. However, tin-rich stoichiometries still suffer from tin oxidation and vacancy formation which often obscures the fundamentally achievable performance, causing high background hole densities, accelerating charge-carrier recombination, lowering charge-carrier mobilities, and blue-shifting absorption onsets through the Burstein−Moss effect. We evaluate impacts on photovoltaic device performance, and conclude with an outlook on remaining challenges and promising future directions in this area.

Revealing ultrafast charge-carrier thermalization in tin-iodide perovskites through novel pump–push–probe terahertz spectroscopy,
A. M. Ulatowski, M. D. Farrar, H. J. Snaith, M. B. Johnston, and L. M. Herz,
ACS Photonics, 8 (2021), p. 2509–2518. [journal | article | SI ]

Tin-iodide perovskites are an important group of semiconductors for photovoltaic applications, promising higher intrinsic charge-carrier mobilities and lower toxicity than their lead-based counterparts. Controllable tin vacancy formation and the ensuing hole doping provide interesting opportunities to investigate dynamic intraband transitions of charge carriers in these materials. Here, we present for the first time an experimental implementation of a novel Optical-Pump–IR-Push–THz-Probe spectroscopic technique and demonstrate its suitability to investigate the intraband relaxation dynamics of charge carriers brought into nonequilibrium by an infrared “push" pulse. We observe a push-induced decrease of terahertz conductivity for both chemically- and photodoped FA_0.83Cs_0.17SnI₃ thin films and show that these effects derive from stimulated THz emission. We use this technique to reveal that newly photogenerated charge carriers relax within the bands of FA_0.83Cs_0.17SnI₃ on a subpicosecond time scale when a large, already fully thermalized (cold) population of charge-carriers is present. Such rapid dissipation of the initial charge-carrier energy suggests that the propensity of tin halide perovskites toward unintentional self-doping resulting from tin vacancy formation makes these materials less suited to implementation in hot-carrier solar cells than their lead-based counterparts.

Chemical control of the octahedral network of solar absorbers from the CuI-AgI-BiI₃ phase space via the discovery of 3D CuAgBiI₅,
H. C. Sansom, L. R. Buizza, M. Zanella, J. T. Gibbon, M. J. Pitcher, M. S. Dyer, T. D. Manning, V. R. Dhanak, L. M. Herz, H. J. Snaith, J. B. Claridge, and M. J. Rosseinsky,
Inorganic Chemistry, 60 (2021), p. 18154–18167. [journal | article | SI ]

A newly reported compound, CuAgBiI₅, is synthesised as powder, crystals, and thin films. The structure consists of a 3D octahedral Ag⁺/Bi³⁺ network as in spinel but the tetrahedral interstitials occupied by Cu⁺ differ from those of a spinel. The 3D octahedral network of CuAgBiI₅ allows us to identify a relationship between octahedral site occupancy (composition) and octahedral motif (structure) across the whole CuI-AgI-BiI₃ phase field, giving the ability to chemically control structural dimensionality. To investigate composition-structure-property relationships, we compare the basic optoelectronic properties of CuAgBiI₅ with those of Cu₂AgBiI₆ (which has a 2D octahedral network) and reveal a surprisingly low sensitivity towards dimensionality of the octahedral network. The absorption onset of CuAgBiI₅ (2.02 eV) barely changes compared with that of Cu₂AgBiI₆ (2.06 eV) indicating no obvious signs of an increase in charge confinement. Such behaviour contrasts with that for lead halide perovskites which show clear confinement effects upon lowering dimensionality of the octahedral network from 3D to 2D. Changes in photoluminescence spectra and lifetimes between the two compounds mostly derive from difference in extrinsic defect densities rather than intrinsic effects. While both materials show good stability, bulk CuAgBiI₅ powder samples are found to be more sensitive degradation under solar irradiation compared to Cu₂AgBiI₆.

Phase segregation in mixed-halide perovskites affects charge-carrier dynamics while preserving mobility,
S. G. Motti, J. B. Patel, R. D. J. Oliver, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Nature Communications, 12 (2021), p. 6955. [journal | article | SI ]

Mixed halide perovskites can provide optimal bandgaps for tandem solar cells which are key to improved costefficiencies, but can still suffer from detrimental illumination-induced phase segregation. Here we employ optical-pump terahertz-probe spectroscopy to investigate the impact of halide segregation on the charge-carrier dynamics and transport properties of mixed halide perovskite films. We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities, and that charge carriers within the I-rich phase are not strongly localised. We further demonstrate enhanced lattice anharmonicity in the segregated I-rich domains, which is likely to support ionic migration. These phonon anharmonicity effects also serve as evidence of a remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains. Our analysis demonstrates how minimal structural transformations during phase segregation have a dramatic effect on the charge-carrier dynamics as a result of charge funnelling. We suggest that because such enhanced recombination is radiative, performance losses may be mitigated by deployment of careful light management strategies in solar cells.

Light absorption and recycling in hybrid metal halide perovskite photovoltaic devices,
J. B. Patel, A. D. Wright, K. B. Lohmann, K. Peng, C. Q. Xia, J. M. Ball, N. K. Noel, T. W. Crothers, J. Wong-Leung, H. J. Snaith, L. M. Herz, and M. B. Johnston,
Adv. Energy Mater., 10 (2020), p. 1903653. [journal | article | SI ]

Production of highly efficient single- and multi-junction metal halide perovskite (MHP) solar cells requires careful optimization of the optical and electrical properties of these devices. Here precise control of CH₃NH₃PbI₃ perovskite layers is demonstrated in solar cell devices through the use of dual source coevaporation. Light absorption and device performance are tracked for incorporated MHP films ranging from ~67 nm to ~1.4 μm thickness and transfer-matrix optical modeling is utilized to quantify optical losses that arise from interference effects. Based on these results, a device with 19.2% steady-state power conversion efficiency is achieved through incorporation of a perovskite film with near-optimum predicted thickness (~709 nm). Significantly, a clear signature of photon reabsorption is observed in perovskite films that have the same thickness (~709 nm) as in the optimized device. Despite the positive effect of photon recycling associated with photon reabsorption, devices with thicker (>750 nm) MHP layers exhibit poor performance owing to competing nonradiative charge recombination in a ”dead-volume” of MHP. Overall, these findings demonstrate the need for fine control over MHP thickness to achieve the highest efficiency cells, and accurate consideration of photon reabsorption, optical interference, and charge transport properties.

Trap states, electric fields, and phase segregation in mixed-halide perovskite photovoltaic devices,
A. J. Knight, J. B. Patel, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Adv. Energy Mater., 10 (2020), p. 1903488. [journal | article | SI ]

Mixed-halide perovskites are essential for use in all-perovskite or perovskite–silicon tandem solar cells due to their tunable bandgap. However, trap states and halide segregation currently present the two main challenges for efficient mixed-halide perovskite technologies. Here photoluminescence techniques are used to study trap states and halide segregation in full mixed-halide perovskite photovoltaic devices. This work identifies three distinct defect species in the perovskite material: a charged, mobile defect that traps charge-carriers in the perovskite, a charge-neutral defect that induces halide segregation, and a charged, mobile defect that screens the perovskite from external electric fields. These three defects are proposed to be MA⁺ interstitials, crystal distortions, and halide vacancies and/or interstitials, respectively. Finally, external quantum efficiency measurements show that photoexcited charge-carriers can be extracted from the iodide-rich low-bandgap regions of the phase-segregated perovskite formed under illumination, suggesting the existence of charge-carrier percolation pathways through grain boundaries where phase-segregation may occur.

Control over crystal size in vapor deposited metal-halide perovskite films,
K. B. Lohmann, J. B. Patel, M. U. Rothmann, C. Q. Xia, R. D. J. Oliver, L. M. Herz, H. J. Snaith, and M. B. Johnston,
ACS Energy Lett., 5 (2020), p. 710–717. [journal | article | SI ]

Understanding and controlling grain growth in metal halide perovskite polycrystalline thin films is an important step in improving the performance of perovskite solar cells. We demonstrate accurate control of crystallite size in CH₃NH₃PbI₃ thin films by regulating substrate temperature during vacuum co-deposition of inorganic (PbI₂) and organic (CH₃NH₃I) precursors. Films co-deposited onto a cold (−2^oC) substrate exhibited large, micrometer-sized crystal grains, while films that formed at room temperature (23^oC) only produced grains of 100 nm extent. We isolated the effects of substrate temperature on crystal growth by developing a new method to control sublimation of the organic precursor, and CH₃NH₃PbI₃ solar cells deposited in this way yielded a power conversion efficiency of up to 18.2%. Furthermore, we found substrate temperature directly affects the adsorption rate of CH₃NH₃I, thus impacting crystal formation and hence solar cell device performance via changes to the conversion rate of PbI₂ to CH₃NH₃PbI₃ and stoichiometry. These findings offer new routes to developing efficient solar cells through reproducible control of crystal morphology and composition.

Understanding the performance-limiting factors of Cs₂AgBiBr₆ double-perovskite solar cells,
G. Longo, S. Mahesh, L. R. V. Buizza, A. D. Wright, A. J. Ramadan, M. Abdi-Jalebi, P. K. Nayak, L. M. Herz, and H. J. Snaith,
ACS Energy Lett., 5 (2020), p. 2200–2207. [journal | article | SI ]

Double perovskites have recently emerged as possible alternatives to lead-based halide perovskites for photovoltaic applications. In particular, Cs₂AgBiBr₆ has been the subject of several studies because of its environmental stability, low toxicity, and its promising optoelectronic features. Despite these encouraging features, the performances of solar cells based on this double perovskite are still low, suggesting severe limitations that need to be addressed. In this work we combine experimental and theoretical studies to show that the short electron diffusion length is one of the major causes for the limited performance of Cs₂AgBiBr₆ solar cells. Using EQE measurements on semitransparent Cs₂AgBiBr₆ solar cells we estimate the electron diffusion length to be only 30 nm and corroborated this value by terahertz spectroscopy. By using photothermal deflection spectroscopy and surface photovoltage measurements we correlate the limited electron diffusion length with a high density of electron traps. Our findings highlight important faults affecting this double perovskite, showing the challenges to overcome and hinting to a possible path to improve the efficiency of Cs₂AgBiBr₆ solar cells.

Post-passivation of multication perovskite with rubidium butyrate,
J. C. Germino, R. Szostak, S. G. Motti, R. F. Moral, P. E. Marchezi, H. S. Seleghini, L. G. Bonato, F. L. de Araujo, T. D. Z. Atvars, L. M. Herz, D. Fenning, A. Hagfeldt, and A. F. Nogueira,
ACS Photonics, 7 (2020), p. 2282–2291. [journal | article | SI ]

Many multication perovskites for highly stable and efficient solar cells benefit from rubidium iodide introduced in the precursor solution. It is well known that Rb⁺ influences positively the optoelectronic and mobility properties and has a direct effect upon crystallization and halide homogenization. As Rb⁺ is often incorporated by adding RbI in the precursor solution, it can be difficult to distinguish the influence of Rb⁺ and I^- separately. Herein, we report a post-passivation of methylammonium-free (CsFA) perovskite films with rubidium butyrate (RbBu). The passivation with RbBu increases the hydrophobicity of the perovskite surface and passivates shallow and deep traps, leading to an increase of charge- carrier lifetimes and diffusion lengths. Consequently, a better photovoltaic performance is also observed. These superior properties are attributed to both surface (halide-vacancy) and grain-boundary passivation by the carboxylate group and Rb⁺, respectively. We found that Rb⁺ itself acts as a direct and powerful passivating agent for multication perovskites, and this is proven by decoupling its contribuition and halide’s contribuition to other important performance parameters (e.g. crystallization, halide vacancies filling, etc).

CsPbBr₃ nanocrystal films: Deviations from bulk vibrational and optoelectronic properties,
S. G. Motti, F. Krieg, A. J. Ramadan, J. B. Patel, H. J. Snaith, M. V. Kovalenko, M. B. Johnston, and L. M. Herz,
Adv. Func. Mater., 30 (2020), p. 1909904. [journal | article | SI ]

Metal‐halide perovskites (MHP) are highly promising semiconductors for light‐emitting and photovoltaic applications. The colloidal synthesis of nanocrystals (NCs) is an effective approach for obtaining nearly defect‐free MHP that can be processed into inks for low‐cost, high‐performance device fabrication. However, disentangling the effects of surface ligands, morphology, and boundaries on charge‐carrier transport in thin films fabricated with these high‐quality NCs is inherently difficult. To overcome this fundamental challenge, terahertz (THz) spectroscopy is employed to optically probe the photoconductivity of CsPbBr₃ NC films. The vibrational and optoelectronic properties of the NCs are compared with those of the corresponding bulk polycrystalline perovskite and significant deviations are found. Charge‐carrier mobilities and recombination rates are demonstrated to vary significantly with the NC size. Such dependences derive from the localized nature of charge carriers within NCs, with local mobilities dominating over interparticle transport. It is further shown that the colloidally synthesized NCs have distinct vibrational properties with respect to the bulk perovskite, exhibiting blue‐shifted optical phonon modes with enhanced THz absorption strength that also manifest as strong modulations in the THz photoconductivity spectra. Such fundamental insights into NC versus bulk properties will guide the optimization of nanocrystalline perovskite thin films for optoelectronic applications.

Metal composition influences optoelectronic quality in mixed-metal lead-tin triiodide perovskite solar absorbers,
M. T. Klug, R. L. Milot, J. Patel, T. Green, H. C. Sansom, M. Farrar, A. J. Ramadan, S. Martani, Z. Wang, B. Wenger, J. M. Ball, L. Langshaw, A. Petrozza, M. B. Johnston, L. M. Herz, and H. Snaith,
Energy Environ. Sci., 13 (2020), p. 1776–1787. [journal | article | SI ]

Current designs for all-perovskite multi-junction solar cells require mixed-metal Pb-Sn compositions to achieve narrower band gaps than are possible with their neat Pb counterparts. The lower band gap range achievable with mixed-metal Pb-Sn perovskites also encompasses the 1.3 to 1.4 eV range that is theoretically ideal for maximising the efficiency of single-junction devices. Here we examine the optoelectronic quality and photovoltaic performance of the ((HC(NH₂)₂)_0.83Cs_0.17)(Pb_1-ySn_y)I₃ family of perovskite materials across the full range of achievable band gaps by substituting between 0.001% and 70% of the Pb content with Sn. We reveal that a compositional range of “defectiveness” exists when Sn comprises between 0.5% and 20% of the metal content, but that the optoelectronic quality is restored for Sn content between 30-50%. When only 1% of Pb content is replaced by Sn, we find that photoconductivity, photoluminescence lifetime, and photoluminescence quantum efficiency are reduced by at least an order of magnitude, which reveals that a small concentration of Sn incorporation produces trap sites that promote non-radiative recombination in the material and limit photovoltaic performance. While these observations suggest that band gaps between 1.35 and 1.5 eV are unlikely to be useful for optoelectronic applications without countermeasures to improve material quality, highly efficient narrower band gap absorber materials are possible at or below 1.33 eV. Through optimising single-junction photovoltaic devices with Sn compositions of 30% and 50%, we respectively demonstrate a 17.6% efficient solar cell with an ideal single-junction band gap of 1.33 eV and an 18.1% efficient low-bandgap device suitable for the bottom absorber in all-perovskite multi-junction cells.

Three-dimensional cross-nanowire networks recover full terahertz state,
K. Peng, D. Jevtics, F. Zhang, S. Sterzl, D. A. Damry, M. U. Rothmann, B. Guilhabert, M. J. Strain, H. H. Tan, L. M. Herz, L. Fu, M. D. Dawson, A. Hurtado, C. Jagadish, and M. B. Johnston,
Science, 368 (2020), p. 510–513. [journal | article | SI ]

Terahertz radiation encompasses a wide band of the electromagnetic spectrum, spanning from microwaves to infrared light, and is a particularly powerful tool for both fundamental scientific research and applications such as security screening, communications, quality control, and medical imaging. Considerable information can be conveyed by the full polarization state of terahertz light, yet to date, most time-domain terahertz detectors are sensitive to just one polarization component. Here we demonstrate a nanotechnology-based semiconductor detector using cross-nanowire networks that records the full polarization state of terahertz pulses. The monolithic device allows simultaneous measurements of the orthogonal components of the terahertz electric field vector without cross-talk. Furthermore, we demonstrate the capabilities of the detector for the study of metamaterials.

Charge-carrier trapping dynamics in bismuth-doped thin films of MAPbBr₃ perovskite,
A. M. Ulatowski, A. D. Wright, B. Wenger, L. R. V. Buizza, S. G. Motti, H. J. Eggimann, K. J. Savill, J. Borchert, H. J. Snaith, M. B. Johnston, and L. M. Herz,
J. Phys. Chem. Lett., 11 (2020), p. 3681–3688. [journal | article | SI ]

Successful chemical doping of metal halide perovskites with small amounts of heterovalent metals has attracted recent research attention because of its potential to improve long-term material stability and tune absorption spectra. However, some additives have been observed to impact negatively on optoelectronic properties, highlighting the importance of understanding charge-carrier behavior in doped metal halide perovskites. Here, we present an investigation of charge-carrier trapping and conduction in films of MAPbBr₃ perovskite chemically doped with bismuth. We find that the addition of bismuth has no effect on either the band gap or exciton binding energy of the MAPbBr₃ host. However, we observe a substantial enhancement of electron-trapping defects upon bismuth doping, which results in an ultrafast chargecarrier decay component, enhanced infrared emission, and a notable decrease of chargecarrier mobility. We propose that such defects arise from the current approach to Bi-doping through addition of BiBr₃, which may enhance the presence of bromide interstitials.

Terahertz conductivity analysis for highly doped thin-film semiconductors,
A. M. Ulatowski, L. M. Herz, and M. B. Johnston,
Journal of Infrared, Millimeter, and Terahertz Waves, 41 (2020), p. 1431–1449. [journal | article | SI ]

The analysis of terahertz transmission through semiconducting thin films has proven to be an excellent tool for investigating optoelectronic properties of novel materials. Terahertz time-domain spectroscopy (THz-TDS) can provide information about phonon modes of the crystal, as well as the electrical conductivity of the sample. When paired with photoexcitation, optical-pump-THz-probe (OPTP) technique can be used to gain an insight into the transient photoconductivity of the semiconductor, revealing the dynamics and the mobility of photoexcited charge carriers. As the relation between the conductivity of the material and the THz transmission function is generally complicated, simple analytical expressions have been developed to enable straightforward calculations of frequency-dependent conductivity from THz-TDS data in the regime of optically thin samples. Here, we assess the accuracy of these approximated analytical formulas in thin films of highly doped semiconductors, finding significant deviations of the calculated photoconductivity from its actual value in materials with background conductivity comparable to 10²Ω^-1cm^-1. We propose an alternative analytical expression, which greatly improves the accuracy of the estimated value of the real photoconductivity, while remaining simple to implement experimentally. Our approximation remains valid in thin films with high dark conductivity of up to 10⁴Ω^-1cm^-1 and provides a very high precision for calculating photoconductivity up to 10⁴Ω^-1cm^-1, and therefore is highly relevant for studies of photoexcited charge-carrier dynamics in electrically doped semiconductors. Using the example of heavily doped thin films of tin-iodide perovskites, we show a simple experimental method of implementing our correction and find that the commonly used expression for photoconductivity could result in an underestimate of charge-carrier mobility of up to a 60%.

Preventing phase segregation in mixed-halide perovskites: A perspective,
A. J. Knight and L. M. Herz,
Energy Environ. Sci., 13 (2020), p. 2024–2046. [journal | article | SI ]

Mixed-halide perovskites are ideal materials for the demanding applications of tandem solar cells and emission-tunable light-emitting diodes (LEDs) because of their high compositional flexibility and optoelectronic performance. However, one major obstacle to their use is the compositional instability some mixed-halide perovskites experience under illumination or charge-carrier injection, during which the perovskite material demixes into regions of differing halide content. Such segregation of halide ions adversely affects the electronic properties of the material and severely limits the prospects of mixed-halide perovskite technology. Accordingly, a considerable amount of research has been performed aiming to uncover the underlying mechanisms and mitigating factors of the halide segregation process. Here we present a perspective of strategies designed to reduce the effects of halide segregation in working mixed-halide perovskite devices, based on recent literature reports. We discuss a multitude of mitigating techniques, and conclude that a combination of stoichiometric engineering, crystallinity control and trap state passivation is clearly imperative for abating halide segregation. In addition, the reduction of halide vacancies and control over illumination and temperature can, to a certain extent, mitigate halide segregation. Less direct approaches, such as a change in atmospheric environment, perovskite incorporation into a nanocrystalline composition, or direct control over the crystallographic structure of the perovskite, may however prove too cumbersome to be of practical use. This perspective paves the way for the design and creation of phase-stable, mixed-halide perovskite materials for photovoltaic and LED applications.

Charge-carrier trapping and radiative recombination in metal halide perovskite semiconductors,
M. J. Trimpl, A. D. Wright, K. Schutt, L. R. V. Buizza, Z. Wang, M. B. Johnston, H. J. Snaith, P. Müller-Buschbaum, and L. M. Herz,
Adv. Func. Mater., 30 (2020), p. 2004312. [journal | article | SI ]

Trap-related charge-carrier recombination fundamentally limits the performance of perovskite solar cells and other optoelectronic devices. While improved fabrication and passivation techniques have reduced trap densities, the properties of trap states and their impact on the charge-carrier dynamics in metal-halide perovskites are still under debate. Here, a unified model is presented of the radiative and nonradiative recombination channels in a mixed formamidinium-cesium lead iodide perovskite, including charge-carrier trapping, de-trapping and accumulation, as well as higher-order recombination mechanisms. A fast initial photoluminescence (PL) decay component observed after pulsed photogeneration is demonstrated to result from rapid localization of free charge carriers in unoccupied trap states, which may be followed by de-trapping, or nonradiative recombination with free carriers of opposite charge. Such initial decay components are shown to be highly sensitive to remnant charge carriers that accumulate in traps under pulsed-laser excitation, with partial trap occupation masking the trap density actually present in the material. Finally, such modelling reveals a change in trap density at the phase transition, and disentangles the radiative and nonradiative charge recombination channels present in FA_0.95Cs_0.05PbI₃, accurately predicting the experimentally recorded PL efficiencies between 50 and 295 K, and demonstrating that bimolecular recombination is a fully radiative process.

Intrinsic quantum confinement in formamidinium lead triiodide perovskite,
A. D. Wright, G. Volonakis, J. Borchert, C. L. Davies, F. Giustino, M. B. Johnston, and L. M. Herz,
Nature Materials, 19 (2020), p. 1201–1206. [journal | article | SI ]

Understanding the electronic energy landscape in metal halide perovskites is essential for further improvements in their promising performance in thin-film photovoltaics. Here, we uncover the presence of above-bandgap oscillatory features in the absorption spectra of formamidinium lead triiodide thin films. We attribute these discrete features to intrinsically occurring quantum confinement effects, for which the related energies change with temperature according to the inverse square of the intrinsic lattice parameter, and with peak index in a quadratic manner. By determining the threshold film thickness at which the amplitude of the peaks is appreciably decreased, and through ab initio simulations of the absorption features, we estimate the length scale of confinement to be 10–20 nm. Such absorption peaks present a new and intriguing quantum electronic phenomenon in a nominally bulk semiconductor, offering intrinsic nanoscale optoelectronic properties without necessitating cumbersome additional processing steps.

Impact of tin fluoride additive on the properties of mixed tin-lead iodide perovskite semiconductors,
K. J. Savill, A. M. Ulatowski, M. D. Farrar, M. B. Johnston, H. J. Snaith, and L. M. Herz,
Adv. Func. Mater., 30 (2020), p. 2005594. [journal | article | SI ]

Mixed tin-lead halide perovskites are promising low-bandgap absorbers for all-perovskite tandem solar cells that offer higher efficiencies than single-junction devices. A significant barrier to higher performance and stability is the ready oxidation of tin, commonly mitigated by various additives whose impact is still poorly understood for mixed tin-lead perovskites. Here, the effects of the commonly used SnF₂ additive are revealed for FA_0.83Cs_0.17Sn_xPb_1−xI₃ perovskites across the full compositional lead-tin range and SnF₂ percentages of 0.1–20% of precursor tin content. SnF₂ addition causes a significant reduction in the background hole density associated with tin vacancies, yielding longer photoluminescence lifetimes, decreased energetic disorder, reduced Burstein–Moss shifts, and higher charge-carrier mobilities. Such effects are optimized for SnF₂ addition of 1%, while for 5% SnF₂ and above, additional nonradiative recombination pathways begin to appear. It is further found that the addition of SnF₂ reduces a tetragonal distortion in the perovskite structure deriving from the presence of tin vacancies that cause strain, particularly for high tin content. The optical phonon response associated with inorganic lattice vibrations is further explored, exhibiting a shift to higher frequency and significant broadening with increasing tin fraction, in accordance with lower effective atomic metal masses and shorter phonon lifetimes.

Efficient energy transfer mitigates parasitic light absorption in molecular charge-extraction layers for perovskite solar cells,
H. J. Eggimann, J. B. Patel, M. B. Johnston, and L. M. Herz,
Nature Communications, 11 (2020), p. 5525. [journal | article | SI ]

Organic semiconductors are commonly used as charge-extraction layers in metal-halide perovskite solar cells. However, parasitic light absorption in the sun-facing front molecular layer, through which sun light must propagate before reaching the perovskite layer, may lower the power conversion efficiency of such devices. Here, we show that such losses may be eliminated through efficient excitation energy transfer from a photoexcited polymer layer to the underlying perovskite. Experimentally observed energy transfer between a range of different polymer films and a methylammonium lead iodide perovskite layer was used as basis for modelling the efficacy of the mechanism as a function of layer thickness, photoluminescence quantum efficiency and absorption coefficient of the organic polymer film. Our findings reveal that efficient energy transfer can be achieved for thin (<=10 nm) organic charge-extraction layers exhibiting high photoluminescence quantum efficiency. We further explore how the morphology of such thin polymer layers may be affected by interface formation with the perovskite.

Atomic-scale microstructure of metal halide perovskite,
M. U. Rothmann, J. S. Kim, J. Borchert, K. B. Lohmann, C. M. O’Leary, A. A. Sheader, L. Clark, H. J. Snaith, M. B. Johnston, P. D. Nellist, and L. M. Herz,
Science, 370 (2020), p. eabb5940. [journal | article | SI ]

Hybrid organic-inorganic perovskites have high potential as materials for solar energy applications, but their microscopic properties are still not well understood. Atomic-resolution scanning transmission electron microscopy has provided invaluable insights for many crystalline solar cell materials, and we used this method to successfully image formamidinium lead triiodide [CH(NH₂)₂PbI₃| thin films with a low dose of electron irradiation. Such images reveal a highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI₂ interfaces, with a striking absence of long-range disorder in the crystal. We found that beam-induced degradation of the perovskite leads to an initial loss of formamidinium [CH(NH₂)²⁺| ions, leaving behind a partially unoccupied perovskite lattice, which explains the unusual regenerative properties of these materials. We further observed aligned point defects and climb-dissociated dislocations. Our findings thus provide an atomic-level understanding of technologically important lead halide perovskites.

Structural and optical properties of Cs₂AgBiBr₆ double perovskite,
L. Schade, A. D. Wright, R. D. Johnson, M. Dollmann, B. Wenger, P. K. Nayak, D. Prabhakaran, L. M. Herz, R. Nicholas, H. J. Snaith, and P. G. Radaelli,
ACS Energy Lett., 4 (2019), p. 299–305. [journal | article | SI ]

We present a comprehensive study of the relationship between the crystal structure and optoelectronic properties of the double perovskite Cs₂AgBiBr₆, which has emerged as a promising candidate for photovoltaic devices. On the basis of single-crystal/powder X-ray diffraction and neutron powder diffraction, we have revealed the presence of a structural phase transition at T_s ≈ 122 K between the room-temperature cubic structure (space group Fm3̅m) and a new low-temperature tetragonal structure (I4/m). From reflectivity measurements we found that the peak exciton energy E_ex ≈ 2.85 eV near the direct gap shifts proportionally to the tetragonal strain, which is consistent with the Eex being primarily controlled by a rotational degree of freedom of the crystal structure, thus by the angle Bi–Ag–Br. We observed the time-resolved photoluminescence kinetics and we found that, among the relaxation channels, a fast one is mainly present in the tetragonal phase, suggesting that its origin may lie in the formation of tetragonal twin domains.

Solution-processed all-perovskite multi-junction solar cells,
D. P. McMeekin, S. Mahesh, N. K. Noel, M. T. Klug, J. Lim, J. H. Warby, J. M. Ball, L. M. Herz, M. B. Johnston, and H. J. Snaith,
Joule, 3 (2019), p. 387–401. [journal | article | SI ]

Multi-junction device architectures can increase the power conversion efficiency (PCE) of photovoltaic (PV) cells beyond the single-junction thermodynamic limit. However, these devices are challenging to produce by solution-based methods, where dissolution of underlying layers is problematic. By employing a highly volatile acetonitrile(CH₃CN)/methylamine(CH₃NH₂) (ACN/MA) solvent-based perovskite solution, we demonstrate fully solution-processed absorber, transport, and recombination layers for monolithic all-perovskite tandem and triple-junction solar cells. By combining FA_0.83Cs_0.17Pb(Br_0.7I_0.3)₃ (1.94 eV) and MAPbI₃ (1.57 eV) junctions, we reach two-terminal tandem PCEs of more than 15% (steady state). We show that a MAPb_0.75Sn_0.25I₃ (1.34 eV) narrow band-gap perovskite can be processed via the ACN/MA solvent-based system, demonstrating the first proof-of-concept, monolithic all-perovskite triple-junction solar cell with an open-circuit voltage reaching 2.83 V. Through optical and electronic modeling, we estimate the achievable PCE of a state-of-the-art triple-junction device architecture to be 26.7%. Our work opens new possibilities for large-scale, low-cost, printable perovskite multi-junction solar cells.

Electronic traps and phase segregation in lead mixed-halide perovskite,
A. Knight, A. D. Wright, J. B. Patel, D. McMeekin, H. J. Snaith, M. B. Johnston, and L. M. Herz,
ACS Energy Lett., 4 (2019), p. 75–84. [journal | article | SI ]

An understanding of the factors driving halide segregation in lead mixed-halide perovskites is required for their implementation in tandem solar cells with existing silicon technology. Here we report that the halide segregation dynamics observed in the photoluminescence from CH₃NH₃Pb(Br_0.5I_0.5)₃ is strongly influenced by the atmospheric environment, and that encapsulation of films with a layer of poly(methyl methacrylate) allows for halide segregation dynamics to be fully reversible and repeatable. We further establish an empirical model directly linking the amount of halide segregation observed in the photoluminescence to the fraction of charge-carriers recombining through trap-mediated channels, and the photon flux absorbed. From such quantitative analysis we show that under pulsed illumination, the frequency of the modulation alone has no influence on the segregation dynamics. Additionally, we extrapolate that working CH₃NH₃Pb(Br_0.5I_0.5)₃ perovskite cells would require a reduction of the trap-related charge-carrier recombination rate to e5 in order for halide segregation to be sufficiently suppressed.

Aromaticity and antiaromaticity in the excited states of porphyrin nanorings,
M. D. Peeks, J. Q. Gong, K. McLoughlin, T. Kobatake, R. Haver, L. M. Herz, and H. L. Anderson,
J. Phys. Chem. Lett., 10 (2019), p. 2017–2022. [journal | article | SI ]

Aromaticity can be a useful concept for predicting the behavior of excited states. Here we show that π-conjugated porphyrin nanorings exhibit size-dependent excited-state global aromaticity and antiaromaticity for rings containing up to eight porphyrin subunits, although they have no significant global aromaticity in their neutral singlet ground states. Applying Baird's rule, even rings ([4n| π-electrons) are aromatic in their lowest excited states, whereas the lowest excited states of odd rings ([4n + 2| π-electrons) are antiaromatic. These predictions are borne out by density functional theory (DFT) studies of the nucleus-independent chemical shift (NICS) in the T1 triplet state of each ring, which reveal the critical importance of the triplet delocalization to the emergence of excited-state aromaticity. The singlet excited states (S1) are explored by measurements of the radiative rate and fluorescence peak wavelength, revealing a subtle odd–even alternation as a function of ring size, consistent with symmetry breaking in antiaromatic excited states.

Heterogeneous photon recycling and charge diffusion enhance charge transport in quasi-2D lead-halide perovskite films,
S. G. Motti, T. Crothers, R. Yang, Y. Cao, R. Li, M. B. Johnston, J. Wang, and L. M. Herz,
Nano Lett., 19 (2019), p. 3953–3960. [journal | article | SI ]

The addition of large hydrophobic cations to lead halide perovskites has significantly enhanced the environmental stability of photovoltaic cells based on these materials. However, the associated formation of two-dimensional structures inside the material can lead to dielectric confinement, higher exciton binding energies, wider bandgaps and limited charge-carrier mobilities. Here we show that such effects are not detrimental to the charge transport for carefully processed films comprising a self-assembled thin layer of quasi-two-dimensional (2D) perovskite interfaced with a 3D MAPbI₃ perovskite layer. We apply a combination of time-resolved photoluminescence and photoconductivity spectroscopy to reveal the charge-carrier recombination and transport through the film profile, when either the quasi-2D or the 3D layers are selectively excited. Through modeling of the recorded dynamics, we demonstrate that while the charge-carrier mobility is lower within the quasi-2D region, charge-carrier diffusion to the 3D phase leads to a rapid recovery in photoconductivity even when the quasi-2D region is initially photoexcited. In addition, the blue-shifted emission originating from quasi-2D regions overlaps significantly with the absorption spectrum of the 3D perovskite, allowing for highly effective “heterogeneous photon recycling”. We show that this combination fully compensates for the adverse effects of electronic confinement, yielding quasi-2D perovskites with highly efficient charge transporting properties.

How β-phase content moderates chain conjugation and energy transfer in polyfluorene films,
H. J. Eggimann, F. L. Roux, and L. M. Herz,
J. Phys. Chem. Lett., 10 (2019), p. 1729–1736. [journal | article | SI ]

Poly(9,9-dioctylfluorene) (PFO) is a blue-light-emitting polymer exhibiting two distinct phases, the disordered “glassy” phase and a more ordered β-phase. We investigate how a systematic increase in the fraction of β-phase present in PFO films controls chain conformation, photoluminescence quantum efficiency (PLQE), and the resonant energy transfer from the glassy- to the β-phase. All films are prepared by the same technique, using paraffin oil as an additive to the spin-coating solution, allowing systematic tuning of the β-phase fraction. The PFO films exhibit high PLQE with values increasing to 0.72 for increasing fractions of β-phase present, with the β-phase chain conformation becoming more planar and including more repeat units. Differences in Förster radii calculated from the overlap of steady-state absorptance and emission spectra and from time-resolved ultrafast photoluminescence transients indicate that exciton diffusion within the glassy-phase plays an important role in the energy transfer process.

The effect of ultraviolet radiation on organic photovoltaic materials and devices,
J. B. Patel, P. Tiwana, N. Seidler, G. Morse, O. Lozman, M. B. Johnston, and L. M. Herz,
ACS Appl. Mater. Interfaces, 11 (2019), p. 21543–21551. [journal | article | SI ]

Organic photovoltaics are a sustainable and cost-effective power-generation technology that may aid the move to zero emission buildings, carbon neutral cities and electric vehicles. Whilst state of the art organic photovoltaics devices can be encapsulated to withstand air and moisture, they are currently still susceptible to light-induced degradation, leading to a decline in long-term efficiency of the devices. In this study, the role of ultraviolet (UV) radiation on a multilayer organic photovoltaic device is systematically uncovered using spectral filtering. By applying long pass filters to remove different parts of the UV portion of the AM1.5G spectrum, two main photodegradation processes are shown to occur in the OPV devices. A UV-activated process is found to cause a significant decrease in the photocurrent across the whole spectrum and is most likely linked to deterioration of the charge extraction layers. In addition, a photodegradation process caused by UV-filtered sun light is found to change the micromorphology of the bulk heterojunction material, leading to a reduction in photocurrent at high photon energies. These findings strongly suggest that fabrication of inherently photostable organic photovoltaic devices will require the replacement of fullerene-based electron transporter materials with alternative organic semiconductors.

Charge-carrier dynamics, mobilities and diffusion lengths of 2D-3D hybrid butylammonium-caesium-formamidinium lead halide perovskites,
L. R. V. Buizza, T. W. Crothers, Z. Wang, J. B. Patel, R. L. Milot, M. B. Johnston, H. J. Snaith, and L. M. Herz,
Adv. Func. Mater., 29 (2019), p. 1902656. [journal | article | SI ]

Perovskite solar cells (PSCs) have improved dramatically over the past decade, increasing in efficiency and gradually overcoming hurdles of temperature- and humidity-induced instability. Materials that combine high charge-carrier lifetimes and mobilities, strong absorption, and good crystallinity of 3D perovskites with the hydrophobic properties of 2D perovskites have become particularly promising candidates for use in solar cells. In order to fully understand the optoelectronic properties of these 2D–3D hybrid systems, the hybrid perovskite BA_x(FA_0.83Cs_0.17)_1-xPb(I_0.6Br_0.4)₃ is investigated across the composition range 0 <= x <= 0.8. Small amounts of butylammonium (BA) are found that help to improve crystallinity and appear to passivate grain boundaries, thus reducing trap-mediated chargecarrier recombination and enhancing charge-carrier mobilities. Excessive amounts of BA lead to poor crystallinity and inhomogeneous film formation, greatly reducing effective charge-carrier mobility. For low amounts of BA, the benevolent effects of reduced recombination and enhanced mobilities lead to charge-carrier diffusion lengths up to 7.7 μm for x=0.167. These measurements pave the way for highly efficient, highly stable PSCs and other optoelectronic devices based on 2D–3D hybrid materials.

Tuning the circumference of six-porphyrin nanorings,
R. Haver, L. Tejerina, H.-W. Jiang, M. Rickhaus, M. Jirasek, I. Gruebner, H. J. Eggimann, L. M. Herz, and H. L. Anderson,
J. Am. Chem. Soc., 141 (2019), p. 7965–7971. [journal | article | SI ]

Most macrocycles are made from a simple repeat unit, resulting in high symmetry. Breaking this symmetry allows fine tuning of the circumference, providing better control of the host-guest behavior and electronic structure. Here we present the template-directed synthesis of two unsymmetrical cyclic porphyrin hexamers with both ethyne (C2) and butadiyne (C4) links, and we compare these nanorings with the symmetrical analogues with six ethyne or six butadiyne links. Inserting two extra carbon atoms into the smaller nanoring causes a spectacular change in binding behavior: the template-affinity increases by a factor of 3×10⁹, to a value of ca. 10³⁸ M^–1 and the mean effective molarity is ca. 830 M. In contrast, removing two carbon atoms from the largest nanoring results in almost no change in its template-affinity. The strain in these nanorings is 90–130 kJ mol^–1, as estimated both from DFT calculation of homodesmotic reactions and from comparing template-affinities of linear and cyclic oligomers. Breaking the symmetry has little effect on the absorption and fluorescence behavior of the nanorings: the low radiative rates that are characteristic of a circular delocalized S1 excited state are preserved in the low-symmetry macrocycles.

Impurity tracking enables enhanced control and reproducibility of hybrid perovskite vapor deposition,
J. Borchert, I. Levchuk, L. C. Snoek, M. U. Rothmann, R. Haver, H. J. Snaith, C. J. Brabec, L. M. Herz, and M. B. Johnston,
ACS Appl. Mater. Interfaces, 11 (2019), p. 28851–28857. [journal | article | SI ]

Metal halide perovskite semiconductors have the potential to enable low-cost, flexible, and efficient solar cells for a wide range of applications. Physical vapor deposition by co-evaporation of precursors is a method that results in very smooth and pinhole-free perovskite thin films and allows excellent control over film thickness and composition. However, for a deposition method to become industrially scalable, reproducible process control and high device yields are essential. Unfortunately, to date, the control and reproducibility of evaporating organic precursors such as methylammonium iodide (MAI) have proved extremely challenging. We show that the established method of controlling the evaporation rate of MAI with quartz microbalances (QMBs) is critically sensitive to the concentration of the impurities MAH₂PO₃ and MAH₂PO₂ that are usually present in MAI after synthesis. Therefore, controlling the deposition rate of MAI with QMBs is unreliable since the concentration of such impurities typically varies from one batch of MAI to another and even during the course of a deposition. However once reliable control of MAI deposition is achieved, we find that the presence of precursor impurities during perovskite deposition does not degrade the solar cell performance. Our results indicate that as long as precursor deposition rates are well controlled, physical vapor deposition will allow high solar cell device yields even if the purity of precursors changes from one run to another.

Growth modes and quantum confinement in ultrathin vapour-deposited MAPbI₃ films,
E. S. Parrott, J. B. Patel, A.-A. Haghighirad, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Nanoscale, 11 (2019), p. 14276–14284. [journal | article | SI ]

Vapour deposition of metal halide perovskite by co-evaporation of precursors has the potential to achieve large-area high-efficiency solar cells on an industrial scale, yet little is known about the growth of metal halide perovskites by this method at the current time. Here, we report the fabrication of MAPbI₃ films with average thicknesses from 2–320 nm by co-evaporation. We analyze the film properties using X-ray diffraction, optical absorption and photoluminescence (PL) to provide insights into the nucleation and growth of MAPbI₃ films on quartz substrates. We find that the perovskite initially forms crystallite islands of around 8 nm in height, which may be the cause of the persistent small grain sizes reported for evaporated metal halide perovskites that hinder device efficiency and stability. As more material is added, islands coalesce until full coverage of the substrate is reached at around 10 nm average thickness. We also find that quantum confinement induces substantial shifts to the PL wavelength when the average thickness is below 40 nm, offering dual-source vapour deposition as an alternative method of fabricating nanoscale structures for LEDs and other devices.

Dual-source co-evaporation of low-bandgap FA_1-xCs_xSn_1-yPb_yI₃ perovskites for photovoltaics,
J. Ball, L. Buizza, H. Sansom, M. Farrar, M. Klug, J. Borchert, J. B. Patel, L. M. Herz, M. B. Johnston, and H. J. Snaith,
ACS Energy Lett., 4 (2019), p. 2748–2756. [journal | article | SI ]

Perovskite halides are well-suited to monolithic multijunction photovoltaics, promising low cost solar-to-electrical power conversion. Critical to all-perovskite multijunction fabrication is the deposition of a low-bandgap absorber without damaging other device layers. Vapour deposition is thus an attractive method, obviating the need for optically lossy protective interlayers, but is challenging for multi-component perovskites. Here, we demonstrate a method to dual-source co-evaporate low-bandgap perovskite films and devices. We used mixtures formed by melting of metal halides as a single-crucible source of Cs, Pb, and Sn cations. Surprisingly, when this melt was co-evaporated with formamidinium iodide (FAI), uniform and dense perovskite films in the family FA_1-xCs_xSn_1-yPb_yI₃ were formed. Inclusion of SnF₂ in the melt helped to regulate the perovskite’s optoelectronic quality leading to a steady-state power conversion efficiency of ~10% in a solar cell. This represents a new processing paradigm for evaporated perovskite alloys, which is an important step towards all-perovskite multijunction photovoltaics.

Charge-carrier cooling and polarization memory loss in formamidinium tin triiodide,
K. J. Savill, M. T. Klug, R. L. Milot, H. J. Snaith, and L. M. Herz,
J. Phys. Chem. Lett., 10 (2019), p. 6038–6047. [journal | article | SI ]

Reports of slow charge-carrier cooling in hybrid metal halide perovskites have prompted hopes of achieving higher photovoltaic cell voltages through hot-carrier extraction. However, observations of long-lived hot charge carriers even at low photoexcitation densities and an orders-of-magnitude spread in reported cooling times have been challenging to explain. Here we present ultrafast time-resolved photoluminescence measurements on formamidinum tin triiodide, showing fast initial cooling over tens of picoseconds and demonstrating that a perceived secondary regime of slower cooling instead derives from electronic relaxation, state-filling, and recombination in the presence of energetic disorder. We identify limitations of some widely used approaches to determine charge-carrier temperature and make use of an improved model which accounts for the full photoluminescence line shape. Further, we do not find any persistent polarization anisotropy in FASnI₃ within 270 fs after excitation, indicating that excited carriers rapidly lose both polarization memory and excess energy through interactions with the perovskite lattice.

Modification of the fluorinated tin oxide/electron-transporting material interface by a strong reductant and its effect on perovskite solar cell efficiency,
F. Pulvirenti, B. Wegner, N. K. Noel, G. Mazzotta, R. Hill, J. B. Patel, L. M. Herz, M. B. Johnston, M. K. Riede, H. J. Snaith, N. Koch, S. Barlow, and S. R. Marder,
Molecular Systems Design & Engineering, 3 (2018), p. 741–747. [journal | article | SI ]

Materials selection is essential to enable the commercialization of perovskite solar cells (PSCs). Replacing metal oxides with organic electron-transporting materials (ETMs) increases the operational stability of solar cells under UV-light exposure and provides a better electronic contact with the perovskite. However, the interface between an organic semiconductor and an electrode material can be detrimental to charge extraction if the electronic alignment is not properly adjusted. Here, we use the organometallic dimer (RhCp*Cp)₂, which behaves as a masked form of the highly reducing (RhCp*Cp)^., to investigate the impact of the electrode/ETM interface on charge collection in PSCs. (RhCp*Cp)₂ not only places the electrode and the ETM in ohmic contact, but it induces interfacial doping of the ETM. Perylene-3,4:9,10-tetracarboxylic bis(benzimidazole) (PTCBI) is chosen as a vacuum-processable alternative to C₆₀ as ETM for two reasons: firstly, PTCBI and fullerenes have similar electron affinities, and secondly, perylene derivatives can be inexpensive and stable to temperature and humidity, as evidenced by finding applications in car paint. By vacuum-depositing (RhCp*Cp)₂ between the electrode and PTCBI, solar cells exhibiting stabilized power conversion efficiencies up to 14.2% can be fabricated, outperforming solar cells with no organometallic dimer (12.7%).

Temperature-dependent refractive index of quartz at terahertz frequencies,
C. L. Davies, J. B. Patel, C. Q. Xia, L. M. Herz, and M. B. Johnston,
Journal of Infrared, Millimeter, and Terahertz Waves, 39 (2018), p. 1236–1248. [journal | article | SI ]

Characterisation of materials often requires the use of a substrate to support the sample being investigated. For optical characterisation at terahertz frequencies, quartz is commonly used owing to its high transmission and low absorption at these frequencies. Knowledge of the complex refractive index of quartz is required for analysis of time-domain terahertz spectroscopy and optical pump terahertz probe spectroscopy for samples on a quartz substrate. Here, we present the refractive index and extinction coefficient for α-quartz between 0.5 THz and 5.5 THz (17-183 cm⁻¹) taken at 10, 40, 80, 120, 160, 200 and 300 K. Quartz shows excellent transmission and is thus an ideal optical substrate over the THz band, apart from the region 3.9 +/- 0.1 THz owing to a spectral feature originating from the lowest energy optical phonon modes. We also present the experimentally measured polariton dispersion of α-quartz over this frequency range.

Photocurrent spectroscopy of perovskite solar cells over a wide temperature range from 15 to 350 K,
J. B. Patel, Q. Lin, O. Zadvorna, C. L. Davies, L. M. Herz, and M. B. Johnston,
J. Phys. Chem. Lett., 9 (2018), p. 263–268. [journal | article | SI ]

Solar cells based on metal halide perovskite thin films show great promise for energy generation in a range of environments from terrestrial installations to space applications. Here we assess the device characteristics of the prototypical perovskite solar cells based on methylammonium lead triiodide (CH₃NH₃PbI₃) over a broad temperature range from 15 to 350 K (-258 to 77^oC). For these devices, we observe a peak in the short-circuit current density and open-circuit voltage at 200 K (−73^oC) with decent operation maintained up to 350 K. We identify the clear signature of crystalline PbI₂ contributing directly to the low-temperature photocurrent spectra, showing that PbI₂ plays an active role (beyond passivation) in CH₃NH₃PbI₃ solar cells. Finally we observe a blue-shift in the photocurrent spectrum with respect to the absorption spectrum at low temperature (15 K), allowing us to extract a lower limit on the exciton binding energy of 9.1 meV for CH₃NH₃PbI₃.

Highly crystalline methylammonium lead tribromide perovskite films for efficient photovoltaic devices,
N. K. Noel, B. Wenger, S. N. Habisreutinger, J. B. Patel, T. Crothers, Z. Wang, R. J. Nicholas, M. B. Johnston, L. M. Herz, and H. J. Snaith,
ACS Energy Lett., 3 (2018), p. 1233–1240. [journal | article | SI ]

The rise of metal-halide perovskite solar cells has captivated the research community, promising to disrupt the current energy landscape. While a sizable percentage of the research done on this class of materials has been focused on the neat and iodide-rich perovskites, bromide-based perovskites can deliver substantially higher voltages because of their relatively wide band gaps of over 2 eV. The potential for efficient, high-voltage devices makes materials such as these incredibly attractive for multijunction photovoltaic applications. Here, we use the acetonitrile/methylamine solvent system to deposit smooth, highly crystalline films of CH₃NH₃PbBr₃. By using choline chloride as a passivating agent for these films, we achieve photoluminescence quantum efficiencies of up to 5.5% and demonstrate charge-carrier mobilities of 17.8 cm²/(V s). Incorporating these films into photovoltaic devices, we achieve scanned power conversion efficiencies of up to 8.9%, with stabilized efficiencies of 7.6%, providing a simple route to realizing efficient, high-voltage CH₃NH₃PbBr₃ planar-heterojunction devices.

Raman spectrum of the organic-inorganic halide perovskite CH₃NH₃PbI₃ from first principles and high-resolution low-temperature raman measurements,
M. A. Pèrez-Osorio, Q. Lin, R. T. Phillips, R. L. Milot, L. M. Herz, M. B. Johnston, and F. Giustino,
J. Phys. Chem. C, 122 (2018), p. 21703–21717. [journal | article | SI ]

We investigate the Raman spectrum of the low-temperature orthorhombic phase of the organic-inorganic halide perovskite CH₃NH₃PbI₃, by combining first-principles calculations with high-resolution low-temperature Raman measurements. We find good agreement between theory and experiment, and successfully assign each of the Raman peaks to the underlying vibrational modes. In the low-frequency spectral range (below 60 cm^-1) we assign the prominent Raman signals at 26, 32, 42 and 49 cm^-1 to the Pb-I-Pb bending modes with either A_g or B_2g symmetry, and the signal at 58 cm^-1 to the librational mode of the organic cation. Owing to their significant intensity, we propose that these peaks can serve as clear markers of the vibrations of the [PbI₃|^- network and of the CH₃NH₃⁺ cations in this perovskite, respectively. In particular, the ratios of the intensities of these peaks might be used to monitor possible deviations from the ideal stoichiometry of CH₃NH₃PbI₃.

High electron mobility and insights into temperature-dependent scattering mechanisms in InAsSb nanowires,
J. L. Boland, F. Amaduzzi, S. Sterzl, H. Potts, L. M. Herz, A. Fontcuberta i Morral, and M. B. Johnston,
Nano Lett., 18 (2018), p. 3703–3710. [journal | article | SI ]

InAsSb nanowires are promising elements for thermoelectric devices, infrared photodetectors, high-speed transistors, as well as thermophotovoltaic cells. By changing the Sb alloy fraction the mid-infrared bandgap energy and thermal conductivity may be tuned for specific device applications. Using both terahertz and Raman noncontact probes, we show that Sb alloying increases the electron mobility in the nanowires by over a factor of 3 from InAs to InAs_0.65Sb_0.35. We also extract the temperature-dependent electron mobility via both terahertz and Raman spectroscopy, and we report the highest electron mobilities for InAs_0.65Sb_0.35 nanowires to date, exceeding 16,000 cm² V^–1 s^–1 at 10 K.

Template-directed synthesis of a conjugated zinc porphyrin nanoball,
J. Cremers, R. Haver, M. Rickhaus, J. Q. Gong, L. Favereau, M. D. Peeks, T. D. Claridge, L. M. Herz, and H. L. Anderson,
J. Am. Chem. Soc., 140 (2018), p. 5352–5355. [journal | article | SI ]

We report the template-directed synthesis of a π-conjugated 14-porphyrin nanoball. This structure consists of two intersecting nanorings containing six and 10 porphyrin units. Fluorescence upconversion spectroscopy experiments demonstrate that electronic excitation delocalizes over the whole three-dimensional π system in less than 0.3 ps if the nanoball is bound to its templates or over 2 ps if the nanoball is empty.

Hybrid perovskites: Prospects for concentrator solar cells,
Q. Lin, Z. Wang, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Advanced Science, 5 (2018), p. 1700792. [journal | article | SI ]

Perovskite solar cells have shown a meteoric rise of power conversion efficiency and a steady pace of improvements in their stability of operation. Such rapid progress has triggered research into approaches that could boost efficiencies beyond the Shockley-Queisser limit stipulated for a single-junction cell under normal solar illumination conditions. The tandem solar cell architecture is one concept here that has recently been successfully implemented. However, the approach of solar concentration has so far not been sufficiently explored for perovskite photovoltaics, despite its frequent use in the area of inorganic semiconductor solar cells. In this Letter, we assess the prospects of hybrid perovskites for use in concentrator solar cells. We theoretically predict solar cell performance parameters as a function of solar concentration levels, based on representative assumptions of charge-carrier recombination and extraction rates in the device. We demonstrate that perovskite solar cells can fundamentally exhibit appreciably higher energy-conversion efficiencies under solar concentration, where they are able to exceed the Shockley-Queisser limit and exhibit strongly elevated open-circuit voltages. We therefore conclude that sufficient material and device stability under increased illumination levels would be the only significant challenge to perovskite concentrator solar cell applications.

Bimolecular recombination in methylammonium lead triiodide perovskite is an inverse absorption process,
C. L. Davies, M. R. Filip, J. B. Patel, T. W. Crothers, C. Verdi, A. D. Wright, R. L. Milot, F. Giustino, M. B. Johnston, and L. M. Herz,
Nature Communications, 9 (2018), p. 293. [journal | article | SI ]

Photovoltaic devices based on metal halide perovskites are rapidly improving in efficiency. Once the Shockley–Queisser limit is reached, charge-carrier extraction will be limited only by radiative bimolecular recombination of electrons with holes. Yet, this fundamental process, and its link with material stoichiometry, is still poorly understood. Here we show that bimolecular charge-carrier recombination in methylammonium lead triiodide perovskite can be fully explained as the inverse process of absorption. By correctly accounting for contributions to the absorption from excitons and electron-hole continuum states, we are able to utilise the van Roosbroeck–Shockley relation to determine bimolecular recombination rate constants from absorption spectra. We show that the sharpening of photon, electron and hole distribution functions significantly enhances bimolecular charge recombination as the temperature is lowered, mirroring trends in transient spectroscopy. Our findings provide vital understanding of band-to-band recombination processes in this hybrid perovskite, which comprise direct, fully radiative transitions between thermalized electrons and holes.

Interplay of structural and optoelectronic properties in formamidinium mixed tin–lead triiodide perovskites,
E. S. Parrott, T. Green, R. L. Milot, M. B. Johnston, H. J. Snaith, and L. M. Herz,
Advanced Functional Materials, 28 (2018), p. 1802803. [journal | article | SI ]

Mixed lead–tin triiodide perovskites are promising absorber materials for low bandgap bottom cells in all-perovskite tandem photovoltaic devices. Key structural and electronic properties of the FAPb_1−xSn_xI₃ perovskite are presented here as a function of lead:tin content across the alloy series. Temperature-dependent photoluminescence and optical absorption measurements are used to identify changes in the bandgap and phase transition temperature. The large bandgap bowing parameter, a crucial element for the attainment of low bandgaps in this system, is shown to depend on the structural phase, reaching a value of 0.84 eV in the low-temperature phase and 0.73 eV at room temperature. The parabolic nature of the bowing at all temperatures is compatible with a mechanism arising from bond bending to accommodate the random placement of unevenly sized lead and tin ions. Charge-carrier recombination dynamics are shown to fall into two regimes. Tin-rich compositions exhibit fast, monoexponential recombination that is almost temperature-independent, in accordance with high levels of electrical doping. Lead-rich compositions show slower, stretched-exponential chargecarrier recombination that is strongly temperature-dependent, in accordance with a multiphonon assisted process. These results highlight the importance of structure and composition for control of bandgap bowing and charge-carrier recombination mechanisms in low bandgap absorbers for all-perovskite tandem solar cells.

High irradiance performance of metal halide perovskites for concentrator photovoltaics,
Z. Wang, Q. Lin, B. Wenger, M. G. Christoforo, Y.-H. Lin, M. T. Klug, M. B. Johnston, L. M. Herz, and H. J. Snaith,
Nature Energy, 3 (2018), p. 855–861. [journal | article | SI ]

Traditionally, III–V multi-junction cells have been used in concentrator photovoltaic (CPV) applications, which deliver extremely high efficiencies but have failed to compete with ‘flat-plate’ silicon technologies owing to cost. Here, we assess the feasibility of using metal halide perovskites for CPVs, and we evaluate their device performance and stability under concentrated light. Under simulated sunlight, we achieve a peak efficiency of 23.6% under 14 Suns (that is, 14 times the standard solar irradiance), as compared to 21.1% under 1 Sun, and measure 1.26 V open-circuit voltage under 53 Suns, for a material with a bandgap of 1.63 eV. Importantly, our encapsulated devices maintain over 90% of their original efficiency after 150 h aging under 10 Suns at maximum power point. Our work reveals the potential of perovskite CPVs, and may lead to new PV deployment strategies combining perovskites with low-concentration factor and lower-accuracy solar tracking systems.

Impact of the organic cation on the optoelectronic properties of formamidinium lead triiodide,
C. L. Davies, J. Borchert, C. Q. Xia, R. L. Milot, H. Kraus, M. B. Johnston, and L. M. Herz,
J. Phys. Chem. Lett., 9 (2018), p. 4502–4511. [journal | article | SI ]

Metal halide perovskites have proven to be excellent light-harvesting materials in photovoltaic devices whose efficiencies are rapidly improving. Here, we examine the temperature-dependent photon absorption, exciton binding energy, and band gap of FAPbI₃ (thin film) and find remarkably different behavior across the β-γ phase transition compared with MAPbI₃. While MAPbI₃ has shown abrupt changes in the band gap and exciton binding energy, values for FAPbI₃ vary smoothly over a range of 100-160K in accordance with a more gradual transition. In addition, we find that the charge-carrier mobility in FAPbI₃ exhibits a clear T^–0.5 trend with temperature, in excellent agreement with theoretical predictions that assume electron–phonon interactions to be governed by the Fröhlich mechanism but in contrast to the T^–1.5 dependence previously observed for MAPbI₃. Finally, we directly observe intraexcitonic transitions in FAPbI₃ at low temperature, from which we determine a low exciton binding energy of only 5.3meV at 10K.

The effects of doping density and temperature on the optoelectronic properties of formamidinium tin triiodide thin films,
R. L. Milot, M. T. Klug, C. L. Davies, Z. Wang, H. Kraus, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Adv. Mater., 30 (2018), p. 1804506. [journal | article | SI ]

Optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI₃) thin films, whose background hole doping density was varied through SnF₂ addition during film fabrication. Monomolecular charge-carrier recombination exhibits both a dopant-mediated part that grows linearly with hole doping density and remnant contributions that remain under tin-enriched processing conditions. At hole densities near 10²⁰ cm^-3, a strong Burstein-Moss effect increases absorption onset energies by ~300meV beyond the band gap energy of undoped FASnI₃ (shown to be 1.2 eV at 5 K and 1.35 eV at room temperature). At very high doping densities (10²⁰ cm^-3), temperature-dependent measurements indicate that the effective charge-carrier mobility is suppressed through scattering with ionized dopants. Once the background hole concentration is nearer 10¹⁹ cm^-3 and below, the charge-carrier mobility increases with decreasing temperature according to ~T^-1.2, suggesting it is limited mostly by intrinsic interactions with lattice vibrations. For the lowest doping concentration of 7.2×10¹⁸ cm^-3, charge-carrier mobilities reach a value of 67 cm²V^-1s^-1 at room temperature and 470 cm²V^-1s^-1 at 50 K. Intra-excitonic transitions observed in the THz-frequency photoconductivity spectra at 5K reveal an exciton binding energy of only 3.1 meV for FASnI₃, in agreement with the low bandgap energy exhibited by this perovskite.

How lattice dynamics moderate the electronic properties of metal-halide perovskites,
L. M. Herz,
J. Phys. Chem. Lett., 9 (2018), p. 6853–6863. [journal | article | SI ]

Metal-halide perovskites have emerged as highly promising semiconductors with excellent optoelectronic properties. This Perspective outlines how the dynamic response of the ionic lattice affects key electronic properties such as exciton binding energies and charge-carrier mobilities in hybrid perovskites. Such links are shown to derive from the frequency-dependence of the dielectric function, which is governed by contributions from electronic interband transitions, polar vibrations of the metal-halide sublattice, organic cation collective reorientations, and ionic movement. The influence of each of these contributions to charge-carrier screening and carrier-lattice interactions is discussed, which allows for general trends with material composition to be revealed. Overall, this Perspective highlights the challenges and questions arising from the peculiar combination of a soft polar metal-halide sublattice interspersed with rotationally mobile dipolar molecules that is encountered in hybrid metal-halide perovskites.

The influence of surfaces on the transient terahertz conductivity and electron mobility of GaAs nanowires,
H. J. Joyce, S. A. Baig, P. Parkinson, C. L. Davies, J. L. Boland, H. H. Tan, C. Jagadish, L. M. Herz, and M. B. Johnston,
J. Phys. D: Appl. Phys., 50 (2017), p. 224001. [journal | article | SI ]

Bare unpassivated GaAs nanowires feature relatively high electron mobilities (400-2100 cm² V⁻¹ s⁻¹) and ultrashort charge carrier lifetimes (1-5 ps) at room temperature. These two properties are highly desirable for high speed optoelectronic devices, including photoreceivers, modulators and switches operating at microwave and terahertz frequencies. When engineering these GaAs nanowire-based devices, it is important to have a quantitative understanding of how the charge carrier mobility and lifetime can be tuned. Here we use optical-pump–terahertzprobe spectroscopy to quantify how mobility and lifetime depend on the nanowire surfaces and on carrier density in unpassivated GaAs nanowires. We also present two alternative frameworks for the analysis of nanowire photoconductivity: one based on plasmon resonance and the other based on Maxwell–Garnett effective medium theory with the nanowires modelled as prolate ellipsoids. We find the electron mobility decreases significantly with decreasing nanowire diameter, as charge carriers experience increased scattering at nanowire surfaces. Reducing the diameter from 50 nm to 30 nm degrades the electron mobility by up to 47%. Photoconductivity dynamics were dominated by trapping at saturable states existing at the nanowire surface, and the trapping rate was highest for the nanowires of narrowest diameter. The maximum surface recombination velocity, which occurs in the limit of all traps being empty, was calculated as 1.3 × 10⁶ cm s⁻¹. We note that when selecting the optimum nanowire diameter for an ultrafast device, there is a trade-off between achieving a short lifetime and a high carrier mobility. To achieve high speed GaAs nanowire devices featuring the highest charge carrier mobilities and shortest lifetimes, we recommend operating the devices at low charge carrier densities.

Cs₂InAgCl₆: A new lead-free halide double perovskite with direct band gap,
G. Volonakis, A. A. Haghighirad, R. L. Milot, W. H. Sio, M. R. Filip, B. Wenger, M. B. Johnston, L. M. Herz, H. J. Snaith, and F. Giustino,
J. Phys. Chem. Lett., 8 (2017), p. 772–778. [journal | article | SI ]

A₂BB'X₆ halide double perovskites based on bismuth and silver A₂BB′X₆ halide double perovskites based on bismuth and silver have recently been proposed as potential environmentally friendly alternatives to lead-based hybrid halide perovskites. In particular, Cs₂BiAgX₆ (X = Cl, Br) have been synthesized and found to exhibit band gaps in the visible range. However, the band gaps of these compounds are indirect, which is not ideal for applications in thin film photovoltaics. Here, we propose a new class of halide double perovskites, where the B³⁺ and B⁺ cations are In³⁺ and Ag⁺, respectively. Our first-principles calculations indicate that the hypothetical compounds Cs₂InAgX₆ (X = Cl, Br, I) should exhibit direct band gaps between the visible (I) and the ultraviolet (Cl). Based on these predictions, we attempt to synthesize Cs₂InAgCl₆ and Cs₂InAgBr₆, and we succeed to form the hitherto unknown double perovskite Cs₂InAgCl₆. X-ray diffraction yields a double perovskite structure with space group Fm3m. The measured band gap is 3.3 eV, and the compound is found to be photosensitive and turns reversibly from white to orange under ultraviolet illumination. We also perform an empirical analysis of the stability of Cs₂InAgX₆ and their mixed halides based on Goldschmidt's rules, and we find that it should also be possible to form Cs₂InAg(Cl_1−xBr_x)₆ for x < 1. The synthesis of mixed halides will open the way to the development of lead-free double perovskites with direct and tunable band gaps.

Towards higher electron mobility in modulation doped GaAs/AlGaAs core shell nanowires,
J. L. Boland, G. Tütüncüoglu, J. Gong, S. Conesa-Boj, C. L. Davies, L. M. Herz, A. Fontcuberta i Morral, and M. B. Johnston,
Nanoscale, 9 (2017), p. 7839–7846. [journal | article | SI ]

Precise control over the electrical conductivity of semiconductor nanowires is a crucial prerequisite for implementation into novel electronic and optoelectronic devices. Advances in our understanding of doping mechanisms in nanowires and their influence on electron mobility and radiative efficiency are urgently required. Here, we investigate the electronic properties of n-type modulation doped GaAs/AlGaAs nanowires via optical pump terahertz (THz) probe spectroscopy and photoluminescence spectroscopy over the temperature range 5K-300K. We directly determine an ionisation energy of 6.7±0.5meV (T = 52K) for the Si donors that create the modulation doping in the AlGaAs shell. We further elucidate the temperature dependence of the electron mobility, photoconductivity lifetime and radiative efficiency, and determine the charge-carrier scattering mechanisms that limit electron mobility. We show that below the donor ionization temperature, charge scattering is limited by interactions with interfaces, leading to an excellent electron mobility of 4360±380cm²V^-1s^-1 at 5 K. Above the ionization temperature, polar scattering via longitudinal optical (LO) phonons dominates, leading to a room temperature mobility of 2220±130cm²V^-1s^-1. In addition, we show that the Si donors effectively passivate interfacial trap states in the nanowires, leading to prolonged photoconductivity lifetimes with increasing temperature, accompanied by an enhanced radiative efficiency that exceeds 10% at room temperature.

The entangled triplet pair state in acene and heteroacene materials,
C. K. Yong, A. J. Musser, S. L. Bayliss, S. Lukman, H. Tamura, O. Bubnova, R. K. Hallani, A. Meneau, R. Resel, M. Maruyama, S. Hotta, L. M. Herz, D. Beljonne, J. E. Anthony, J. Clark, and H. Sirringhaus,
Nature Communications, 8 (2017), p. 15953. [journal | article | SI ]

Entanglement of states is one of the most surprising and counter-intuitive consequences of quantum mechanics, with potent applications in cryptography and computing. In organic materials, one particularly significant manifestation is the spin-entangled triplet-pair state, which mediates the spin-conserving fission of one spin-0 singlet exciton into two spin-1 triplet excitons. Despite long theoretical and experimental exploration, the nature of the triplet-pair state and inter-triplet interactions have proved elusive. Here, we use a range of organic semiconductors which undergo singlet exciton fission to reveal the photophysical properties of entangled triplet-pair states. We find that the triplet-pair is bound with respect to free triplets with an energy that is largely materialindependent (~30meV). During its lifetime, the component triplets behave cooperatively as a singlet and emit light through a Herzberg-Teller-type mechanism, resulting in vibronically structured photoluminescence. In photovoltaic blends, charge transfer can occur from the bound triplet pairs with >100% photon-to-charge conversion efficiency.

Crystallization kinetics and morphology control of mixed-cation lead mixed-halide perovskite via tunability of the colloidal precursor solution,
D. P. McMeekin, Z. Wang, W. Rehman, F. Pulvirenti, J. B. Patel, N. K. Noel, S. R. Marder, M. B. Johnston, L. M. Herz, and H. J. Snaith,
Adv. Mater., 29 (2017), p. 1607039. [journal | article | SI ]

The meteoric rise of the field of perovskite solar cells has been fueled by the ease with which a wide range of high-quality materials can be fabricated via simple solution processing methods. However, to date, little effort has been devoted to understanding the precursor solutions, and the role of additives such as hydrohalic acids upon film crystallization and final optoelectronic quality. Here, a direct link between the colloids concentration present in the [HC(NH₂)₂|_0.83Cs_0.17Pb(Br_0.2I_0.8)₃ precursor solution and the nucleation and growth stages of the thin film formation is established. Using dynamic light scattering analysis, the dissolution of colloids over a time span triggered by the addition of hydrohalic acids is monitored. These colloids appear to provide nucleation sites for the perovskite crystallization, which critically impacts morphology, crystal quality, and optoelectronic properties. Via 2D X-ray diffraction, highly ordered and textured crystals for films prepared from solutions with lower colloidal concentrations are observed. This increase in material quality allows for a reduction in microstrain along with a twofold increase in chargecarrier mobilities leading to values exceeding 20cm² V⁻¹ s⁻¹. Using a solution with an optimized colloidal concentration, devices that reach current–voltage measured power conversion efficiency of 18.8% and stabilized efficiency of 17.9% are fabricated.

Influence of interface morphology on hysteresis in vapor-deposited perovskite solar cells,
J. B. Patel, J. Wong-Leung, S. V. Reenen, N. Sakai, J. T. W. Wang, E. S. Parrott, M. Liu, H. J. Snaith, L. M. Herz, and M. B. Johnston,
Adv. Electron. Mater., 3 (2017), p. 1600470. [journal | article | SI ]

Hysteresis in the current–voltage characteristics of vapor-deposited perovskite solar cells is shown to originate from an amorphous region of CH₃NH₃PbI₃ at the interface with the device's electron transport layer. Interface engineering is used to produce highly crystalline perovskite material at this interface which results in hysteresis-free evaporated planar heterojunction solar cells.

Photovoltaic mixed-cation lead mixed-halide perovskites: links between crystallinity, photo-stability and electronic properties,
W. Rehman, D. P. McMeekin, J. B. Patel, R. L. Milot, M. B. Johnston, H. J. Snaith, and L. M. Herz,
Energy Environ. Sci., 10 (2017), p. 361–369. [journal | article | SI ]

Lead mixed halide perovskites are highly promising semiconductors for both multi-junction photovoltaic and light emitting applications due to their tunable band gaps, with emission and absorption energies spanning the UV-visible to near IR regions. However, many such perovskites exhibit unwanted halide segregation under photo-illumination, the cause of which is still unclear. In our study, we establish crucial links between crystal phase stability, photostability and optoelectronic properties of the mixed-cation lead mixed-halide perovskite Cs_yFA_(1-y)Pb(Br_xI_(1-x))₃. We demonstrate a region for caesium content between 0.10 < y < 0.30 which features high crystalline quality, long charge-carrier lifetimes and high charge-carrier mobilities. Importantly, we show that for such high-quality perovskites, photo-induced halide segregation is strongly suppressed, suggesting that high crystalline quality is a prerequisite for good optoelectronic quality and band gap stability. We propose that regions of short-range crystalline order aid halide segregation, possibly by releasing lattice strain between iodide rich and bromide rich domains. For an optimized caesium content, we explore the orthogonal halide-variation parameter space for Cs_0.17FA_0.83Pb(Br_xI_(1-x))₃ perovskites. We demonstrate excellent charge-carrier mobilities (11–40 cm²V^-1s^-1) and diffusion lengths (0.8-4.4 μm) under solar conditions across the full iodide–bromide tuning range. Therefore, the addition of caesium yields a more photo-stable perovskite system whose absorption onsets can be tuned for bandgap-optimized tandem solar cells.

Large-area, highly uniform evaporated formamidinium lead triiodide thin films for solar cells,
J. Borchert, R. L. Milot, J. B. Patel, C. L. Davies, A. D. Wright, L. M. Maestro, H. J. Snaith, L. M. Herz, and M. B. Johnston,
ACS Energy Letters, 2 (2017), p. 2799–2804. [journal | article | SI ]

Perovskite thin-film solar cells are one of the most promising emerging renewable energy technologies because of their potential for low-cost, large-area fabrication combined with high energy conversion efficiencies. Recently, formamidinium lead triiodide (FAPbI₃) and other formamidinium (CH(NH₂)₂) based perovskites have been explored as interesting alternatives to methylammonium lead triiodide (MAPbI₃) because they exhibit better thermal stability. However, at present a major challenge is the scale-up of perovskite solar cells from small test-cells to full solar modules. We show that coevaporation is a scalable method for the deposition of homogeneous FAPbI₃ thin films over large areas. The method allows precise control over film thickness and results in highly uniform, pinhole-free layers. Our films exhibited a high charge-carrier mobility of 26cm²V⁻¹s⁻¹, excellent optical properties, and a bimolecular recombination constant of 7×10⁻¹¹cm³s⁻¹. Solar cells fabricated using these vapor-deposited layers within a regular device architecture produced stabilized power conversion efficiencies of up to 14.2%. Thus, we demonstrate that efficient FAPbI₃ solar cells can be vapor-deposited, which opens up a pathway toward large-area stable perovskite photovoltaics.

Band-tail recombination in hybrid lead iodide perovskite,
A. D. Wright, R. L. Milot, G. E. Eperon, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Adv. Func. Mater., 27 (2017), p. 1700860. [journal | article | SI ]

Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here we investigate the low-temperature photoluminescence of formamidinium lead triiodide (HC(NH₂)₂PbI₃), observing a power-law time dependence in the emission intensity, and an additional low-energy emission peak which exhibits an anomalous relative Stokes shift. Using a rate-equation model and a Monte Carlo simulation, we reveal that both phenomena arise from the existence of an exponential trap-density tail with characteristic energy scale of  3 meV. Charge-carrier recombination from sites deep within the density of tail states is found to cause emission with energy downshifted by up to several tens of meV. Hence such phenomena may in part be responsible for open-circuit voltage losses commonly observed in these materials. In this high-quality hybrid perovskite, trap states therefore predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated e.g. with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose band-structure picture is moderated by a modest degree of energetic disorder.

2D-3D hetero-structured butylammonium-Cs-formamidinium lead trihalide perovskites for stable and efficient solar cells,
Z. Wang, Q. Lin, F. P. Chmiel, N. Sakai, L. M. Herz, and H. J. Snaith,
Nature Energy, 2 (2017), p. 17135. [journal | article | SI ]

Three-dimensional (3D) organic-inorganic perovskite solar cells have undergone a meteoric rise in cell efficiency to >22%. However, the perovskite absorber layer is prone to degradation in water, oxygen and UV light. Two-dimensional (2D) Ruddlesden−Popper layered perovskites have exhibited promising environmental stability, but perform less well in solar cells, possibly due to the inhibition of out-of-plane charge transport by the insulating spacer cations. Alternatively, moving away from methylammonium, to the mixed cation formamidinium-caesium based perovskites has led to considerably enhancement of the stability of 3D perovskite absorber layers. Here, we report highly efficient and stable perovskite solar cells based on a self-assembled butylammonium-Cs-formamidinium mixed-cation lead mixed-halide perovskite photoactive layer. Long-chain alkyl-ammonium halides added to the formamidinium-cesium based perovskite precursor solution strongly enhances the crystallinity of the 3D perovskite phase, while also inducing the formation of new layered-phases in the films. By carefully regulating the composition, we are able to achieve ”plate-like" layered perovskite crystallites standing up between the host 3D perovskite grains. This spontaneously forming heterostructure allows the efficient charge carrier transport in the 3D perovskite phase, while reducing charge recombination via fortuitous grain boundary passivation. We also observe reduced current-voltage hysteresis and improved device stability, which we correlate to enhanced crystallinity and reduced crystal defects in the 3D perovskite phase. With the optimized composition, we achieved a power conversion efficiency of 20.6% (stabilised efficiency of 19.5%) from a narrow bandgap (1.61 eV) perovskite solar cell and of 17.2% (stabilised efficiency of 17.3%) from a wider bandgap (1.72 eV) perovskite solar cell optimised for tandem applications. In addition to enhanced efficiency, the addition of butylammonium greatly enhances the long-term stability of the devices. For the first time, our cells sustain more than 80% of their ”post burn-in" efficiency after 1,000 hrs of aging under simulated full spectrum sun light measured in an ambient environment without encapsulation. With additional sealing with a glass/polymer-foil/glass laminate, we extend this lifetime to close to 4,000 hrs. Our work illustrates that engineering heterostructures between 2D and 3D perovskite phases is both possible, and can lead to enhancement of both performance and stability of perovskite solar cells.

Near-infrared and short-wavelength infrared photodiodes based on dye-perovskite composites,
Q. Lin, Z. Wang, M. Young, J. B. Patel, R. L. Milot, L. M. Maestro, R. R. Lunt, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Adv. Func. Mater., 27 (2017), p. 1702485. [journal | article | SI ]

Organohalide perovskites have emerged as promising light-sensing materials because of their superior optoelectronic properties and low-cost processing methods. Recently, perovskite-based photodetectors have successfully been demonstrated as both broadband and narrowband varieties. However, the photodetection bandwidth in perovskite-based photodetectors has so far been limited to the near-infrared regime owing to the relatively wide band gap of hybrid organohalide perovskites. In particular, short-wavelength infrared photodiodes operating beyond 1μm have not yet been realized with organohalide perovskites. In this study, narrow band gap organic dyes are combined with hybrid perovskites to form composite films as active photoresponsive layers. Tuning the dye loading allows for optimization of the spectral response characteristics and excellent charge-carrier mobilities near 11cm²V⁻¹s⁻¹, suggesting that these composites combine the light-absorbing properties or IR dyes with the outstanding charge-extraction characteristics of the perovskite. This study demonstrates the first perovskite photodiodes with deep near-infrared and short-wavelength infrared response that extends as far as 1.6 μm. All devices are solution-processed and exhibit relatively high responsivity, low dark current, and fast response at room temperature, making this approach highly attractive for next-generation light-detection techniques.

Charge-carrier mobilities in metal halide perovskites: Fundamental mechanisms and limits,
L. M. Herz,
ACS Energy Lett., 2 (2017), p. 1539–1548. [journal | article | SI ]

Perovskite photovoltaic cells have seen a remarkable rise in power conversion efficiencies over a period of only a few years. Much of this performance is underpinned by the favourable charge-carrier mobilities in metal-halide perovskites (MHPs), which are remarkably high for materials with such facile and versatile processing routes. This Perspective outlines the mechanisms that set a fundamental upper limit to charge-carrier mobility values in MHPs, and reveals how they may be tuned through changes in stoichiometry. In addition, extrinsic effects such as grain size, energetic disorder and self-doping are discussed for specific MHPs in the context of remedies designed to avoid them.

Photon reabsorption masks intrinsic bimolecular charge-carrier recombination in CH₃NH₃PbI₃ perovskite,
T. W. Crothers, R. L. Milot, J. B. Patel, E. S. Parrott, J. Schlipf, P. Müller-Buschbaum, M. B. Johnston, and L. M. Herz,
Nano Lett., 17 (2017), p. 5782–5789. [journal | article | SI ]

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH₃NH₃PbI₃ are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50 and 533 nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8×10⁻¹⁰cm³s⁻¹ is common to all samples irrespective of film thickness. Hence, we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.

Preface for special topic: Perovskite solar cells—a research update,
L. Schmidt-Mende and L. M. Herz,
APL Mater., 4 (2016), p. 091201. [journal | article | SI ]

Over the last few years, tremendous progress has been made in the research field of perovskite solar cells. Not only are record power conversion efficiencies now exceeding 20%, but our understanding about the different mechanisms leading to this extraordinary performance has improved phenomenally. The aim of this special issue is to review the current state-of-the-art understanding of perovskite solar cells. Most of the presented articles are research updates giving a succinct overview over different aspects concerning perovskite solar cells.

Formation dynamics of CH₃NH₃PbI₃ perovskite following two-step layer deposition,
J. B. Patel, R. L. Milot, A. D. Wright, L. M. Herz, and M. B. Johnston,
J. Phys. Chem. Lett., 7 (2016), p. 96–102. [journal | article | SI ]

Hybrid metal-halide perovskites have emerged as a leading class of semiconductors for optoelectronic devices because of their desirable material properties and versatile fabrication methods. However, little is known about the chemical transformations that occur in the initial stages of perovskite crystal formation. Here we follow the real-time formation dynamics of MAPbI₃ from a bilayer of lead iodide (PbI₂) and methylammonium iodide (MAI) deposited through a two-step thermal evaporation process. By lowering the substrate temperature during deposition, we are able to initially inhibit intermixing of the two layers. We subsequently use infrared and visible light transmission, X-ray diffraction, and photoluminescence lifetime measurements to reveal the room-temperature transformations that occur in vacuum and ambient air, as MAI diffuses into the PbI₂ lattice to form MAPbI₃. In vacuum, the transformation to MAPbI₃ is incomplete as unreacted MAI is retained in the film. However, exposure to moist air allows for conversion of the unreacted MAI to MAPbI₃, demonstrating that moisture is essential in making MAI more mobile and thus aiding perovskite crystallization. These dynamic processes are reflected in the observed charge-carrier lifetimes, which strongly fluctuate during periods of large ion migration but steadily increase with improving crystallinity.

Structured organic–inorganic perovskite toward a distributed feedback laser,
M. Saliba, S. M. Wood, J. B. Patel, P. K. Nayak, J. Huang, J. A. Alexander-Webber, B. Wenger, S. D. Stranks, M. T. Hörantner, J. T.-W. Wang, R. J. Nicholas, L. M. Herz, M. B. Johnston, S. M. Morris, H. J. Snaith, and M. K. Riede,
Adv. Mater., 28 (2016), p. 923–929. [journal | article | SI ]

A general strategy for the in-plane structuring of organic-inorganic perovskite films is presented. The method is used to fabricate an industrially relevant distributed feedback (DFB) cavity, which is a critical step towards all-electrially pumped injection laser diodes. This approach opens the prospects of perovskite materials for much improved optical control in LEDs, solar cells and also toward applications as optical devices.

Increased photoconductivity lifetime in GaAs nanowires by controlled n-type and p-type doping,
J. L. Boland, A. Casadei, G. Tütüncüoglu, F. Matteini, C. L. Davies, F. Jabeen, H. J. Joyce, L. M. Herz, A. Fontcuberta i Morral, and M. B. Johnston,
ACS Nano, 10 (2016), p. 4219–4227. [journal | article | SI ]

Controlled doping of GaAs nanowires is crucial for the development of nanowire-based electronic and optoelectronic devices. Here, we present a noncontact method based on time-resolved terahertz photoconductivity for assessing n- and p-type doping efficiency in nanowires. Using this technique, we measure extrinsic electron and hole concentrations in excess of 10¹⁸ cm^–3 for GaAs nanowires with n-type and p-type doped shells. Furthermore, we show that controlled doping can significantly increase the photoconductivity lifetime of GaAs nanowires by over an order of magnitude: from 0.13 ns in undoped nanowires to 3.8 and 2.5 ns in n-doped and p-doped nanowires, respectively. Thus, controlled doping can be used to reduce the effects of parasitic surface recombination in optoelectronic nanowire devices, which is promising for nanowire devices, such as solar cells and nanowire lasers.

Breaking the symmetry in molecular nanorings,
J. Q. Gong, L. Favereau, H. L. Anderson, and L. M. Herz,
J. Phys. Chem. Lett., 7 (2016), p. 332–338. [journal | article | SI ]

Because of their unique electronic properties, cyclic molecular structures ranging from benzene to natural light-harvesting complexes have received much attention. Rigid π-conjugated templated porphyrin nanorings serve as excellent model systems here because they possess well-defined structures that can readily be controlled and because they support highly delocalized excitations. In this study, we have deliberately modified a series of six-porphyrin nanorings to examine the impact of lowering the rotational symmetry on their photophysical properties. We reveal that as symmetry distortions increase in severity along the series of structures, spectral changes and an enhancement of radiative emission strength occur, which derive from a transfer of oscillator strength into the lowest (k = 0) state. We find that concomitantly, the degeneracy of the dipole-allowed first excited (k = +/- 1) state is lifted, leading to an ultrafast polarization switching effect in the emission from strongly symmetry-broken nanorings.

Hybrid perovskites for photovoltaics: charge-carrier recombination, diffusion and radiative efficiencies,
M. B. Johnston and L. M. Herz,
Acc. Chem. Res., 49 (2016), p. 146–154. [journal | article | SI ]

Photovoltaic (PV) devices that harvest the energy provided by the sun have great potential as renewable energy sources, yet uptake has been hampered by the increased cost of solar electricity compared with fossil fuels. Hybrid metal halide perovskites have recently emerged as low-cost active materials in PV cells with power conversion efficiencies now exceeding 20%. Rapid progress has been achieved over only a few years through improvements in materials processing and device design. In addition, hybrid perovskites appear to be good light emitters under certain conditions, raising the prospect of applications in low-cost light-emitting diodes and lasers. Further optimization of such hybrid perovskite devices now needs to be supported by a better understanding of how light is converted into electrical currents, and vice versa. This Account provides an overview of charge-carrier recombination and mobility mechanisms encountered in such materials. Optical-Pump Terahertz-Probe (OPTP) photoconductivity spectroscopy is an ideal tool here, as it allows the dynamics of mobile charge carriers inside the perovskite to be monitored following excitation with a short laser pulse whose photon energy falls into the range of the solar spectrum. We first review our insights gained from transient OPTP and photoluminescence spectroscopy on the mechanisms dominating charge-carrier recombination in these materials. We discuss that mono-molecular charge-recombination predominantly originates from trapping of charges, with trap depths being relatively shallow (tens of meV) for hybrid lead iodide perovskites. Bimolecular recombination arises from direct band-to-band electron-hole recombination and is found to be in significant violation of the simple Langevin model. Auger recombination exhibits links with electronic band-structure, in accordance with its requirement for energy and momentum conservation for all charges involved. We further discuss charge-carrier mobility values extracted from OPTP measurements, and their dependence on perovskite composition and morphology. The significance of the reviewed charge-carrier recombination and mobility parameters is subsequently evaluated in terms of the charge-carrier diffusion lengths and radiative efficiencies that may be obtained for such hybrid perovskites. We particularly focus on calculating such quantities in the limit of ultra-low trap-related recombination, which has not yet been demonstrated but could be reached through further advances in material processing. We find that for thin films of hybrid lead iodide perovskites with typical charge-carrier mobilities of ~30cm²/(Vs), charge-carrier diffusion lengths at solar (AM1.5) irradiation are unlikely to exceed ~10μm even if all trap-related related recombination is eliminated. We further examine the radiative efficiency for hybrid lead halide perovskite films and show that if high efficiencies are to be obtained for intermediate charge-carrier densities (n~10¹⁴cm^-3) trap-related recombination lifetimes will have to be enhanced well into the microsecond range.

Synthesis of five-porphyrin nanorings using ferrocene and corannulene templates,
P. Liu, Y. Hisamune, M. D. Peeks, B. Odell, J. Q. Gong, L. M. Herz, and H. L. Anderson,
Angew. Chem. Int. Ed., 55 (2016), p. 8358–8362. [journal | article | SI ]

The smallest and most strained member of a family of π-conjugated cyclic porphyrin oligomers has been synthesized using penta-pyridyl templates based on ferrocene and corannulene cores. Both templates are effective for directing the synthesis of the butadiyne-linked cyclic porphyrin pentamer, despite the fact that the radii of their N5 donor sets are too small by 0.5 Å and 0.9 Å, respectively (from DFT geometry optimization). The five-porphyrin nanoring exhibits a highly structured absorption spectrum and its fluorescence spectrum extends to 1200 nm, reflecting strong π-conjugation and Herzberg-Teller vibronic coupling.

Effect of structural phase transition on charge-carrier lifetimes and defects in CH₃NH₃SnI₃ perovskite,
E. S. Parrott, R. L. Milot, T. Stergiopoulos, H. J. Snaith, M. B. Johnston, and L. M. Herz,
J. Phys. Chem. Lett., 7 (2016), p. 1321–1326. [journal | article | SI ]

Methylammonium tin triiodide (MASnI₃) has been successfully employed in lead-free perovskite solar cells, but overall power-conversion efficiencies are still significantly lower than for lead-based perovskites. Here we present photoluminescence (PL) spectra and time-resolved PL from 8 to 295 K and find a marked improvement in carrier lifetime and a substantial reduction in PL line width below ~110 K, indicating that the cause of the hindered performance is activated at the orthorhombic to tetragonal phase transition. Our measurements therefore suggest that targeted structural change may be capable of tailoring the relative energy level alignment of defects (e.g., tin vacancies) to reduce the background dopant density and improve charge extraction. In addition, we observe for the first time an above-gap emission feature that may arise from higher-lying interband transitions, raising the prospect of excess energy harvesting.

A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells,
D. P. McMeekin, G. Sadoughi, W. Rehman, G. E. Eperon, M. Saliba, M. T. Hörantner, A. Haghighirad, N. Sakai, L. Korte, B. Rech, M. B. Johnston, L. M. Herz, and H. J. Snaith,
Science, 351 (2016), p. 151–155. [journal | article | SI ]

Metal halide perovskite photovoltaic cells could potentially boost the efficiency of commercial silicon photovoltaic modules from ~20 toward 30% when used in tandem architectures. An optimum perovskite cell optical band gap of ~1.75 electron volts (eV) can be achieved by varying halide composition, but to date, such materials have had poor photostability and thermal stability. Here we present a highly crystalline and compositionally photostable material, [HC(NH₂)₂|_0.83Cs_0.17Pb(I_0.6Br_0.4)₃, with an optical band gap of ~1.74 eV, and we fabricated perovskite cells that reached open-circuit voltages of 1.2 volts and power conversion efficiency of over 17% on small areas and 14.7% on 0.715 cm² cells. By combining these perovskite cells with a 19%-efficient silicon cell, we demonstrated the feasibility of achieving >25%-efficient four-terminal tandem cells.

Size-independent energy transfer in biomimetic nanoring complexes,
P. Parkinson, N. Kamonsutthipaijit, H. L. Anderson, and L. M. Herz,
ACS Nano, 10 (2016), p. 5933–5940. [journal | article | SI ]

Supramolecular antenna-ring complexes are of great interest due to their presence in natural light-harvesting complexes. While such systems are known to provide benefits through robust and efficient energy funneling, the relation-ship between molecular structure, strain (governed by nuclear co-ordinates and motion) and energy dynamics (arising from electronic behavior) is highly complex. We present a synthetic antenna-nanoring system based on a series of conjugated porphyrin chromophores ideally suited to explore such effects. By systematically varying the size of the acceptor nanoring, we reveal the interplay between antenna-nanoring binding, local strain and energy dynamics on the picosecond timescale. Binding of the antenna unit creates a local strain in the nanoring, and this strain was measured as a function of the size of the nanoring, by UV-vis-NIR titration, providing information on the conformational flexibility of the system. Strikingly, the energy transfer rate is independent of nanoring size, indicating the existence of strain-localized acceptor states, spread over about six porphyrin units, arising from the non-covalent antenna-nanoring association.

Charge carrier dynamics in organic-inorganic metal halide perovskites,
L. M. Herz,
Annu. Rev. Phys. Chem., 67 (2016), p. 65–89. [journal | article | SI ]

Hybrid organic-inorganic metal halide perovskites have recently emerged as exciting new light-harvesting and charge-transporting materials for efficient photovoltaic devices. Yet knowledge of the nature of the photo-generated excitations and their subsequent dynamics is only just emerging. This article reviews the current state of the field, focusing first on a description of the crystal and electronic band structure that gives rise to the strong optical transitions that enable light harvesting. An overview is presented of the numerous experimental approaches towards determining values for exciton binding energies, which appear to be small (few meV to few 10s of meV) and depend significantly on temperature because of associated changes in the dielectric function. Experimental evidence for charge-carrier relaxation dynamics within the first few picoseconds after excitation is discussed in terms of thermalization, cooling and many-body effects. Charge-carrier recombination mechanisms are reviewed, encompassing trap-assisted non-radiative recombination that is highly specific to processing conditions, radiative bi-molecular (electron-hole) recombination, and non-radiative many-body (Auger) mechanisms.

Radiative monomolecular recombination boosts amplified spontaneous emission in HC(NH₂)₂SnI₃ perovskite films,
R. L. Milot, G. E. Eperon, T. Green, H. J. Snaith, M. B. Johnston, and L. M. Herz,
J. Phys. Chem. Lett., 7 (2016), p. 4178–4184. [journal | article | SI ]

Hybrid metal-halide perovskites have potential as cost-effective gain media for laser technology because of their superior optoelectronic properties. Although lead-halide perovskites have been most widely studied to date, tin-based perovskites have been proposed as a less toxic alternative. In this Letter, we show that amplified spontaneous emission (ASE) in formamidinium tin triiodide (FASnI₃) thin films is supported by an observed radiative monomolecular charge recombination pathway deriving from its unintentional doping. Such a radiative component will be active even at the lowest charge-carrier densities, opening a pathway for ultralow light-emission thresholds. Using time-resolved THz photoconductivity analysis, we further show that the material has an unprecedentedly high charge-carrier mobility of 22 cm² V⁻¹ s⁻¹ favoring efficient transport. In addition, FASnI₃ exhibits strong radiative bimolecular recombination and Auger rates that are over an order of magnitude lower than for lead-halide perovskites. In combination, these properties reveal that tin-halide perovskites are highly suited to light-emitting devices.

Perovskite-perovskite tandem photovoltaics with ideal bandgaps,
G. E. Eperon, T. Leijtens, K. A. Bush, R. Prasanna, T. Green, J. T.-W. Wang, D. P. McMeekin, G. Volonakis, R. L. Milot, R. May, A. Palmstrom, D. J. Slotcavage, R. A. Belisle, J. B. Patel, E. S. Parrott, R. J. Sutton, W. Ma, F. Moghadam, B. Conings, A. Babayigit, H.-G. Boyen, S. Bent, F. Giustino, L. M. Herz, M. B. Johnston, M. D. McGehee, and H. J. Snaith,
Science, 354 (2016), p. 861–865. [journal | article | SI ]

We demonstrate four and two-terminal perovskite-perovskite tandem solar cells with ideally matched bandgaps. We develop an infrared absorbing 1.2eV bandgap perovskite, FA_0.75Cs_0.25Sn_0.5Pb_0.5I₃, that can deliver 14.8% efficiency. By combining this material with a wider bandgap FA_0.83Cs_0.17Pb(I_0.5Br_0.5)₃ material, we reach monolithic two terminal tandem efficiencies of 17.0% with over 1.65 volts open-circuit voltage. We also make mechanically stacked four terminal tandem cells and obtain 20.3% efficiency. Crucially, we find that our infrared absorbing perovskite cells exhibit excellent thermal and atmospheric stability, unprecedented for Sn based perovskites. This device architecture and materials set will enable “all perovskite” thin film solar cells to reach the highest efficiencies in the long term at the lowest costs.

Electron-phonon coupling in hybrid lead halide perovskites,
A. D. Wright, C. Verdi, R. L. Milot, G. E. Eperon, M. A. Pérez-Osorio, H. J. Snaith, F. Giustino, M. B. Johnston, and L. M. Herz,
Nature Communications, 7 (2016), p. 11755. [journal | article | SI ]

Phonon scattering limits charge-carrier mobilities and governs emission line broadening in hybrid metal halide perovskites. Establishing how charge-carriers interact with phonons in these materials is therefore essential for the development of high-efficiency perovskite photovoltaics and low-cost lasers. Here, we investigate the temperature dependence of emission line broadening in the four commonly-studied formamidinium and methylammonium perovskites, HC(NH₂)₂PbI₃, HC(NH₂)₂PbBr₃, CH₃NH₃PbI₃ and CH₃NH₃PbBr₃, and discover that scattering from longitudinal optical phonons via the Fröhlich interaction is the dominant source of electron-phonon coupling near room temperature, with scattering off acoustic phonons negligible. We determine energies for the interacting longitudinal optical phonon modes to be 11.5 and 15.3meV, and Fröhlich coupling constants of approximately 40 and 60meV for the lead iodide and bromide perovskites, respectively. Our findings correlate well with first-principles calculations based on many-body perturbation theory, which underlines the suitability of an electronic bandstructure picture for describing charge-carriers in hybrid perovskites.

Charge-carrier dynamics in 2D hybrid metal−halide perovskites,
R. L. Milot, R. J. Sutton, G. E. Eperon, A. A. Haghighirad, J. M. Hardigree, L. Miranda, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Nano Letters, 16 (2016), p. 7001–7007. [journal | article | SI ]

Hybrid metal−halide perovskites are promising new materials for use in solar cells; however, their chemical stability in the presence of moisture remains a significant drawback. Quasi two-dimensional (2D) perovskites that incorporate hydrophobic organic interlayers offer improved resistance to degradation by moisture, currently still at the cost of overall cell efficiency. To elucidate the factors affecting the optoelectronic properties of these materials, we have investigated the charge transport properties and crystallographic orientation of mixed methylammonium (MA)−phenylethylammonium (PEA) lead iodide thin films as a function of the MA-to-PEA ratio and, thus, the thickness of the “encapsulated’’ MA lead−halide layers. We find that monomolecular charge-carrier recombination rates first decrease with increasing PEA fraction, most likely as a result of trap passivation, but then increase significantly as excitonic effects begin to dominate for thin confined layers. Bimolecular and Auger recombination rate constants are found to be sensitive to changes in electronic confinement, which alters the density of states for electronic transitions. We demonstrate that effective charge-carrier mobilities remain remarkably high (near 10 cm²V^-1 s^-1) for intermediate PEA content and are enhanced for preferential orientation of the conducting lead iodide layers along the probing electric field. The trade-off between trap reduction, electronic confinement, and layer orientation leads to calculated charge-carrier diffusion lengths reaching a maximum of 2.5 μm for intermediate PEA content (50%).

Ultrafast delocalization of excitation in synthetic light-harvesting nanorings,
C.-K. Yong, P. Parkinson, D. V. Kondratuk, W.-H. Chen, A. Stannard, A. Summerfield, J. Sprafke, M. O'Sullivan, P. Beton, H. L. Anderson, and L. M. Herz,
Chemical Science, 6 (2015), p. 181. [journal | article | SI ]

Rings of chlorophyll molecules harvest sunlight remarkably efficiently during photosynthesis in purple bacteria. The key to their efficiency lies in their highly delocalized excited states that allow for ultrafast energy migration. Here we show that a family of synthetic nanorings mimic the ultrafast energy transfer and delocalization observed in nature. π-Conjugated nanorings with diameters of up to 10 nm, consisting of up to 24 porphyrin units, are found to exhibit excitation delocalization within the first 200 fs of light absorption. Transitions from the first singlet excited state of the circular nanorings are dipole-forbidden as a result of symmetry constraints, but these selection rules can be lifted through static and dynamic distortions of the rings. The increase in the radiative emission rate in the larger nanorings correlates with an increase in static disorder expected from Monte Carlo simulations. For highly symmetric rings, the radiative rate is found to be enhanced with increasing temperature. Although this type of thermally activated superradiance has been theoretically predicted in circular chromophore arrays, it has not previously been observed in any natural or synthetic systems. As expected, the activation energy for emission increases when a nanoring is fixed in a circular conformation by coordination to a radial template. These nanorings offer extended chromophores with high excitation delocalization that is remarkably stable against thermally induced disorder. Such findings open new opportunities for exploring coherence effects in nanometer molecular rings and for implementing these biomimetic light-harvesters in man-made devices.

Charge selective contacts, mobile ions and anomalous hysteresis in organic-inorganic perovskite solar cells,
Y. Zhang, M. Liu, G. E. Eperon, T. Leijtens, D. P. McMeekin, M. Saliba, W. Zhang, M. D. Bastiani, L. M. Herz, M. B. Johnston, H. Lin, and H. Snaith,
Materials Horizons, 2 (2015), p. 315. [journal | article | SI ]

High-efficiency perovskite solar cells typically employ an organic-inorganic metal halide perovskite material as light absorber and charge transporter, sandwiched between a p-type electron-blocking organic hole-transporting layer and an n-type hole-blocking electron collection titania compact layer. Some device configurations also include a thin mesoporous layer of TiO₂ or Al₂O₃ which is infiltrated and capped with the perovskite absorber. Herein, we demonstrate that it is possible to fabricate planar and mesoporous perovskite solar cells devoid of an electron selective hole-blocking titania compact layer, which momentarily exhibit power conversion efficiencies (PCEs) of over 13%. This performance is however not sustained and is related to the previously observed anomalous hysteresis in perovskite solar cells. The “compact layer-free” meso-superstructured perovskite devices yield a stabilised PCE of only 2.7% while the compact layer-free planar heterojunction devices display no measureable steady state power output when devoid of an electron selective contact. In contrast, devices including the titania compact layer exhibit stabilized efficiency close to that derived from the current voltage measurements. We propose that under forward bias the perovskite diode becomes polarised, providing a beneficial field, allowing accumulation of positive and negative space charge near the contacts, which enables more efficient charge extraction. This provides the required built-in potential and selective charge extraction at each contact to temporarily enable efficient operation of the perovskite solar cells even in the absence of s charge selective n- and p-type contact layers. The polarisation of the material is consistent with long range migration and accumulation of ionic species within the perovskite to the regions near the contacts. When the external field is reduced under working conditions, the ions can slowly diffuse away from the contacts redistributing throughout the film, reducing the field asymmetry and the effectiveness of the operation of the solar cells. We note that in light of recent publications showing high efficiency in devices devoid of charge selective contacts, this work reaffirms the absolute necessity to measure and report the stabilized power output under load when characterizing perovskite solar cells.

Structure-directed exciton dynamics in templated molecular nanorings,
J. Q. Gong, P. Parkinson, D. V. Kondratuk, G. Gil-Ramírez, H. L. Anderson, and L. M. Herz,
J. Phys. Chem. C, 119 (2015), p. 6414. [journal | article | SI ]

Conjugated polymers with cyclic structures are interesting because their symmetry leads to unique electronic properties. Recent advances in Vernier templating now allow large shape-persistent fully conjugated porphyrin nanorings to be synthesized, exhibiting unique electronic properties. We examine the impact of different conformations on exciton delocalization and emission depolarization in a range of different porphyrin nanoring topologies with comparable spatial extent. Low photoluminescence anisotropy values are found to occur within the first few hundred femtoseconds after pulsed excitation, suggesting ultrafast delocalization of excitons across the nanoring structures. Molecular dynamics simulations show that further polarization memory loss is caused by out-of-plane distortions associated with twisting and bending of the templated nanoring topologies.

Modulation doping of GaAs/AlGaAs core–shell nanowires with effective defect passivation and high electron mobility,
J. L. Boland, S. Conesa-Boj, P. Parkinson, G. Tütüncüoglu, F. Matteini, D. Rüffer, A. Casadei, F. Amaduzzi, F. Jabeen, C. L. Davies, H. J. Joyce, L. M. Herz, A. F. i Morral, and M. B. Johnston,
Nano Lett., 15 (2015), p. 1336. [journal | article | SI ]

Reliable doping is required to realize many devices based on semiconductor nanowires. Group III–V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al_0.33Ga_0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 × 10¹⁶ cm^–3 was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 +/- 300 cm² V^–1 s^–1 and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 +/- 0.3 and 2.4 +/- 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.

Fast charge-carrier trapping in TiO₂ nanotubes,
C. Wehrenfennig, C. M. Palumbiny, H. J. Snaith, M. B. Johnston, L. Schmidt-Mende, and L. M. Herz,
J. Phys. Chem. C, 119 (2015), p. 9159. [journal | article | SI ]

One-dimensional semiconductors such as nanowires and nanotubes are attractive materials for incorporation in photovoltaic devices as they potentially offer short percolation pathways to charge-collecting contacts. We report the observation of free-electron lifetimes in TiO₂ nanotubes of the order of tens of picoseconds. These lifetimes are surprisingly short compared to those determined in films of TiO₂ nanoparticles. Samples of ordered nanotube arrays with several different tube wall thicknesses were fabricated by anodization and have been investigated by means of optical-pump-terahertz-probe (OPTP) spectroscopy, which allows measurement of transient photoinduced conductivity with picosecond resolution. Our results indicate a two-stage decay of the photoexcited electron population. We attribute the faster component to temporary immobilization of charge in shallow trap states, from which electrons can detrap again by thermal excitation. The slower component most likely reflects irreversible trapping in states deeper below the conduction band edge. Free-electron lifetimes associated with shallow trapping appear to be independent of the tube wall thickness and have very similar values for electrons directly photoexcited in the material and for those injected from an attached photoexcited dye. These results suggest that trap states are not predominantly located at the surface of the tubes. The effective THz charge-carrier mobility in the TiO₂ nanotubes is determined (0.1–0.4 cm²/(Vs)) and found to be within the same range as carrier mobilities reported for TiO₂ nanoparticles. Implications for the relative performance of these nanostructures in dye-sensitized solar cells are discussed.

Enhanced amplified spontaneous emission in perovskites using a flexible cholesteric liquid crystal reflector,
S. D. Stranks, S. M. Wood, K. Wojciechowski, F. Deschler, M. Saliba, H. Khandelwal, J. B. Patel, S. Elston, L. M. Herz, M. B. Johnston, A. P. Schenning, M. G. Debije, M. Riede, S. M. Morris, and H. J. Snaith,
Nano Lett., 15 (2015), p. 4935. [journal | article | SI ]

Organic-inorganic perovskites are highly promising solar cell materials, with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically-pumped amplified spontaneous emission (ASE) at 780 nm from a 50-nm-thick film of CH₃NH₃PbI₃ perovskite that is sandwiched within a cavity comprised of a thin-film (7 μm) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, resulting in a factor of 2 reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out-of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelength-tuneable ‘mirror-less’ single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.

Low ensemble disorder in quantum well tube nanowires,
C. L. Davies, P. Parkinson, N. Jiang, J. L. Boland, S. Conesa-Boj, H. H. Tan, C. Jagadish, L. M. Herz, and M. B. Johnston,
Nanoscale, 7 (2015), p. 20531–20538. [journal | article | SI ]

We have observed very low disorder in high quality quantum well tubes (QWT) in GaAs-Al_0.44Ga_0.56As core-multishell nanowires. Room-temperature photoluminescence spectra were measured from 150 single nanowires enabling a full statistical analysis of both intra- and inter-nanowire disorder. By modelling individual nanowire spectra, we assigned a quantum well tube thickness, a core disorder parameter and a QWT disorder parameter to each nanowire. A strong correlation was observed between disorder in the GaAs cores and disorder in the GaAs QWTs, which indicates that variations in core morphology effectively propagate to the shell layers. This highlights the importance of high quality core growth prior to shell deposition. Furthermore, variations in QWT thicknesses for different facet directions was found to be a likely cause of intra-wire disorder, highlighting the need for accurate shell growth.

Rapid energy transfer enabling control of emission polarization in perylene-bisimide donor-acceptor triads,
C. Menelaou, J. Schiphorst, A. M. Kendhale, P. Parkinson, M. G. Debije, A. Schenning, and L. M. Herz,
J. Phys. Chem. Lett., 6 (2015), p. 1170. [journal | article | SI ]

Materials showing rapid intramolecular energy transfer and polarization switching are of interest for both their fundamental photophysics and potential for use in real-world applications. Here, we report two donor–acceptor–donor triad dyes based on perylene-bisimide subunits, with the long axis of the donors arranged either parallel or perpendicular to that of the central acceptor. We observe rapid energy transfer (<2 ps) and effective polarization control in both dye molecules in solution. A distributed-dipole Förster model predicts the excitation energy transfer rate for the linearly arranged triad but severely underestimates it for the orthogonal case. We show that the rapid energy transfer arises from a combination of through-bond coupling and through-space transfer between donor and acceptor units. As they allow energy cascading to an excited state with controllable polarization, these triad dyes show high potential for use in luminescent solar concentrator devices.

Identification of a triplet pair intermediate in singlet exciton fission in solution,
H. L. Stern, A. J. Musser, S. Gelinas, P. Parkinson, L. M. Herz, M. J. Bruzek, J. Anthony, R. H. Friend, and B. J. Walker,
Proc. Natl. Acad. Sci. U.S.A., 112 (2015), p. 7656. [journal | article | SI ]

Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley–Queisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spin-singlet configuration, but such a state has never been optically observed. In solution, we show that the dynamics of fission are diffusion limited and enable the isolation of an intermediate species. In concentrated solutions of bis(triisopropylsilylethynyl)[TIPS|—tetracene we find rapid (<100 ps) formation of excimers and a slower (10 ns) break up of the excimer to two triplet exciton-bearing free molecules. These excimers are spectroscopically distinct from singlet and triplet excitons, yet possess both singlet and triplet characteristics, enabling identification as a triplet pair state. We find that this triplet pair state is significantly stabilized relative to free triplet excitons, and that it plays a critical role in the efficient endothermic singlet fission process.

A molecular nanotube with three-dimensional π-conjugation,
P. Neuhaus, A. Cnossen, J. Q. Gong, L. M. Herz, and H. L. Anderson,
Angew. Chem. Int. Ed., 54 (2015), p. 7344. [journal | article | SI ]

A π-conjugated twelve-porphyrin tube is synthesized in 32% yield by a template-directed coupling reaction that joins together six porphyrin dimers, forming twelve new C[BOND|C bonds. The nanotube has two bound templates, enclosing an internal volume of approximately 4.5 nm³. Its UV/Vis/NIR absorption and fluorescence spectra resemble those of a previously reported six-porphyrin ring, but are red-shifted by approximately 300 cm⁻¹, reflecting increased conjugation. Ultrafast fluorescence spectroscopy demonstrates extensive excited-state delocalization. Transfer of electronic excitation from an initially formed state polarized in the direction of the nanotube axis (z axis) to an excited state polarized in the xy plane occurs within 200 fs, resulting in a negative fluorescence anisotropy on excitation at 742 nm.

Vibrational properties of the organic–inorganic halide perovskite CH₃NH₃PbI₃ from theory and experiment: Factor group analysis, first-principles calculations, and low-temperature infrared spectra,
M. A. Perez-Osorio, R. L. Milot, M. R. Filip, J. B. Patel, L. M. Herz, M. B. Johnston, and F. Giustino,
J. Phys. Chem. C, 119 (2015), p. 25703. [journal | article | SI ]

In this work, we investigate the vibrational properties of the hybrid organic/inorganic halide perovskite MAPbI₃ (MA = CH₃NH₃) in the range 6–3500 cm^–1 by combining first-principles density-functional perturbation theory calculations and low-temperature infrared (IR) absorption measurements on evaporated perovskite films. By using a group factor analysis, we establish the symmetry of the normal modes of vibration and predict their IR and Raman activity. We validate our analysis via explicit calculation of the IR intensities. Our calculated spectrum is in good agreement with our measurements. By comparing theory and experiment, we are able to assign most of the features in the IR spectrum. Our analysis shows that the IR spectrum of MAPbI₃ can be partitioned into three distinct regions: the internal vibrations of the MA cations (800–3100 cm^–1), the cation librations (140–180 cm^–1), and the internal vibrations of the PbI₃ network (<100 cm^–1). The low-frequency region of the IR spectrum is dominated by Pb–I stretching modes of the PbI₃ network with B_u symmetry and librational modes of the MA cations. In addition, we find that the largest contributions to the static dielectric constant arise from Pb–I stretching and Pb–I–Pb rocking modes, and that one low-frequency B_2u Pb–I stretching mode exhibits a large LO–TO splitting of 50 cm^–1.

Self-assembly of russian doll concentric porphyrin nanorings,
S. A. L. Rousseaux, J. Q. Gong, R. Haver, B. Odell, T. D. W. Claridge, L. M. Herz, and H. L. Anderson,
J. Am. Chem. Soc., 137 (2015), p. 12713. [journal | article | SI ]

Electronic communication between concentric macrocycles with wavefunctions which extend around their circumferences can lead to remarkable behavior, as illustrated by multi-walled carbon nanotubes and photosynthetic chlorophyll arrays. However it is difficult to hold one π-conjugated molecular ring inside another. Here we show that ring-in-ring complexes, consisting of a 6-porphyrin ring locked inside a 12-porphyrin ring, can be assembled by placing different metals in the two rings (zinc and aluminum). A bridging ligand with carboxylate and imidazole binding sites forms spokes between the two rings, resulting in a highly cooperative supramolecular self-assembly process. Excitation is transferred from the inner 6-ring to the outer 12-ring of this Russian doll complex within 40ps. These complexes lead to a form of template-directed synthesis in which one nanoring promotes formation of a larger concentric homologous ring; here the effective template is an eight-component non-covalent assembly. Russian doll templating provides a new approach to amplifying the size of a covalent nanostructure.

Temperature-dependent charge-carrier dynamics in CH₃NH₃PbI₃ perovskite thin films,
R. L. Milot, G. E. Eperon, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Adv. Func. Mater., 25 (2015), p. 6218. [journal | article | SI ]

The photoluminescence, transmittance, charge-carrier recombination dynamics, mobility, and diffusion length of CH₃NH₃PbI₃ are investigated in the temperature range from 8K to 370K. Profound changes in the optoelectronic properties of this prototypical photovoltaic material are observed across the two structural phase transitions occurring at 160K and 310K. Drude-like THz photoconductivity spectra at all temperatures above 80K suggest that charge localization effects are absent in this range. The monomolecular charge-carrier recombination rate generally increases with rising temperature, indicating a mechanism dominated by ionized-impurity mediated recombination. Deduced activation energies E_a associated with ionization are found to increase markedly from the room-temperature tetragonal (E_a~20meV) to the higher-temperature cubic (E_a~200meV) phase adopted above 310K. Conversely, the bimolecular rate constant decreases with rising temperature as charge-carrier mobility declines, while the Auger rate constant is highly phase specific, suggesting a strong dependence on electronic band structure. The charge-carrier diffusion length gradually decreases with rising temperature from about 3μm at -93^oC to 1.2μm at 67^oC but remains well above the optical absorption depth in the visible. These results demonstrate that there are no fundamental obstacles to the operation of cells based on CH₃NH₃PbI₃ under typical field conditions.

Six-coordinate zinc porphyrins for template-directed synthesis of spiro-fused nanorings,
L. Favereau, A. Cnossen, J. B. Kelber, J. Q. Gong, R. M. Oetterli, J. Cremers, L. M. Herz, and H. L. Anderson,
J. Am. Chem. Soc., 137 (2015), p. 14256. [journal | article | SI ]

Five-coordinate geometry is the standard binding mode of zinc porphyrins with pyridine ligands. Here we show that pseudo-octahedral six-coordinate zinc porphyrin complexes can also be formed in solution, by taking advantage of the chelate effect. UV–vis–NIR titrations indicate that the strength of this second coordination is ca. 6–8 kJ mol^–1. We have used the formation of six-coordinate zinc porphyrin complexes to achieve the template-directed synthesis of a 3D π-conjugated spiro-fused array of 11 porphyrin units, covalently connected in a nontrivial topology. Time-resolved fluorescence anisotropy experiments show that electronic excitation delocalizes between the two perpendicular nanorings of this spiro-system within the experimental time-resolution of 270 fs.

Charge-carrier dynamics and mobilities in formamidinium lead mixed-halide perovskites,
W. Rehman, R. L. Milot, G. E. Eperon, C. Wehrenfennig, J. L. Boland, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Adv. Mater., 27 (2015), p. 7938–7944. [journal | article | SI ]

The mixed-halide perovskite FAPb(Br_yI_1-y)₃ is attractive for colour-tunable and tandem solar cells. Bimolecular and Auger charge-carrier recombination rate constants strongly correlate with the Br content y suggesting a link with electronic structure. FAPbBr₃ and FAPbI₃ exhibit charge-carrier mobilities of 14cm²V^-1s^-1 and 27cm²V^-1s^-1 and diffusion lengths exceeding 1micron while mobilities across the mixed Br/I system depend on crystalline phase disorder.

Dependence of dye regeneration and charge collection on the pore-filling fraction in solid-state dye-sensitized solar cells,
C. T. Weisspfennig, D. J. Hollman, C. Menelaou, S. D. Stranks, H. J. Joyce, M. B. Johnston, H. J. Snaith, and L. M. Herz,
Adv. Func. Mater., 24 (2014), p. 668. [journal | article | SI ]

Solid-state dye-sensitized solar cells rely on effective infiltration of a solid-state hole-transporting material into the pores of a nanoporous TiO₂ network to allow for dye regeneration and hole extraction. Using microsecond transient absorption spectroscopy and femtosecond photoluminescence upconversion spectroscopy, the hole-transfer yield from the dye to the hole-transporting material 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) is shown to rise rapidly with higher pore-filling fractions as the dye-coated pore surface is increasingly covered with hole-transporting material. Once a pore-filling fraction of ≈30% is reached, further increases do not significantly change the hole-transfer yield. Using simple models of infiltration of spiro-OMeTAD into the TiO₂ porous network, it is shown that this pore-filling fraction is less than the amount required to cover the dye surface with at least a single layer of hole-transporting material, suggesting that charge diffusion through the dye monolayer network precedes transfer to the hole-transporting material. Comparison of these results with device parameters shows that improvements of the power-conversion efficiency beyond 30% pore filling are not caused by a higher hole-transfer yield, but by a higher charge-collection efficiency, which is found to occur in steps. The observed sharp onsets in photocurrent and power-conversion efficiencies with increasing pore-filling fraction correlate well with percolation theory, pre- dicting the points of cohesive pathway formation in successive spiro-OMeTAD layers adhered to the pore walls. From percolation theory it is predicted that, for standard mesoporous TiO₂ with 20nm pore size, the photocurrent should show no further improvement beyond an 83% pore-filling fraction.

The impact of molecular charge-transfer states on photocurrent generation in solid state DSSCs employing low band-gap dyes,
S. S. K. Raavi, P. Docampo, C. Wehrenfennig, M. J. P. Alcocer, G. Sadoughi, L. M. Herz, H. J. Snaith, and A. Petrozza,
J. Phys. Chem. C, 118 (2014), p. 16825. [journal | article | SI ]

“Push-pull” structures have been considered a winning strategy for the design of fully organic molecules as sensitizers in dye-sensitized solar cells (DSC). In this work we show that the presence of a molecular excited state with a strong charge transfer character may be critical for charge generation when the total energy of the photoexcitation is too low to intercept accepting states in the TiO₂ photoanode. Though hole transfer to the Spiro-OMeTAD can be very fast, an electron-hole pair is likely to form at the organic interface, resulting in a possible trap-like excitation. This leads to poor photocurrent generation in the solid state DSC device. We demonstrate that it is possible to overcome this issue by fabricating SnO₂ based solid state DSC. The resulting solar cell shows, for the first time, that a SnO₂-based ss-DSC can outperform a TiO₂-based one when a perylene-based, low band gap, push-pull dye is used as sensitizer.

An ultrafast carbon nanotube terahertz polarisation modulator,
C. J. Docherty, S. D. Stranks, S. N. Habisreutinger, H. J. Joyce, L. M. Herz, R. J. Nicholas, and M. B. Johnston,
J. Appl. Phys., 115 (2014), p. 203108. [journal | article | SI ]

We demonstrate ultrafast modulation of terahertz radiation by unaligned optically pumped single-walled carbon nanotubes. Photoexcitation by an ultrafast optical pump pulse induces transient terahertz absorption in nanowires aligned parallel to the optical pump. By controlling the polarisation of the optical pump, we show that terahertz polarisation and modulation can be tuned, allowing sub-picosecond modulation of terahertz radiation. Such speeds suggest potential for semiconductor nanowire devices in terahertz communication technologies.

Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells,
G. E. Eperon, S. D. Stranks, C. Menelaou, M. B. Johnston, L. M. Herz, and H. J. Snaith,
Energy Environ. Sci., 7 (2014), p. 982. [journal | article | SI ]

Perovksite-based solar cells have attracted significant recent interest, with power conversion efficiencies in excess of 15% already superceding a number of established thin-film solar cell technologies. Most work has focused on a methylammonium lead trihalide perovksites, with a bandgaps of ~1.55eV and greater. Here, we explore the effect of replacing the methylammonium cation in this perovskite, and show that with the slightly larger formamidinium cation, we can synthesise formamidinium lead trihalide perovskites with a bandgap tunable between 1.48 and 2.23eV. We take the 1.48 eV-bandgap perovskite as most suited for single junction solar cells, and demonstrate long-range electron and hole diffusion lengths in this material, making it suitable for planar heterojunction solar cells. We fabricate such devices, and due to the reduced bandgap we achieve high short-circuit currents of >23 mA cm^-2, resulting in power conversion efficiencies of up to 14.2%, the highest efficiency yet for solution processed planar heterojunction perovskite solar cells. Formamidinium lead triiodide is hence promising as a new candidate for this class of solar cell.

High charge carrier mobilities and lifetimes in organo lead trihalide perovskites,
C. Wehrenfennig, G. E. Eperon, M. B. Johnston, H. J. Snaith, and L. M. Herz,
Adv. Mater., 26 (2014), p. 1584. [journal | article | SI ]

Organolead trihalide perovskites are shown to exhibit the best of both worlds – charge carrier mobilities around 10cm²V⁻¹s⁻¹ and low bi-molecular charge recombination constants. The ratio of the two is found to defy the Langevin limit of kinetic charge capture by over four orders of magnitude. This fundamental mechanism causes long (micron) charge-pair diffusion lengths crucial for flat-heterojunction solar cells.

Charge carrier recombination channels in the low-temperature phase of organic-inorganic lead halide perovskite thin films,
C. Wehrenfennig, M. Liu, H. J. Snaith, M. B. Johnston, and L. M. Herz,
APL Materials, 2 (2014), p. 081513. [journal | article | SI ]

The optoelectronic properties of the mixed hybrid lead halide perovskite CH₃NH₃PbI_3−xClx have been subject to numerous recent studies related to its extraordinary capabilities as an absorber material in thin film solar cells. While the greatest part of the current research concentrates on the behavior of the perovskite at room temperature, the observed influence of phonon-coupling and excitonic effects on charge carrier dynamics suggests that low-temperature phenomena can give valuable additional insights into the underlying physics. Here, we present a temperature-dependent study of optical absorption and photoluminescence (PL) emission of vapor-deposited CH₃NH₃PbI_3−xClx exploring the nature of recombination channels in the high- and the low-temperature phase of the material. On cooling, we identify an up-shift of the absorption onset by about 0.1 eV at about 100 K, which is likely to correspond to the known tetragonal-to-orthorhombic transition of the pure halide CH₃NH₃PbI₃. With further decreasing temperature a second PL emission peak emerges in addition to the peak from the high-temperature phase. The transition on heating is found to occur at about 140 K, i.e. revealing significant hysteresis in the system. While PL decay lifetimes are found to be independent of temperature above the transition, significantly accelerated recombination is observed in the low-temperature phase. Our data suggests that small inclusions of domains adopting the high-temperature phase are responsible for this behavior rather than a spontaneous increase in the intrinsic rate constants. These observations show that even sparse lower-energy sites can have a strong impact on material performance, acting as charge recombination centres that may detrimentally affect photovoltaic performance but that may also prove useful for optoelectronic applications such as lasing by enhancing population inversion.

Homogeneous emission line broadening in the organo lead halide perovskite CH₃NH₃PbI_3−xCl_x,
C. Wehrenfennig, M. Liu, H. J. Snaith, M. B. Johnston, and L. M. Herz,
J. Phys. Chem. Lett., 5 (2014), p. 1300. [journal | article | SI ]

The organic−inorganic hybrid perovskites methylammonium lead iodide (CH₃NH₃PbI₃) and the partially chlorine-substituted mixed halide CH₃NH₃PbI_3−xCl_x emit strong and broad photoluminescence (PL) around their band gap energy of ∼1.6 eV. However, the nature of the radiative decay channels behind the observed emission and, in particular, the spectral broadening mechanisms are still unclear. Here we investigate these processes for high-quality vapor-deposited films of CH₃NH₃PbI_3−xCl_x using time- and excitation-energy dependent photoluminescence spectroscopy. We show that the PL spectrum is homogenously broadened with a line width of 103 meV most likely as a consequence of phonon coupling effects. Further analysis reveals that defects or trap states play a minor role in radiative decay channels. In terms of possible lasing applications, the emission spectrum of the perovskite is sufficiently broad to have potential for amplification of light pulses below 100 fs pulse duration.

Effect of nanocrystalline domains in photovoltaic devices with benzodithiophene based donor-acceptor co-polymers,
C. Menelaou, S. Tierney, N. Blouin, W. Mitchell, P. Tiwana, I. McKerracher, C. Jagadish, M. Carrasco, and L. M. Herz,
J. Phys. Chem. C, 118 (2014), p. 17351. [journal | article | SI ]

We have investigated the effects of thin-film morphology on the photovolatic performance for a series of donor-acceptor copolymers based on benzodithiophene donor and benzothiadiazole acceptor units. Photovoltaic devices incorporating polymer:fullerene blends show highest efficiencies (up to 6%) for those polymers exhibiting the least degree of crystallinity in X-ray diffraction patterns and a corresponding lowest surface roughness in thin films. We find that the existence of such crystalline domains in thin polymer films correlates well with spectral signatures of polymer chain aggregates already present in solution prior to casting of the film. Polymer solubility and casting conditions therefore appear to be crucial factors for enhancing efficiencies of photovoltaic devices based on such donor-acceptor copolymers. To examine why the presence of crystallite domains lowers device efficiencies, we measured exciton diffusion lengths by modelling the time-dependent photoluminescence from thin polymer films deposited on an exciton quencher layer of TiO₂. We find that exciton diffusion lengths in these materials are substantial (4-7.5 nm) and show some variation with polymer crystallinity. However, ultrafast (1 ps) quenching of the polymer emission from polymer:PCBM blends indicates that the vast majority of excitons rapidly reach the charge-dissociating interface and hence exciton diffusion does not represent a limiting factor. We therefore conclude that the subsequent charge extraction and lifetimes must be adversely affected by the presence of crystalline domains. We suggest that the formed crystallites are too small to offer significant enhancements in long-range charge carrier mobility, but instead introduce domain boundaries which impede charge extraction. For this class of materials, polymer designs are therefore required that target high solubility and chain entropy, leading to amorphous film formation.

Dichroic perylene bisimide triad displaying energy transfer in switchable luminescent solar concentrators,
J. ter Schiphorst, A. M. Kendhale, M. G. Debije, C. Menelaou, L. M. Herz, and A. P. H. J. Schenning,
Chem. Mater., 26 (2014), p. 3876. [journal | article | SI ]

Combining positive and negative dichroic fluorophores for advanced light management in luminescent solar concentrators,
M. G. Debije, C. Menelaou, L. M. Herz, and A. P. H. J. Schenning,
Adv. Optical Mater., 2 (2014), p. 687. [journal | article | SI ]

This work describes the unexpected alignment of a common perylene bisimide fluorescent dye (Lumogen Red305) in a host liquid crystal matrix. The negative dichroic fluorophore orients with the primary absorption/emission dipole, corresponding to the physical long axis of the perylene bisimide core, perpendicular to the director of a host liquid crystal. A second absorption dipole, which lies perpendicular to the primary dipole and corresponding to the physical short axis of the perylene core, lies parallel to the host liquid crystal. Individual illumination of the two absorbing optical transition dipole moments of the Red305 dye results in a single linearly polarized emission. When Red305 is combined with a second positive dichroic Coumarin-type fluorophore that aligns in the conventional manner in a liquid crystal host, that is, with an absorption dipole along the physical long axis of the fluorophore and the director of the host liquid crystal, advanced light management is possible, such as electrically switchable colors and directing emission light to different edges in a luminescent solar concentrator device.

Lead-free organic-inorganic tin halide perovskites for photovoltaic applications,
N. K. Noel, S. D. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A. A. Haghighirad, A. Sadhanal, G. E. Eperon, S. K. Pathak, M. B. Johnston, A. Petrozza, L. M. Herz, and H. J. Snaith,
Energy Environ. Sci., 7 (2014), p. 3061. [journal | article | SI ]

Already exhibiting solar to electrical power conversion efficiencies of over 16%, organic-inorganic lead halide perovskite solar cells are one of the most promising emerging contenders in the drive to provide a cheap and clean source of energy. One concern however, is the potential toxicology issue of lead, a key component in the archetypical material. The most likely substitute is tin, which like lead, is also a group 14 metal. While organic-inorganic tin halide perovskites have shown good semiconducting behaviour, the instability of tin in its 2+ oxidation state has thus far proved to be an overwhelming challenge. Here we report the first completely lead-free, CH₃NH₃SnI₃ perovskite solar cell processed on a mesoporous TiO₂ scaffold, reaching efficiencies of over 6% under 1 sun illumination. Remarkably, we achieve open circuit voltages over 0.88 V from a material which has a 1.23 eV band gap.

Charge-carrier dynamics in vapour-deposited films of the organolead halide perovskite CH₃NH₃PbI_3−xCl_x,
C. Wehrenfennig, M. Liu, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Energy Environ. Sci., 7 (2014), p. 2269. [journal | article | SI ]

We determine high charge carrier mobilities > 33 cm² V⁻¹ s⁻¹ and bi-molecular recombination rates about five orders of magnitude below the prediction of Langevin’s model for vapour-deposited CH₃NH₃PbI_3−xCl_x using ultrafast THz spectroscopy. At charge carrier densities below 10¹⁷ cm^-3 intrinsic diffusion lengths are shown to approach 3 microns, limited by slow mono-molecular decay processes.

Ultrafast transient terahertz conductivity of monolayer MoS₂ and WSe₂ grown by chemical vapor deposition,
C. J. Docherty, P. Parkinson, H. J. Joyce, M.-H. Chiu, C.-H. Chen, M.-Y. Lee, L.-J. Li, L. M. Herz, and M. B. Johnston,
ACS Nano, 8 (2014), p. 11147–11153. [journal | article | SI ]

We have measured ultrafast charge carrier dynamics in monolayers and trilayers of the transition metal dichalcogenides MoS₂ and WSe₂ using a combination of time-resolved photoluminescence and terahertz spectroscopy. We recorded a photoconductivity and photoluminescence response time of just 350 fs from CVD-grown monolayer MoS₂, and 1 ps from trilayer MoS₂ and monolayer WSe₂. Our results indicate the potential of these materials as high-speed optoelectronic materials.

Ultrafast energy transfer in biomimetic multistrand nanorings,
P. Parkinson, C. E. I. Knappke, N. Kamonsutthipaijit, K. Sirithip, J. D. Matichak, H. L. Anderson, and L. M. Herz,
J. Am. Chem. Soc., 136 (2014), p. 8217. [journal | article | SI ]

We report the synthesis of LH2-like supramolecular double- and triple-strand complexes based upon porphyrin nanorings. Energy transfer from the antenna dimers to the π-conjugated nanoring occurs on a sub-picosecond timescale, rivaling transfer rates in natural light harvesting systems. The presence of a second nanoring acceptor doubles the transfer rate, providing strong evidence for multi-directional energy funneling. The behavior of these systems is particularly intriguing because the local nature of the interaction may allow energy transfer into states that are, for cyclic nanorings, symmetry forbidden in the far-field. These complexes are versatile synthetic models for natural light harvesting systems.

Electron mobilities approaching bulk limits in "surface-free" GaAs nanowires,
H. J. Joyce, P. Parkinson, N. Jiang, C. J. Docherty, Q. Gao, H. H. Tan, C. Jagadish, L. M. Herz, and M. B. Johnston,
Nano Lett., 14 (2014), p. 5989. [journal | article | SI ]

Achieving bulk-like charge carrier mobilities in semiconductor nanowires is a major challenge facing the development of nanowire-based electronic devices. Here we demonstrate that engineering the GaAs nanowire surface by overcoating with optimized AlGaAs shells is an effective means of obtaining exceptionally high carrier mobilities and lifetimes. We performed measurements of GaAs/AlGaAs core-shell nanowires using optical pump-terahertz probe spectroscopy: a noncontact and accurate probe of carrier transport on ultrafast timescales. The carrier lifetimes and mobilities both improved significantly with increasing AlGaAs shell thickness. Remarkably, optimized GaAs/AlGaAs core-shell nanowires exhibited electron mobilities up to 3000 cm²V^-1s^-1, reaching over 65% of the electron mobility typical of high quality undoped bulk GaAs at equivalent photoexcited carrier densities. This points to the high interface quality and the very low levels of ionized impurities and lattice defects in these nanowires. The improvements in mobility were concomitant with drastic improvements in photoconductivity lifetime, reaching 1.6 ns. Comparison of photoconductivity and photoluminescence dynamics indicates that that midgap GaAs surface states, and consequently surface band-bending and depletion, are effectively eliminated in these high quality heterostructures.

Chromophores in molecular nanorings: When is a ring a ring?,
P. Parkinson, D. Kondratuk, C. Menelaou, J. Gong, H. L. Anderson, and L. M. Herz,
J. Phys. Chem. Lett., 5 (2014), p. 4356. [journal | article | SI ]

The topology of a conjugated molecule plays a significant role in controlling both the electronic properties and the conformational manifold that the molecule may explore. Fully π-conjugated molecular nanorings are of particular interest, as their lowest electronic transition may be strongly suppressed as a result of symmetry constraints. In contrast, the simple Kasha model predicts an enhancement in the radiative rate for corresponding linear oligomers. Here we investigate such effects in linear and cyclic conjugated molecules containing between 6 and 42 butadiyne-linked porphyrin units (corresponding to 600 C–C bonds) as pure monodisperse oligomers. We demonstrate that as the diameter of the nanorings increases beyond ∼10 nm, its electronic properties tend toward those of a similarly sized linear molecule as a result of excitation localization on a subsegment of the ring. However, significant differences persist in the nature of the emitting dipole polarization even beyond this limit, arising from variations in molecular curvature and conformation.

Side chains control dynamics and self-sorting in fluorescent organic nanoparticles,
A. Kaeser, I. Fischer, R. Abbel, P. Besenius, D. Dasgupta, M. A. J. Gillisen, G. Portale, A. L. Stevens, L. M. Herz, and A. P. H. J. Schenning,
ACS Nano, 7 (2013), p. 408. [journal | article | SI ]

To develop fluorescent organic nanoparticles with tailored properties for imaging and sensing, full control over the size, fluorescence, stability, dynamics, and supramolecular organization of these particles is crucial. We have designed, synthesized, and fully characterized 12 nonionic fluorene cooligomers that formed self-assembled fluorescent nanoparticles in water. In these series of molecules, the ratio of hydrophilic ethylene glycol and hydrophobic alkyl side chains was systematically altered to investigate its role on the above-mentioned properties. The nanoparticles consisting of π-conjugated oligomers containing polar ethylene glycol side chains were less stable and larger in size, while nanoparticles self-assembled from oligomers containing nonpolar pendant chains were more stable, smaller, and generally had a higher fluorescence quantum yield. Furthermore, the dynamics of themolecules between the nanoparticles was enhanced if the number of hydrophilic side chains increased. Energy transfer studies between naphthalene and benzothiadiazole fluorene co-oligomers with the same side chains showed no exchange of molecules between the particles for the apolar molecules. For the more polar systems, the exchange of molecules between nanoparticles took place at room temperature or after annealing. Self-assembled nanoparticles consisting of π-conjugated oligomers having different side chains caused self-sorting, resulting either in the formation of domains within particles or the formation of separate nanoparticles. Our results show that we can control the stability, fluorescence, dynamics, and self-sorting properties of the nanoparticles by simply changing the nature of the side chains of the π-conjugated oligomers. These findings are not only important for the field of selfassembled nanoparticles but also for the construction of well-defined multicomponent supramolecular materials in general.

Electronic properties of GaAs, InAs and InP nanowires studied by terahertz spectroscopy,
H. J. Joyce, C. J. Docherty, Q. Gao, H. H. Tan, C. Jagadish, J. Lloyd-Hughes, L. M. Herz, and M. B. Johnston,
Nanotechnology, 24 (2013), p. 214006. [journal | article | SI ]

We have performed a comparative study of ultrafast charge carrier dynamics in a range of III–V nanowires using optical pump–terahertz probe spectroscopy. This versatile technique allows measurement of important parameters for device applications, including carrier lifetimes, surface recombination velocities, carrier mobilities and donor doping levels. GaAs, InAs and InP nanowires of varying diameters were measured. For all samples, the electronic response was dominated by a pronounced surface plasmon mode. Of the three nanowire materials, InAs nanowires exhibited the highest electron mobilities of 6000 cm² V⁻¹ s⁻¹, which highlights their potential for high mobility applications, such as field effect transistors. InP nanowires exhibited the longest carrier lifetimes and the lowest surface recombination velocity of 170 cm s⁻¹. This very low surface recombination velocity makes InP nanowires suitable for applications where carrier lifetime is crucial, such as in photovoltaics. In contrast, the carrier lifetimes in GaAs nanowires were extremely short, of the order of picoseconds, due tothe high surface recombination velocity, which was measured as 5.4×10⁵ cm s⁻¹.Theses findings will assist in the choice of nanowires for different applications, and identify the challenges in producing nanowires suitable for future electronic and optoelectronic devices.

Optimizing the energy offset between dye and hole-transporting material in solid-state dye-sensitized solar cells,
C. T. Weisspfennig, M. Lee, J. Teuscher, P. Docampo, S. D. Stranks, H. J. Joyce, H. Bergmann, I. Bruder, D. Kondratiuk, M. B. Johnston, H. J. Snaith, and L. M. Herz,
J. Phys. Chem. C, 117 (2013), p. 19850. [journal | article | SI ]

The power-conversion efficiency of solid-state dye-sensitized solar cells can be optimized by reducing the energy offset between the highest occupied molecular orbital (HOMO) levels of dye and hole-transporting material (HTM) to minimize the loss-in-potential. Here, we report a study of three novel HTMs with HOMO levels slightly above and below the one of the commonly used HTM 2,2',7,7'- tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) to systematically explore this possibility. Using transient absorption spectroscopy and employing the ruthenium based dye Z907 as sensitizer, it is shown that, despite one new HTM showing a 100% hole-transfer yield, all devices based on the new HTMs performed worse than those incorporating spiro-OMeTAD. We further demonstrate that the design of the HTM has an additional impact on the electronic density of states present at the TiO₂ electrode surface and hence influences not only hole- but also electron-transfer from the sensitizer. These results provide insight into the complex influence of the HTM on charge transfer and provide guidance for the molecular design of new materials.

Direct observation of charge-carrier heating at WZ–ZB InP nanowire heterojunctions,
C. K. Yong, J. Wong-Leung, H. J. Joyce, J. Lloyd-Hughes, Q. Gao, H. H. Tan, C. Jagadish, M. B. Johnston, and L. M. Herz,
Nano Lett., 13 (2013), p. 4280. [journal | article | SI ]

We have investigated the dynamics of hot charge carriers in InP nanowire ensembles containing a range of densities of zinc-blende inclusions along the otherwise wurtzite nanowires. From time-dependent photoluminescence spectra, we extract the temperature of the charge carriers as a function of time after nonresonant excitation. We find that charge-carrier temperature initially decreases rapidly with time in accordance with efficient heat transfer to lattice vibrations. However, cooling rates are subsequently slowed and are significantly lower for nanowires containing a higher density of stacking faults. We conclude that the transfer of charges across the type II interface is followed by release of additional energy to the lattice, which raises the phonon bath temperature above equilibrium and impedes the carrier cooling occurring through interaction with such phonons. These results demonstrate that type II heterointerfaces in semiconductor nanowires can sustain a hot charge-carrier distribution over an extended time period. In photovoltaic applications, such heterointerfaces may hence both reduce recombination rates and limit energy losses by allowing hot-carrier harvesting.

Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber,
S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, and H. J. Snaith,
Science, 342 (2013), p. 341. [journal | article | SI ]

Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH₃NH₃PbI_3-xCl_x) and triiodide (CH₃NH₃PbI₃) perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of ~100 nanometers. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a critical parameter to optimize for future perovskite absorber development.

Energy transfer processes along a supramolecular chain of pi-conjugated molecules,
S. A. Schmid, R. Abbel, A. P. H. J. Schenning, E. W. Meijer, and L. M. Herz,
Phil. Trans. R. Soc. A, 370 (2012), p. 3787. [journal | article | SI ]

We have investigated the energy transfer dynamics in a supramolecular linear polymer chain comprising oligofluorene (OF) energy donor units linked by quadruple hydrogenbonding groups, and oligophenylene (OPV) chain ends that act as energy acceptors. Using femtosecond spectroscopy, we followed the dynamics of energy transfer from the main chain of OF units to the OPV chain ends and simulated these data taking a Monte Carlo approach that included different extents of electronic wave function delocalization for the energy donor and acceptor. Best correlations between experimental and theoretical results were obtained for the assumption of electronic coupling occurring between a localized donor dipole moment and a delocalized acceptor moment. These findings emphasize that geometric relaxation following initial excitation of the donor needs to be taken into account, as it leads to a localization of the donor’s excited state wave function prior to energy transfer. In addition, our simulations show that the energy transfer from the main chain to the ends is dominated by an interplay between slow and spatially limited exciton migration along the OF segments comprising the main chain and the comparatively faster hetero-transfer to the end-cap acceptors from directly adjoining OF segments. These results clearly support the description of host–guest energy transfer in linear polymer chains as a two-step mechanism with exciton diffusion in the host being a prerequisite to energy transfer to the guest.

Noncontact measurement of charge carrier lifetime and mobility in GaN nanowires,
P. Parkinson, C. Dodson, H. J. Joyce, K. A. Bertness, N. A. Sanford, L. M. Herz, and M. B. Johnston,
Nano Lett., 12 (2012), p. 4600. [journal | article | SI ]

The first noncontact photoconductivity measurements of gallium nitride nanowires (NWs) are presented, revealing a high crystallographic and optoelectronic quality achieved by use of catalyst-free molecular beam epitaxy. In comparison with bulk material, the NWs exhibit a long conductivity lifetime (>2 ns) and a high mobility (820 +/- 120 cm²/(V s)). This is due to the weak influence of surface traps with respect to other III−V semiconducting NWs and to the favorable crystalline structure of the NWs achieved via strain-relieved growth.

Unraveling the function of an MgO interlayer in both electrolyte and solid-state SnO₂ based dye-sensitized solar cells,
P. Docampo, P. Tiwana, N. Sakai, H. Miura, L. Herz, T. Murakami, and H. J. Snaith,
J. Phys. Chem. C, 116 (2012), p. 22840. [journal | article | SI ]

The coating of n-type mesoporous metal oxides with nanometer thick dielectric shells is a route that has proven to be successful at enhancing the efficiency of some families of dye-sensitized solar cells. The primary intention is to introduce a “surface passivation layer” to inhibit recombination between photoinduced electrons and holes across the dye-sensitized interface. However, the precise function of these dielectric interlayers is often ambiguous. Here, the role of a thin MgO interlayer conformally deposited over mesoporous SnO₂ in liquid electrolyte and solid-state dye-sensitized solar cells is investigated. For both families of devices the open-circuit voltage is increased by over 200 mV; however, the short-circuit photocurrent is increased for the solid-state cells, but reduced for the electrolyte based devices. Through electronic and spectroscopic characterization we deduce that there are four distinct influences of the MgO interlayer: It increases dye-loading, slows down recombination, slows down photoinduced electron transfer, and results in a greater than 200 mV shift in the conduction band edge, with respect to the electrolyte redox potential. The compilation of these four factors have differing effects and magnitudes in the solid-state and electrolyte DSCs but quantitatively account for the difference in device performances observed for both systems with and without the MgO shells. To the best of our knowledge, this is the most comprehensive account of the role of dielectric shells in dye-sensitized solar cells and will enable much better interfacial design of photoelectrodes for DSCs.

Nanoengineering coaxial carbon nanotube -- dual-polymer heterostructures,
S. D. Stranks, C.-K. Yong, J. A. Alexander-Webber, C. Weisspfennig, M. B. Johnston, L. M. Herz, and R. J. Nicholas,
ACS Nano, 6 (2012), p. 6058. [journal | article | SI ]

We describe studies of new nanostructured materials consisting of carbon nanotubes wrapped in sequential coatings of two different semiconducting polymers, namely, poly(3- hexylthiophene) (P3HT) and poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT). Using absorption spectroscopy and steady-state and ultrafast photoluminescence measurements, we demonstrate the role of the different layer structures in controlling energy levels and charge transfer in both solution and film samples. By varying the simple solution processing steps, we can control the ordering and proportions of the wrapping polymers in the solid state. The resulting novel coaxial structures open up a variety of new applications for nanotube blends and are particularly promising for implementation into organic photovoltaic devices. The carbon nanotube template can also be used to optimize both the electronic properties and morphology of polymer composites in a much more controlled fashion than achieved previously, offering a route to producing a new generation of polymer nanostructures.

Ultrafast dynamics of exciton formation in semiconductor nanowires,
C. K. Yong, H. J. Joyce, J. Lloyd-Hughes, Q. Gao, H. H. Tan, C. Jagadish, M. B. Johnston, and L. M. Herz,
Small, 8 (2012), p. 1725. [journal | article | SI ]

The dynamics of free electron–hole pairs and excitons in GaAs–AlGaAs–GaAs core–shell–skin nanowires is investigated using femtosecond transient photoluminescence spectroscopy at 10K. Following nonresonant excitation, a bimolecular interconversion of the initially generated electron–hole plasma into an exciton population is observed. This conducting-to-insulating transition appears to occur gradually over electron–hole charge pair densities of 2--4 × 10¹⁶ cm⁻³. The smoothness of the Mott transition is attributed to the slow carrier-cooling during the bimolecular interconversion of free charge carriers into excitons and to the presence of chemical-potential fluctuations leading to inhomogeneous spectral characteristics. These results demonstrate that high-quality nanowires are model systems for investigating fundamental scientific effects in 1D heterostructures.

Ultralow surface recombination velocity in InP nanowires probed by terahertz spectroscopy,
H. J. Joyce, J. Wong-Leung, C.-K. Yong, C. J. Docherty, S. Paiman, Q. Gao, H. H. Tan, C. Jagadish, J. Lloyd-Hughes, L. M. Herz, and M. B. Johnston,
Nano Lett., 12 (2012), p. 5325. [journal | article | SI ]

Using transient terahertz photoconductivity measurements, we have made noncontact, room temperature measurements of the ultrafast charge carrier dynamics in InP nanowires. InP nanowires exhibited a very long photoconductivity lifetime of over 1 ns, and carrier lifetimes were remarkably insensitive to surface states despite the large nanowire surface area-to-volume ratio. An exceptionally low surface recombination velocity (170 cm/s) was recorded at room temperature. These results suggest that InP nanowires are prime candidates for optoelectronic devices, particularly photovoltaic devices, without the need for surface passivation. We found that the carrier mobility is not limited by nanowire diameter but is strongly limited by the presence of planar crystallographic defects such as stacking faults in these predominantly wurtzite nanowires. These findings show the great potential of very narrow InP nanowires for electronic devices but indicate that improvements in the crystallographic uniformity of InP nanowires will be critical for future nanowire device engineering.

Morphology-dependent energy transfer dynamics in fluorene-based amphiphile nanoparticles,
A. L. Stevens, A. Kaeser, A. P. H. J. Schenning, and L. M. Herz,
ACS Nano, 6 (2012), p. 4777. [journal | article | SI ]

Nanoparticles are interesting systems to study because of their large range of potential uses in biological imaging and sensing. We investigated molecular nanoparticles formed by fast injection of a small volume of molecularly dissolved fluorene-derivative amphiphilic molecules into a polar solvent, which resulted in solid spherical particles of 80 nm diameter with high stability. Energy transfer studies were carried out on two-component nanoparticles that contained mixtures of donor and acceptor amphiphiles of various fractions. We conducted time-resolved photoluminescence measurements on the two-component nanoparticles in order to determine whether the fundamental donor–acceptor interaction parameter (the Förster radius) depends on the acceptor concentration. The Förster radius was found to be large for very low incorporated acceptor fractions (<0.1%), but it declined with increasing concentration. These changes were concomitant with shifts in the acceptor emission and absorption circular dichroism spectra that indicated an increasing clustering of acceptors into domains as their fraction was raised. In addition, for acceptor fractions below 2% the extracted Förster radii were found to be significantly larger than predicted from donor–acceptor spectral overlap calculations, in accordance with efficient excitation diffusion within the donor matrix, aiding the overall transfer to acceptors. We conclude that energy transfer in two-component nanoparticles shows a complex interplay between phase segregation of the constituent donor and acceptor molecules and excitation diffusion within their domains.

The origin of an efficiency improving “light soaking” effect in SnO₂ based solid-state dye-sensitized solar cells,
P. Tiwana, P. Docampo, M. B. Johnston, L. Herz, and H. Snaith,
Energy Environ. Sci., 5 (2012), p. 9566. [journal | article | SI ]

We have observed a strong “light-soaking” effect in SnO₂ based solid-state dye-sensitized solar cells (SDSCs). As measured under AM 1.5G sun light, white LED illumination with no UV component, and red LED illumination, the device short-circuit photocurrent and efficiency increase significantly over 20-30 minutes, until steady-state is achieved. Employing transient photovoltage and photocurrent measurements we demonstrate that while the electron transport rate does increase with light-soaking, there is also an accompanying increase in the recombination rate. The overall result is that the charge collection efficiency gets slightly poorer as the exposure to light increases, clearly not accounting for the increase in photocurrent. To probe the initial electron injection into SnO₂, we have conducted optical-pump terahertz-probe spectroscopy studies on dye sensitized mesoporous SnO₂ films. We observe a monotonic speeding-up of electron transfer from the photoexcited dye into the semiconductor accompanied with a rise in the overall photoconductivity signal at 1 ns, as a function of the measurement time, where the exposure is due to the pulse train at 550 nm wavelength. This can be explained by a positive shift in the conduction band edge, or an increase in the density of states (DoS) below the band edge induced by continued exposure to light. To check this hypothesis, capacitance measurements have been conducted on SnO₂ SDSCs illuminated with red light (no UV) which show an increase in DoS as a result of light soaking. The increased availability of states into which electrons can be transferred justifies the increase in both the charge injection rate, and ensuing photocurrent, and is also corroborated by a slight drop in cell open-circuit voltage. The cause for the shift in surface potential or increase in density of states is not clear, but we postulate that it is due to the photoinduced charging of the SnO₂ inducing a rearrangement of charged species or loss of surface oxygen at the dye-sensitized heterojunction.

Extreme sensitivity of graphene photoconductivity to environmental gases,
C. J. Docherty, C.-T. Lin, H. J. Joyce, R. J. Nicholas, L. M. Herz, L.-J. Li, and M. B. Johnston,
Nature Commun., 3 (2012), p. 1228. [journal | article | SI ]

Graphene is a single layer of covalently bonded carbon atoms, which was discovered only 8 years ago and yet has already attracted intense research and commercial interest. Initial research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The potential for using the unique properties of graphene in terahertzfrequency electronics is particularly exciting; however, initial experiments probing the terahertz-frequency response of graphene are only just emerging. Here we show that the photoconductivity of graphene at terahertz frequencies is dramatically altered by the adsorption of atmospheric gases, such as nitrogen and oxygen. Furthermore, we observe the signature of terahertz stimulated emission from gas-adsorbed graphene. Our findings highlight the importance of environmental conditions on the design and fabrication of high-speed, graphene-based devices.

Strong carrier lifetime enhancement in GaAs nanowires coated with semiconducting polymer,
C. K. Yong, K. Noori, Q. Gao, H. J. Joyce, H. H. Tan, C. Jagadish, F. Giustino, M. B. Johnston, and L. M. Herz,
Nano Lett., 12 (2012), p. 6293. [journal | article | SI ]

The ultrafast charge carrier dynamics in GaAs/conjugated polymer type II heterojunctions are investigated using time-resolved photoluminescence spectroscopy at 10 K. By probing the photoluminescence at the band edge of GaAs, we observe strong carrier lifetime enhancement for nanowires blended with semiconducting polymers. The enhancement is found to depend crucially on the ionization potential of the polymers with respect to the Fermi energy level at the surface of the GaAs nanowires. We attribute these effects to electron doping by the polymer which reduces the unsaturated surfacestate density in GaAs. We find that when the surface of nanowires is terminated by native oxide, the electron injection across the interface is greatly reduced and such surface doping is absent. Our results suggest that surface engineering via π-conjugated polymers can substantially improve the carrier lifetime in nanowire hybrid heterojunctions with applications in photovoltaics and nanoscale photodetectors.

Directing energy transfer in discrete one-dimensional oligonucleotide-templated assemblies,
A. Ruiz-Carretero, P. G. A. Janssen, A. L. Stevens, M. Surin, L. M. Herz, and A. P. H. J. Schenning,
Chem. Commun., 47 (2011), p. 884. [journal | article | SI ]

Monodisperse DNA-templated one dimensional dye assemblies have been constructed in which the energy transfer can be directed. Fluorescence experiments suggest an optimum transfer efficiency for stacks of 30 bases long.

Surface energy relay between cosensitized molecules in solid-state dye-sensitized solar cells,
M. D. Brown, P. Parkinson, T. Torres, H. Miura, L. M. Herz, and H. J. Snaith,
J. Phys. Chem. C, 115 (2011), p. 23204. [journal | article | SI ]

We employ cosensitization of a visible absorbing organic sensitizer and a near IR absorbing Zn-phthalocyanine complex to significantly enhance the optical bandwidth in spiro-OMeTAD based solid-state DSCs. The cosensitized cells exhibit greatly enhanced performance, with full sun AM1.5 power conversion efficiencies of 4.7%, as compared to 3.9% for the best monosensitized device. Unexpectedly, further to broadening the spectral response, the addition of the near IR sensitizer greatly enhances the spectral response in the visible region. Through both electronic and spectroscopic investigations, we demonstrate that resonant energy transfer occurs from the visible to the near IR sensitizer. This unforeseen charge generation route works in conjunction with direct electron transfer from the visible sensitizer, improving the overall charge generation efficiency and explaining the panchromatic enhancements with the cosensitized system. This previously unobserved mechanism for charge generation relaxes the design criteria for visible absorbing sensitizers, providing a second, and possibly primary, channel for efficient charge generation.

Ultrafast charge separation at a polymer−single-walled carbon nanotube molecular junction,
S. D. Stranks, C. Weisspfennig, P. Parkinson, M. B. Johnston, L. M. Herz, and R. J. Nicholas,
Nano Lett., 11 (2011), p. 66. [journal | article | SI ]

We have investigated the charge photogeneration dynamics at the interface formed between single-walled carbon nanotubes (SWNTs) and poly(3-hexylthiophene) (P3HT) using a combination of femtosecond spectroscopic techniques. We demonstrate that photoexcitation of P3HT forming a single molecular layer around a SWNT leads to an ultrafast (430 fs) charge transfer between the materials. The addition of excess P3HT leads to long-term charge separation in which free polarons remain separated at room temperature. Our results suggest that SWNT-P3HT blends incorporating only small fractions (1%) of SWNTs allow photon-to-charge conversion with efficiencies comparable to those for conventional (60:40) P3HT−fullerene blends, provided that small-diameter tubes are individually embedded in the P3HT matrix.

Energy transfer in single-stranded DNA-templated stacks of naphthalene chromophores,
A. L. Stevens, P. G. A. Janssen, A. Ruiz-Carretero, M. Surin, A. P. H. J. Schenning, and L. M. Herz,
J. Phys. Chem. C, 115 (2011), p. 10550. [journal | article | SI ]

We have investigated energy transfer in a novel self-assembled DNA hybrid structure composed of diaminopurine-equipped naphthalene derivatives that are hydrogenbonded along a single-stranded oligothymine template. By performing time-resolved measurements of the naphthalene donor luminescence decay in the absence and presence of a cyanine Cy3.5 acceptor bonded covalently to the 50 end of the oligothymine, we have examined the role of temperature and DNA template length on energy transfer from donors to the acceptor. We find that energy transfer rates decline with increasing temperature over a fairly narrow (+/-5^oC) range over which changes in circular dichroism and donor luminescence lifetime indicate that the chiral assemblies are dissociating. In addition, the transfer rates exhibit a complex dependence on template length, increasing from initially low values for 10 bases toward an optimum for 30 bases and then declining again toward 60 bases.We find that for short (~10 bases) templates, incomplete filling and disorder reduces the overall transfer efficiency, while longer assemblies are more ordered but suffer from larger donor--acceptor separations, resulting in the observed peak at intermediate template length. In order to replicate the observed transfer dynamics, we have constructed a model assuming Förster energy transfer occurs between donors and acceptors whose geometric arrangement had been determined through molecular dynamics simulations of the whole assembly structure. For short DNA templates, the model is found to overestimate the transfer rates because it does not include effects of incomplete complex assembly and stacking faults. In contrast, the model underestimates the transfer rates for long, ordered assemblies indicating that additional mechanisms, such as diffusion of excitations along the donor stacks, need to be included. These results suggest that efficient energy transfer, in excess of that expected from simple Förster calculations, is feasible even for long DNA-templated assemblies of π-stacked conjugated chromophores. Such structures may therefore act as molecular wires transporting energy from one end to another.

Belt-shaped π-systems: Relating geometry to electronic structure in a six-porphyrin nanoring,
J. K. Sprafke, D. V. Kondratuk, M. Wykes, A. L. Thompson, M. Hoffmann, R. Drevinskas, W.-H. Chen, C. K. Yong, J. Kärnbratt, J. E. Bullock, M. Malfois, M. R. Wasielewski, B. Albinsson, L. M. Herz, D. Zigmantas, D. Beljonne, and H. L. Anderson,
J. Am. Chem. Soc., 133 (2011), p. 17262. [journal | article | SI ]

Linear π-conjugated oligomers have been widely investigated, but the behavior of the corresponding cyclic oligomers is poorly understood, despite the recent synthesis of π-conjugated macrocycles such as [n|cycloparaphenylenes and cyclo[n|thiophenes, . Here we present an efficient template-directed synthesis of a π-conjugated butadiyne-linked cyclic porphyrin hexamer directly from the monomer. Small-angle X-ray scattering data show that this nanoring is shape-persistent in solution, even without its template, whereas the linear porphyrin hexamer is relatively flexible. The crystal structure of the nanoring-template complex shows that most of the strain is localized in the acetylenes; the porphyrin units are slightly curved, but the zinc coordination sphere is undistorted. The electrochemistry, absorption and fluorescence spectra indicate that the HOMO-LUMO gap of the nanoring is less than that of the linear hexamer, and less than that of the corresponding polymer. The nanoring exhibits six one-electron reductions and six one-electron oxidations, most of which are well resolved. Ultra-fast fluorescence anisotropy measurements show that absorption of light generates an excited state that is delocalized over the whole π-system within a time of less than 0.5 ps. The fluorescence spectrum is amazingly structured and red-shifted. A similar, but less dramatic, red-shift has been reported in the fluorescence spectra of cycloparaphenylenes, and was attributed to a high exciton binding energy, however the exciton binding energy of the porphyrin nanoring is similar to those of linear oligomers. Quantum-chemical excited state calculations show that the fluorescence spectrum of the nanoring can be fully explained in terms of vibronic Herzberg-Teller (HT) intensity borrowing.

Electron mobility and injection dynamics in mesoporous ZnO, SnO₂, and TiO₂ films used in dye-sensitized solar cells,
P. Tiwana, P. Docampo, M. B. Johnston, H. J. Snaith, and L. M. Herz,
ACS Nano, 5 (2011), p. 5158. [journal | article | SI ]

High-performance dye-sensitized solar cells are usually fabricated using nanostructured TiO₂ as a thin-film electron-collecting material. However, alternative metal-oxides are currently being explored that may offer advantages through ease of processing, higher electron mobility, or interface band energetics. We present here a comparative study of electron mobility and injection dynamics in thin films of TiO₂, ZnO, and SnO₂ nanoparticles sensitized with Z907 ruthenium dye. Using time-resolved terahertz photoconductivity measurements, we show that, for ZnO and SnO₂ nanoporous films, electron injection from the sensitizer has substantial slow components lasting over tens to hundreds of picoseconds, while for TiO₂, the process is predominantly concluded within a few picoseconds. These results correlate well with the overall electron injection efficiencies we determine from photovoltaic cells fabricated from identical nanoporous films, suggesting that such slow components limit the overall photocurrent generated by the solar cell. We conclude that these injection dynamics are not substantially influenced by bulk energy level offsets but rather by the local environment of the dye nanoparticle interface that is governed by dye binding modes and densities of states available for injection, both of which may vary from site to site. In addition, we have extracted the electron mobility in the three nanoporous metal-oxide films at early time after excitation from terahertz conductivity measurements and compared these with the time-averaged, long-range mobility determined for devices based on identical films. Comparison with established values for single-crystal Hall mobilities of the three materials shows that, while electron mobility values for nanoporous TiO₂ films are approaching theoretical maximum values, both early time, short distance and interparticle electron mobility in nanoporous ZnO or SnO₂ films offer considerable scope for improvement.

Impact of nuclear lattice relaxation on the excitation energy transfer along a chain of π-conjugated molecules,
S. A. Schmid, R. Abbel, A. P. H. Schenning, E. W. Meijer, and L. M. Herz,
Phys. Rev. B, 81 (2010), p. 085438. [journal | article | SI ]

We have investigated the extent to which delocalization of the ground-state and excited-state wave functions of a π-conjugated molecule affects the excitation energy transfer (EET) between such molecules. Using femtosecond photoluminescence spectroscopy, we experimentally monitored the EET along well-defined supramolecular chains of extended conjugated molecules. Comparison with Monte Carlo simulations reveals that only a model incorporating a localized emitter and delocalized absorber wave function accurately reproduces these data. Our findings demonstrate that self-localization of the initially excited state, following fast relaxation of the nuclear lattice, has a significant impact on the EET dynamics in molecular assemblies.

Ultrafast terahertz conductivity dynamics in mesoporous TiO₂: Influence of dye sensitization and surface treatment in solid-state dye-sensitized solar cells,
P. Tiwana, P. Parkinson, M. B. Johnston, H. J. Snaith, and L. M. Herz,
J. Phys. Chem. C, 114 (2010), p. 1365. [journal | article | SI ]

We have used optical-pump terahertz-probe spectroscopy to explore the photoinduced conductivity dynamics in mesoporous anatase TiO₂ films, commonly employed as the electron-transporting electrode in dye-sensitized solar cells. We find an intrinsic mobility value of 0.1 cm²/(V s) and diffusion length of 20 nm for electron motion through the TiO₂ matrix. The photoconductivity dynamics in TiO₂ films, both before and after sensitization with a ruthenium bypyridyl complex termed Z907, were examined in order to study the charge injection, trapping, and recombination time scales. We observe a biphasic charge injection from Z907, with a fast sub-500 fs component, followed by a slower 70−200 ps component. This is followed by photoconductivity decay over the first few nanoseconds, predominantly reflecting charge carrier trapping. In addition, we have utilized terahertz spectroscopy to investigate the influence of treating the titania surface with TiCl₄ on early-time charge dynamics. In the solar cells, surface treatment of the mesoporous TiO₂ with TiCl₄ is critical to enable efficient operation. Here, we find that neither early-time charge mobility nor charge injection rate or decay times are significantly affected by the treatment, which suggests that it may, instead, have an impact on phenomena occurring on longer time scales.

Dynamic terahertz polarization in single-walled carbon nanotubes,
X. L. Xu, P. Parkinson, K.-C. Chuang, M. B. Johnston, R. J. Nicholas, and L. M. Herz,
Phys. Rev. B, 82 (2010), p. 085441. [journal | article | SI ]

We have investigated the anisotropic dynamic dielectric response of aligned and well-isolated single-walled carbon nanotubes using optical-pump terahertz (THz)-probe techniques. The polarization anisotropy measurements demonstrate that the THz radiation interacts only with radiation polarized parallel to the nanotubes which have been selectively excited by a polarized pump pulse thus allowing controlled THz polarization to be achieved from unaligned nanotubes.

Role of ultrafast torsional relaxation in the emission from polythiophene aggregates,
P. Parkinson, C. Müller, N. Stingelin, M. B. Johnston, and L. M. Herz,
J. Phys. Chem. Lett., 1 (2010), p. 2788. [journal | article | SI ]

An understanding of aggregation effects in semiconducting polymers is essential for their use in optoelectronic devices; however, the dynamic evolution of such interchain states is not well understood. Here, we have investigated a blend of semiconducting poly(3-hexylthiophene) (P3HT) with an electronically inert ultrahigh-molecular-weight polyethylene (UHMW-PE) matrix that is shown to allow precise control over the extent to which the P3HT chains aggregate. We determined the singlet exciton population within isolated and aggregated P3HT regions using femtosecond time-resolved photoluminescence measurements and found a strong ultrafast decay pathway in the aggregated case only. Comparison of the emission from the two lowest vibronic bands demonstrates a changeover from an initial vibrationally “hot” photoexcited state to a geometrically relaxed aggregate state within ~13ps, corresponding to time scales for torsional relaxation in these materials. We conclude that formation of an aggregate excited state in conjugated polymers is mediated by vibrational relaxation from a low-symmetry to a high-symmetry ordered state for the ensemble.

Terahertz excitonic response of isolated single-walled carbon nanotubes,
X. Xu, K. Chuang, R. J. Nicholas, M. B. Johnston, and L. M. Herz,
J. Phys. Chem. C, 113 (2009), p. 18106. [journal | article | SI ]

We have investigated the ultrafast far-infrared transmission of isolated single-walled carbon nanotubes using optical-pump THz-probe spectroscopy. The THz dielectric response is dominated by excitons with an initial, rapid decay due to Auger recombination followed by a slow decay of isolated single excitons. Frequency-dependent analysis of the photoinduced dielectric function suggest an internal excitonic excitation at 11 meV with further low-frequency (0.6 and 1.4 THz) absorption features at high densities ascribed to exciton complexes. A featureless conductivity bleaching is attributed to an exciton-induced reduction in the mobility of free carriers caused by phase-space filling.

Carrier lifetime and mobility enhancement in nearly defect-free core-shell nanowires measured using time-resolved terahertz spectroscopy,
P. Parkinson, H. J. Joyce, Q. Gao, H. H. Tan, X. Zhang, J. Zou, C. Jagadish, L. M. Herz, and M. B. Johnston,
Nano Lett., 9 (2009), p. 3349. [journal | article | SI ]

We have used transient terahertz photoconductivity measurements to assess the efficacy of two-temperature growth and core-shell encapsulation techniques on the electronic properties of GaAs nanowires. We demonstrate that two-temperature growth of the GaAs core leads to an almost doubling in charge-carrier mobility and a tripling of carrier lifetime. In addition, overcoating the GaAs core with a larger-bandgap material is shown to reduce the density of surface traps by 82%, thereby enhancing the charge conductivity.

Analyzing the molecular weight distribution in supramolecular polymers,
S. A. Schmid, R. Abbel, A. P. H. Schenning, E. W. Meijer, R. P. Sijbesma, and L. M. Herz,
J. Am. Chem. Soc., 131 (2009), p. 17696. [journal | article | SI ]

We have investigated the formation process of supramolecular linear polymer chains and its influence on the resulting chain length distribution function. For this purpose, we explored the migration of excitation energy between oligofluorene units coupled together through quadruple hydrogen-bonding groups to form linear chains that are terminated by oligophenylene vinylene end-caps acting as energy traps. The energy transfer dynamics from the main chain to the chain end was monitored experimentally using time-resolved PL spectroscopy and compared to an equivalent Monte Carlo simulation incorporating information on the structure of the chains, the transition transfer rates, and various weight distribution trial functions. We find that the assumption of a Flory distribution of chain lengths leads to excellent agreement between experimental and simulated data for a wide range of end-cap concentrations. On the other hand, both a Poisson function and a simplified assumption of a monodisperse distribution significantly underestimate the presence of long chains in the ensemble. Our results therefore show that supramolecular polymerization is a steplike process equivalent to polycondensation reactions in linear covalent polymers. These findings emphasize that equal reactivity of the supramolecular building blocks leads to a dynamic growth process for the supramolecular chain involving all chain components at all times.

Polarization anisotropy dynamics for thin films of a conjugated polymer aligned by nanoimprinting,
S. A. Schmid, K. H. Kim, M. H. Chang, Z. Zheng, W. T. S. Huck, R. H. Friend, J. S. Kim, and L. M. Herz,
Phys. Rev. B, 77 (2008), p. 115338. [journal | article | SI ]

Time-integrated and femtosecond time-resolved photoluminescence spectroscopy has been used to study the dynamic emission polarization anisotropy for thin films of a conjugated polymer whose chains had been aligned through a nanoimprinting technique. The results indicate a high degree of chain alignment, with the presence of a small fraction of unaligned chain domains in film regions far from the imprinted surface. The time-averaged emission from aligned domains is found to be slightly shifted to higher photon energies compared to that from more disordered film regions. This effect is attributed to a subtly different chain packing geometry in the more aligned regions of the film, which leads to a reduced exciton diffusivity and inhibits energetic relaxation of the exciton in the inhomogeneously broadened density of states. While for an unaligned reference film, exciton migration results in a nearly complete depolarization of the emission over the first 300 ps, for the aligned films, interchain exciton hopping from unaligned to aligned domains is found to increase the anisotropy over the same time scale. In addition, excitons generated in aligned film domains were found to be slightly more susceptible to nonradiative quenching effects than those in disordered regions deeper inside the film, suggesting a marginally higher defect density near the nanoimprinted surface of the aligned film.

Exciton dissociation in polymer field-effect transistors studied using terahertz spectroscopy,
J. Lloyd-Hughes, T. Richards, H. Sirringhaus, M. B. Johnston, and L. M. Herz,
Phys. Rev. B, 77 (2008), p. 125203. [journal | article | SI ]

We have used terahertz time-domain spectroscopy to investigate photoinduced charge generation in conjugated polymer field-effect transistors. Our measurements show that excitons dissociate in the accumulation layer under the application of a gate voltage, with a quantum efficiency of  0.1 for an average gate field of ~1×10⁸ V m^-1. The transistor history is found to affect the exciton dissociation efficiency, which decreases as holes are increasingly trapped in the accumulation layer. The quantum efficiency of charge formation from excitons is compared with the two contrasting models proposed by Onsager and Arkhipov based on the assumption that field-induced exciton dissociation is assisted by the Brownian diffusive motion or an initial excess energy supplied by excited vibrational modes, respectively.

Enhanced π-conjugation around a porphyrin[6| nanoring,
M. Hoffmann, J. Kärnbratt, M. H. Chang, L. M. Herz, B. Albinsson, and H. L. Anderson,
Angew. Chem. Int. Ed., 47 (2008), p. 4993. [journal | article | SI ]

Belt-shaped chromophores provide fascinating insights into electronic pi delocalization over curved surfaces with radially oriented p orbitals. Examples include the cyclic para-phenylacetylenes and the [4₆|paracyclophanedodecayne of Tsuji and coworkers, as well as fullerenes and carbon nanotubes. Avariety of belt-shaped porphyrin arrays have been synthesized; however, the vast majority of them lacks a complete π-conjugation pathway around the whole macrocycle. Recently we reported the synthesis of a belt-shaped D8h symmetric porphyrin[8| nanoring on an octadentate template. Herein we present an efficient synthesis of an even more strained pi conjugated D₆h porphyrin[6| nanoring 1, by template-directed trimerization of a porphyrin dimer 2 on a hexapyridyl template 3 (Scheme 1).

Conductivity of nanoporous inp membranes investigated using terahertz spectroscopy,
S. K. E. Merchant, J. Lloyd-Hughes, L. Sirbu, I. M. Tiginyanu, P. Parkinson, L. M. Herz, and M. B. Johnston,
Nanotechnology, 19 (2008), p. 395704. [journal | article | SI ]

We have investigated the terahertz conductivity of extrinsic and photoexcited electrons in nanoporous indium phosphide (InP) at different pore densities and orientations. The form of electronic transport in the film was found to differ significantly from that for bulk InP. While photo-generated electrons showed Drude-like transport, the behaviour for extrinsic electrons deviated significantly from the Drude model. Time-resolved photoconductivity measurements found that carrier recombination was slow, with lifetimes exceeding 1 ns for all porosities and orientations. When considered together, these findings suggest that the surfaces created by the nanopores strongly alter the dynamics of both extrinsic and photoexcited electrons.

Mesoscopic order and the dimensionality of long-range resonance energy transfer in supramolecular semiconductors,
C. Daniel, F. Makereel, L. M. Herz, F. J. M. Hoeben, P. Jonkheijm, A. P. H. J. Schenning, E. W. Meijer, and C. Silva,
J. Chem. Phys., 129 (2008), p. 104701. [journal | article | SI ]

We present time-resolved photoluminescence measurements on two series of oligo-p-phenylenevinylene materials that self-assemble into supramolecular nanostructures with thermotropic reversibility in dodecane. One set of derivatives form chiral helical stacks, while the second set form less organized “frustrated” stacks. Here we study the effects of supramolecular organization on the resonance energy transfer rates. We measure these rates in nanoassemblies formed with mixed blends of oligomers and compare them with the rates predicted by Förster theory. Our results and analysis show that control of supramolecular order in the nanometer length scale has a dominant effect on the efficiency and dimensionality of resonance energy transfer.

Dynamics of excited-state conformational relaxation and electronic delocalization in conjugated porphyrin oligomers,
M. H. Chang, M. Hoffmann, H. L. Anderson, and L. M. Herz,
J. Am. Chem. Soc., 130 (2008), p. 10171. [journal | article | SI ]

We have investigated the influence of nuclear geometric relaxation on the extent of the excited-state electronic delocalization in conjugated zinc porphyrin oligomers using ultrafast transient photoluminescence spectroscopy. By use of metal-coordinating templates that force the oligomers into specific geometries in solution we are able to distinguish clearly between relaxation effects arising from the two vibrational modes that preferentially couple to the electronic transitions in such materials, i.e. carbon-carbon bond stretches and inter-ring torsions. We find that light absorption generates an excited state that is initially strongly delocalized along the oligomer but contracts rapidly following vibrational relaxation of the nuclei along C--C stretch coordinates on the subpicosecond time scale. We are able to monitor such excitonic self-trapping effects by observing the extent to which the concomitant ultrafast rotation of the transition dipole moment is found to correlate with the degree of bending induced in the molecular backbone. We further demonstrate that interporphyrin torsional relaxation leads to a subsequent increase in the excited-state electronic delocalization on a longer time scale (~100 ps). Such dynamic planarization of the molecular backbone is evident from the time-dependent increase in the overall emission intensity and red-shift in the peak emission energy that can be observed for wormlike flexible porphyrin octamers but not for torsionally rigidified cyclic or double-strand octamer complexes. These results therefore indicate that, following excitation, the initially highly delocalized excited-state wave function first contracts and then expands again along the conjugated backbone in accordance with the time periods for the vibrational modes coupled to the electronic transition.

Efficient generation of charges via below-gap photoexcitation of polymer-fullerene blend films investigated by terahertz spectroscopy,
P. Parkinson, J. Lloyd-Hughes, M. B. Johnston, and L. M. Herz,
Phys. Rev. B, 78 (2008), p. 115321. [journal | article | SI ]

Using optical-pump terahertz-probe spectroscopy, we have investigated the time-resolved conductivity dynamics of photoexcited polymer-fullerene bulk heterojunction blends for two model polymers: poly[3-hexylthiophene| (P3HT) and poly[-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene| (MDMO-PPV) blended with [6,6|-phenyl-C₆₁ butyric acid methyl ester (PCBM). The observed terahertz-frequency conductivity is characteristic of dispersive charge transport for photoexcitation both at the π-π^* absorption peak (560 nm for P3HT) and significantly below it (800 nm). The photoconductivity at 800 nm is unexpectedly high, which we attribute to the presence of a charge-transfer complex. We report the excitation-fluence dependence of the photoconductivity over more than four orders of magnitude, obtained by utilizing a terahertz spectrometer based upon on either a laser oscillator or an amplifier source. The time-averaged photoconductivity of the P3HT:PCBM blend is over 20 times larger than that of P3HT, indicating that long-lived hole polarons are responsible for the high photovoltaic efficiency of polymer:fullerene blends. At early times (~ps) the linear dependence of photoconductivity upon fluence indicates that interfacial charge transfer dominates as an exciton decay pathway, generating charges with mobility of at least ~0.1 cm²V^-1s^-1. At later times, a sublinear relationship shows that carrier-carrier recombination effects influence the conductivity on a longer time scale (>1 μs) with a bimolecular charge annihilation constant for the blends that is approximately two to three orders of magnitude smaller than that typical for neat polymer films.

Intermolecular interaction effects on the ultrafast depolarization of the optical emission from conjugated polymers,
M. H. Chang, M. J. Frampton, H. L. Anderson, and L. M. Herz,
Phys. Rev. Lett., 98 (2007), p. 027402. [journal | article | SI ]

We have investigated the effect of interchain interactions on the ultrafast depolarization of the photoluminescence from solid films of a conjugated polymer. Accurate control was exercised over the interchain separation by threading of the conjugated chains with insulating macrocycles or complexation with an inert host polymer. Our measurements indicate that excitation into the higher electronic states of a chain aggregate is followed by a fast (<100 fs) relaxation into lower excited states with an associated rotation of the transition dipole moment. These findings emphasize the need for consideration of initial excitonic delocalization across more than one polymeric chain.

Dimensionality-dependent energy transfer in polymer-intercalated SnS₂,
P. Parkinson, E. Aharon, M. H. Chang, C. Dosche, G. L. Frey, A. Köhler, and L. M. Herz,
Phys. Rev. B, 75 (2007), p. 165206. [journal | article | SI ]

We have investigated the influence of dimensionality on the excitation-transfer dynamics in a conjugated polymer blend. Using time-resolved photoluminescence spectroscopy, we have measured the transfer transients for both a three-dimensional blend film and for quasi-two-dimensional monolayers formed through intercalation of the polymer blend between the crystal planes of an inorganic SnS₂ matrix. We compare the experimental data with a simple, dimensionality-dependent model based on electronic coupling between electronic transition moments taken to be point dipoles. Within this approximation, the energy-transfer dynamics is found to adopt a three-dimensional character in the solid film and a two-dimensional nature in the monolayers present in the SnS₂-polymer nanocomposite.

Theory of non-condon emission from the interchain exciton in conjugated polymer aggregates,
E. R. Bittner, S. Karabunarliev, and L. M. Herz,
J. Chem. Phys., 126 (2007), p. 191102. [journal | article | SI ]

The authors present here a simple analysis that explains the apparent strengthening of electron phonon interaction upon aggregation in conjugated polymer materials. The overall scheme is that of an intermolecular Herzberg-Teller effect whereby sidebands of a forbidden transition are activated by oppositely phased vibrations. The authors show that upon aggregation, the 0--0 emission becomes symmetry forbidden and the apparent redshift and remaining vibronic structure are due to sideband (0--1,0--2, etc.) emission. At higher temperatures, the 0--0 peak is due to thermal population in a higher lying even-parity vibronic state rather than direct emission from the odd-paritied lowest intermolecular vibronic state.

Monte Carlo simulation of exciton bimolecular annihilation dynamics in supramolecular semiconductor architectures,
C. Daniel, S. Westenhoff, F. Makereel, R. H. Friend, D. Beljonne, L. M. Herz, and C. Silva,
J. Phys. Chem. C, 111 (2007), p. 19111. [journal | article | SI ]

We present a simulation of exciton dynamics in supramolecular assemblies of an oligo-p-phenylenevinylene derivative monofunctionalised with a quadruple hydrogen-bonding group (MOPV). MOPV molecules form helical stacks in dodecane solution through solvophobic and π-π interactions with thermotropic reversibility. We apply a model of incoherent excitation hopping using a Monte Carlo scheme to extract microscopic physical quantities relevant to energy diffusion and bimolecular annihilation processes within isolated nanostructures. We compare the simulation to ultrafast spectroscopic data, namely photoinduced absorption transients at various excitation fluences, their polarization anisotropy, and the dynamic photoluminescence red-shift. We observe that energy diffusion and bimolecular annihilation processes can be described with the same microscopic model based on a Förster-like model that takes into account the spatial extent of the excited state; these two processes are interconnected via the same underlying physics. We extract a high diffusion coefficient (~0.08 cm²s^-1) over the first few picoseconds following excitation, which plays an important role in dictating the bimolecular annihilation dynamics.

Transient terahertz conductivity of GaAs nanowires,
P. Parkinson, J. Lloyd-Hughes, Q. Gao, H. H. Tan, C. Jagadish, M. B. Johnston, and L. M. Herz,
Nano Lett., 7 (2007), p. 2162. [journal | article | SI ]

The time-resolved conductivity of isolated GaAs nanowires is investigated by optical-pump terahertz-probe time-domain spectroscopy. The electronic response exhibits a pronounced surface plasmon mode that forms within 300 fs before decaying within 10 ps as a result of charge trapping at the nanowire surface. The mobility is extracted using the Drude model for a plasmon and found to be remarkably high, being roughly one-third of that typical for bulk GaAs at room temperature.

Influence of mesoscopic ordering on the photoexcitation transfer dynamics in supramolecular assemblies of oligo-p-phenylenevinylene,
M. H. Chang, F. J. M. Hoeben, P. Jonkheijm, A. P. H. J. Schenning, E. W. Meijer, C. Silva, and L. M. Herz,
Chem. Phys. Lett., 418 (2006), p. 196. [journal | article | SI ]

We have investigated the influence of molecular arrangement on the transfer rates of photoexcitations along supramolecular assemblies of hydrogen-bonded oligo-p-phenylenevinylene (OPV) molecules for two different packing geometries. For well-defined, helical stacks of monofunctional OPVs fast (~50 ps) photoluminescence depolarization and excitation transfer to dopants was observed, in agreement with semi-coherent exciton diffusion. For disordered assemblies of bifunctional OPVs incorporating a spacer to link adjacent molecules, depolarization and energy transfer dynamics occur on a longer time scale (~nanosecond). This strongly suggests that such spacers need to be tuned carefully as they may otherwise interfere with the π-stacking thereby reducing the intermolecular electronic coupling.

Influence of copolymer interface orientation on the optical emission of polymeric semiconductor heterojunctions,
P. Sreearunothai, A. C. Morteani, I. Avilov, J. Cornil, D. Beljonne, R. H. Friend, R. T. Phillips, C. Silva, and L. M. Herz,
Phys. Rev. Lett., 96 (2006), p. 117403. [journal | article | SI ]

We have examined the Coulombic interactions at the interface in a blend of two copolymers with intramolecular charge-transfer character and optimized band offsets for photoinduced charge generation. The combination of both time-resolved measurements of photoluminescence, and quantum-chemical modeling of the heterojunction allows us to show that relative orientation across the heterojunction can lead to either a repulsive barrier (~65 meV) or an attractive interaction which can enhance the chargetransfer processes. We conclude that polymer orientation at the heterojunction can be as important as energy-band offsets in determining the dynamics of charge separation and optical emission.

Charge trapping in polymer transistors probed by terahertz spectroscopy and scanning probe potentiometry,
J. Lloyd-Hughes, T. Richards, H. Sirringhaus, E. Castro-Camus, L. M. Herz, and M. B. Johnston,
Appl. Phys. Lett., 89 (2006), p. 112101. [journal | article | SI ]

Terahertz time-domain spectroscopy and scanning probe potentiometry were used to investigate charge trapping in polymer field-effect transistors fabricated on a silicon gate. The hole density in the transistor channel was determined from the reduction in the transmitted terahertz radiation under an applied gate voltage. Prolonged device operation creates an exponential decay in the differential terahertz transmission, compatible with an increase in the density of trapped holes in the polymer channel. Taken in combination with scanning probe potentionmetry measurements, these results indicate that device degradation is largely a consequence of hole trapping, rather than of changes to the mobility of free holes in the polymer.

Photoexcitation dynamics in thin films of insulated molecular wires,
M. H. Chang, M. J. Frampton, H. L. Anderson, and L. M. Herz,
Appl. Phys. Lett., 89 (2006), p. 232110. [journal | article | SI ]

A study is presented on how encapsulation of conjugated polymer chains affects the motion of photoexcitations and the formation of interchain aggregates in solid films. It is shown that threading of a poly(diphenylene vinylene) backbone inside insulating cyclodextrins (rotaxination) and/or complexation of the chains with poly(ethylene oxide) are effective means of preventing the diffusion of excitons to nonradiative defect sites. Ultrafast time-resolved photoluminescence data reveal that excitation transfer between encapsulated chains is still possible and, for the case of rotaxination, is likely to be facilitated through close packing of end groups belonging to adjacent chains.

Exciton migration in rigid-rod conjugated polymers: An improved Förster model,
E. Hennebicq, G. Pourtois, G. D. Scholes, L. M. Herz, D. M. Russell, C. Silva, S. Setayesh, A. C. Grimsdale, K. Müllen, J.-L. Brédas, and D. Beljonne,
J. Am. Chem. Soc., 127 (2005), p. 4744. [journal | article | SI ]

The dynamics of interchain and intrachain excitation energy transfer taking place in a polyindenofluorene endcapped with perylene derivatives is explored by means of ultrafast spectroscopy combined with correlated quantum-chemical calculations. The experimental data indicate faster exciton migration in films with respect to solution as a result of the emergence of efficient channels involving hopping between chains in close contact. These findings are supported by theoretical simulations based on an improved Förster model. Within this model, the rates are expressed according to the Fermi golden rule on the basis of (i) electronic couplings that take account of the detailed shape of the excited-state wave functions (through the use of a multicentric monopole expansion) and (ii) spectral overlap factors computed from the simulated acceptor absorption and donor emission spectra with explicit coupling to vibrations (considered within a displaced harmonic oscillator model); inhomogeneity is taken into account by assuming a distribution of chromophores with different conjugation lengths. The calculations predict faster intermolecular energy transfer as a result of larger electronic matrix elements and suggest a two-step mechanism for intrachain energy transfer with exciton hopping along the polymer backbone as the limiting step. Injecting the calculated hopping rates into a set of master equations allows the modeling of the dynamics of exciton transport along the polyindenofluorene chains and yields ensemble-averaged energy-transfer rates in good agreement with experiment.

Excitation migration along oligophenylenevinylene-based chiral stacks: Delocalization effects on transport dynamics,
D. Beljonne, E. Hennebicq, C. Daniel, L. M. Herz, C. Silva, G. D. Scholes, F. J. M. Hoeben, P. Jonkheijm, A. P. H. J. Schenning, S. C. J. Meskers, R. T. Phillips, R. H. Friend, and E. W. Meijer,
J. Phys. Chem B, 109 (2005), p. 10594. [journal | article | SI ]

Atomistic models based on quantum-chemical calculations are combined with time-resolved spectroscopic investigations to explore the migration of electronic excitations along oligophenylenevinylene-based chiral stacks. It is found that the usual Pauli master equation (PME) approach relying on uncoherent transport between individual chromophores underestimates the excitation diffusion dynamics, monitored here by the time decay of the transient polarization anisotropy. A better agreement to experiment is achieved when accounting for excitation delocalization among acceptor molecules, as implemented in a modifie version of the PME model. The same models are applied to study light harvesting and trapping in guest-host systems built from oligomers of different lengths.

The effects of supramolecular assembly on exciton decay rates in organic semiconductors,
C. Daniel, F. Makereel, L. M. Herz, F. J. M. Hoeben, P. Jonkheijm, A. P. H. J. Schenning, E. W. Meijer, R. H. Friend, and C. Silva,
J. Chem. Phys., 123 (2005), p. 084902. [journal | article | SI ]

We present time-resolved photoluminescence measurements on two series of oligop-phenylenevinylene (OPV) materials that are functionalized with quadruple hydrogen-bonding groups. These form supramolecular assemblies with thermotropic reversibility. The morphology of the assemblies depends on the way that the oligomers are functionalized; monofunctionalized OPVs (MOPVs) form chiral, helical stacks while bifunctionalized OPVs (BOPVs) form less organized structures. These are therefore model systems to investigate the effects of supramolecular assembly, the effects of morphology, and the dependence of oligomer length on the radiative and nonradiative rates of π-conjugated materials. The purpose of this work is to use MOPV and BOPV derivatives as model systems to study the effect of intermolecular interactions on the molecular photophysics by comparing optical properties in the dissolved phase and the supramolecular assemblies. A simple photophysical analysis allows us to extract the intrinsic radiative and nonradiative decay rates and to unravel the consequences of interchromophore coupling with unprecedented detail. We find that interchromophore coupling strongly reduces both radiative and intrinsic nonradiative rates and that the effect is more pronounced in short oligomers.

Chirality-dependent boron-mediated growth of nitrogen-doped single-walled carbon nanotubes,
G. Wiltshire, L.-J. Li, L. M. Herz, R. J. Nicholas, M. Glerup, J.-L. Sauvajol, and A. N. Khlobystov,
Phys. Rev. B, 72 (2005), p. 205431. [journal | article | SI ]

A change in the relative abundance of single-walled carbon nanotubes, due to the presence of both nitrogen and boron during synthesis, has been identified through Raman and absorption spectroscopy. Raman spectroscopy shows that for two specific branches boron mediates the growth of smaller-diameter zigzag or near-zigzag nanotubes. We combine our experimental results with an improved Kataura model to identify two of the preferentially grown species as (16,0) and (14,1).

Resonance energy transfer dynamics in hydrogen-bonded oligo-p-phenylenevinylene nanostructures,
C. Daniel, L. M. Herz, D. Beljonne, F. J. M. Hoeben, P. Jonkheijm, A. P. H. J. Schenning, E. W. Meijer, R. T. Phillips, and C. Silva,
Synth. Met., 147 (2004), p. 29–35. [journal | article | SI ]

Oligo-p-phenylenevinylene (OPV) materials monofunctionalised with ureido-s-triazine form chiral, helical stacks in dodecane solution. Here, we investigate resonance energy transfer dynamics in supramolecular stacks of OPVs consisting of three phenyl rings (MOPV3) doped with similar oligomers containing four phenyl rings (MOPV4). Broad spectral overlap between the MOPV3 fluorophores and MOPV4 chromophores results in efficient energy transfer from MOPV3 to MOPV4. We observe resonance energy transfer following two distinct regimes. The first is evident by growth of MOPV4 photoluminescence on a timescale of approximately 50 ps, mediated by rapid exciton diffusion in MOPV3 within the stack. In the second regime, dynamics of localised excitons on nanosecond timescales are dominated by direct resonance energy transfer to MOPV4 chromophores. Global analysis of the photoluminescence decay of MOPV3 in blends with varying MOPV4 composition on times > 2 ns is consistent with quasi-one-dimensional resonance energy transfer with Foerster radius of 8 nm.

Towards supramolecular electronics,
A. P. H. J. Schenning, P. Jonkheijm, F. J. M. Hoeben, J. van Herrikhuyzen, S. C. J. Meskers, E. W. Meijer, L. M. Herz, C. Daniel, C. Silva, R. T. Phillips, R. H. Friend, D. Beljonne, A. Miura, S. De Feyter, M. Zdanowska, H. Uji-i, F. C. De Schreyver, Z. Chen, F. Wuerthner, M. Mas-Torrent, D. den Boer, M. Durkut, and P. Hadley,
Synth. Met., 147 (2004), p. 43–48. [journal | article | SI ]

We have demonstrated that it is possible to program pi-conjugated molecules to self-assemble into cylindrical aggregates in solution. By incorporating energy or electron traps in our stacks, energy and electron transfer processes in these one-dimensional assemblies have been studied in solution. The transfer of the single OPV cylinders from solution to a solid support as isolated objects was only possible when specific concentrations and specific solid supports were used. So far, however, we have not been able to measure any current through our fibers.

Time-dependent energy transfer rates in a conjugated polymer guest-host system,
L. M. Herz, C. Silva, A. C. Grimsdale, K. Müllen, and R. T. Phillips,
Phys. Rev. B, 70 (2004), p. 165207. [journal | article | SI ]

We have investigated the energy transfer dynamics in films of a conjugated polyindenofluorene host doped with covalently attached perylene guests. By performing time-resolved measurements of the host luminescence decay under site-selective excitation conditions, we have examined the influence of exciton migration within the host on the temporal evolution of the host-guest energy transfer. We find that highly mobile excitons created at the peak of the host's inhomogeneous density of states transfer to guests considerably faster than more localized excitons created in the low-energy tail, indicating a strong contribution of exciton migration to the overall energy transfer. These effects are significantly more pronounced at low temperature (7 K) than at ambient temperature, suggesting that for the latter, up-hill migration of excitons in the host and a broadening of their homogeneous linewidth may prevent truly site-selective excitation of localized excitons. In the asymptotic long-time limit, the observed dynamics are compatible with long-range single-step Forster energy transfer. However, at early times (less than or similar to 10 ps) after excitation, the behavior notably deviates from this description, suggesting that diffusion- assisted energy transfer is more important in this regime. The measured changes in excitation transfer rates with temperature and excitation energy correlate well with those observed for the dynamic energy shifts of the vibronic emission peaks from the undoped polymer. Our results therefore indicate that energy-transfer rates in polymeric guest-host systems are strongly time-dependent, owing to a contribution both from exciton relaxation through incoherent hopping within the host's density of states and direct Förster energy transfer.

Exciton regeneration at polymeric semiconductor heterojunctions,
A. C. Morteani, P. Sreearunothai, L. M. Herz, R. H. Friend, and C. Silva,
Phys. Rev. Lett., 92 (2004), p. 247402. [journal | article | SI ]

Control of the band-edge offsets at heterojunctions between organic semiconductors allows efficient operation of either photovoltaic or light-emitting diodes. We investigate systems where the exciton is marginally stable against charge separation and show via E-field-dependent time-resolved photoluminescence spectroscopy that excitons that have undergone charge separation at a heterojunction can be efficiently regenerated. This is because the charge transfer produces a geminate electron-hole pair (separation 2.2-3.1 nm) which may collapse into an exciplex and then endothermically (E_a=100-200 meV) back transfer towards the exciton.

Morphology-dependent energy transfer within polyfluorene thin films,
A. L. T. Khan, P. Sreearunothai, L. M. Herz, M. J. Banach, and A. Köhler,
Phys. Rev. B, 69 (2004), p. 085201. [journal | article | SI ]

We have performed a detailed study of the photoluminescence from thin films of blue-light-emitting poly(9,9-dioctylfluorene) containing different fractions of planarized (beta-phase) chains within the glassy polymer film. By choosing solvents with a range of polarities and boiling points we were able to cast films with reliable control of the relative amounts of beta-phase chains present. We analyzed the emission spectra in terms of Franck-Condon progressions and found that, at low temperatures (8 K), the luminescence can be modeled accurately by considering two distinct contributions from the two phases present in the film. The Huang-Rhys parameter for the beta phase is shown to be approximately half the value obtained for the glassy phase, in agreement with a more delocalized exciton in the beta phase. Time-resolved photoluminescence measurements on a film containing roughly 25% of beta phase reveal a fast transfer of excitations from the glassy to the beta phase, indicating that the two phases are well intermixed. Assuming the transfer dynamics to be governed by dipole-dipole coupling, we obtain a Förster radius of 8.2 (0.6) nm, significantly larger than the radius typically found for excitation transfer within the glassy phase. These results are consistent with the large spectral overlap between the emission of the glassy phase and the absorption of the beta phase and explain why the latter dominates the emission even from films containing only a small fraction of beta-phase chains.

Efficient energy transfer in mixed columnar stacks of hydrogen-bonded oligo(p-phenylene vinylene)s in solution,
F. J. M. Hoeben, L. M. Herz, C. Daniel, P. Jonkheijm, A. P. H. J. Schenning, C. Silva, S. C. J. Meskers, D. Beljonne, R. T. Phillips, R. H. Friend, and E. W. Meijer,
Angew. Chem. Int. Edit., 43 (2004), p. 1976–1979. [journal | article | SI ]

Long-range ordering in well-defined aggregates of pi-conjugated structures is the key feature in energy-transfer processes in photosynthetic systems as well as electronic devices based on organic compounds. All the studies on artificial antenna–target systems have indicated the need for precise control of distance and orientation. However, detailed insight into the subtleties of the organizational demands of these artificial systems is lacking. Dynamic structures in solution are not shape persistent, while semi-crystalline bulk samples lack the uniform positioning of chromophores. Our design of a modular supramolecular approach enables us to create molecular stacks which are both dynamic and well-defined.

Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers,
C. Daniel, L. M. Herz, C. Silva, F. J. M. Hoeben, P. Jonkheijm, A. P. H. J. Schenning, and E. W. Meijer,
Phys. Rev. B, 68 (2003), p. 235212. [journal | article | SI ]

We present femtosecond transient absorption measurements on pi- conjugated supramolecular assemblies in a high-pump-fluence regime. Oligo(p-phenylenevinylene) monofunctionalized with ureido-s-triazine (MOPV) self-assembles into chiral stacks in dodecane solution below 75 ^oC at a concentration of 4x10^-4 M. We observe exciton bimolecular annihilation in MOPV stacks at high excitation fluence, indicated by the fluence- dependent decay of 1 ¹B_u-exciton spectral signatures and by the sublinear fluence dependence of time- and wavelength- integrated photoluminescence (PL) intensity. These two characteristics are much less pronounced in MOPV solution where the phase equilibrium is shifted significantly away from supramolecular assembly, slightly below the transition temperature. A mesoscopic rate-equation model is applied to extract the bimolecular annihilation rate constant from the excitation fluence dependence of transient absorption and PL signals. The results demonstrate that the bimolecular annihilation rate is very high with a square-root dependence in time. The exciton annihilation results from a combination of fast exciton diffusion and resonance energy transfer. The supramolecular nanostructures studied here have electronic properties that are intermediate between molecular aggregates and polymeric semiconductors.

Exciton dynamics in supramolecular assemblies of p-phenylenevinylene oligomers,
L. M. Herz, C. Daniel, C. Silva, F. J. M. Hoeben, A. P. H. J. Schenning, E. W. Meijer, R. H. Friend, and R. T. Phillips,
Synth. Met., 139 (2003), p. 839–842. [journal | article | SI ]

We have investigated the dynamics of photoexcitations in chiral assemblies of p-phenylenevinylene oligomers functionalized with hydrogen-bonding motifs. In the regime of low excitation densities, the luminescence transients are influenced by the migration of excitons to defect sites, indicative of fast diffusivity of excitons along the molecular assemblies. In addition, at high excitation densities, bimolecular exciton annihilation is shown to result in the fast depopulation of the stacks' excitonic energy levels.

Low-energy vibrational modes in phenylene oligomers studied by THz time-domain spectroscopy,
M. B. Johnston, L. M. Herz, A. L. T. Khan, A. Köhler, A. G. Davies, and E. H. Linfield,
Chem. Phys. Lett., 377 (2003), p. 256–262. [journal | article | SI ]

Low-energy vibrational modes have been investigated in polycrystalline biphenyl, para-terphenyl, para- quaterphenyl and para-sexiphenyl using THz time-domain spectroscopy (THz-TDS). A number of both internal and external infrared-active modes were observed for wavenumbers ranging between 20 and 80cm^-1. The temperature dependence of these modes is consistent with structural phase transitions occurring in the molecular crystal, indicating that THz-TDS is a sensitive probe of the conformation of conjugated molecular systems.

Fast exciton diffusion in chiral stacks of conjugated p-phenylene vinylene oligomers,
L. M. Herz, C. Daniel, C. Silva, F. J. M. Hoeben, A. P. H. J. Schenning, E. W. Meijer, R. H. Friend, and R. T. Phillips,
Phys. Rev. B, 68 (2003), p. 045203. [journal | article | SI ]

The ultrafast dynamics of photoexcitations have been studied in chiral stacks of conjugated p-phenylene vinylene molecules functionalized with hydrogen-bonding groups. The results indicate that in solution, pi-pi interactions between the molecules give rise to fast exciton diffusion along the stacking axis of the assemblies. The chiral nature of the assemblies is found to cause a rotation of the dipole moment of excitons propagating along the stacks as indicated by time-resolved measurements of the photoluminescence polarization anisotropy. The observed exciton diffusion and energy relaxation dynamics in the molecular stacks are shown to be very similar to those found in conjugated polymer films. Moreover, it is demonstrated that through changes in the temperature of the surrounding solvent, the stacks can be dissociated reversibly as shown by a marked reduction in the diffusivity of excitons.

Quantum computing - fine lines from dots,
L. M. Herz and R. T. Phillips,
Nat. Mater., 1 (2002), p. 212–213. [journal | article | SI ]

Quantum dots are candidates for quantum computing applications, but the coherence time of their quantum states must be improved. Recent optical measurements on single quantum dots indicate that the local environment plays a large role.

Exciton and polaron dynamics in a step-ladder polymeric semiconductor: the influence of interchain order,
C. Silva, D. M. Russell, A. S. Dhoot, L. M. Herz, C. Daniel, N. C. Greenham, A. C. Arias, S. Setayesh, K. Müllen, and R. H. Friend,
J. Phys.-Condes. Matter, 14 (2002), p. 9803–9824. [journal | article | SI ]

We present combined results of femtosecond transient photoluminescence (PL), femtosecond transient absorption and quasi-steady-state photoinduced absorption spectroscopy on the organic semiconductor poly-6, 6', 12, 12'-tetraalkyl-2, 8- indenofluorene (PIF). By control of interchain order via the choice of the side-chain substituents, we have investigated its effect on exciton and polaron dynamics in this model, electronic material. We show that interfaces between ordered and disordered domains play a significant role in the photophysics. At high photoexcitation fluence, a high yield (~ 10%) of polarons is only observed in the ordered semiconductor. This process arises from two-step photoexcitation, first to the lowest exciton, and then to a high-energy state of opposite symmetry. In contrast, triplet exciton population is generated via sequential excitation with smaller yield (<1%) in both ordered and disordered materials. In the low fluence regime, triplet excitons are found to arise from evolution of polarons generated with low efficiency (also <1%) by diffusion-limited processes. The triplet generation yield is strongly dependent on order, with the disordered material displaying a higher yield. Polaron decay is found to be thermally activated, with a higher activation energy and lower room-temperature recombination rate in the ordered material. Furthermore, we do not find that emissive keto defects play a defining role in the PL properties of our PIF samples. Instead, absorption features of aggregate-like species, which we believe to lead to sub-gap emission, are evident in the photocurrent action spectrum of the more ordered PIF derivative.

Interchain vs. intrachain energy transfer in acceptor-capped conjugated polymers,
D. Beljonne, G. Pourtois, C. Silva, E. Hennebicq, L. M. Herz, R. H. Friend, G. D. Scholes, S. Setayesh, K. Müllen, and J. L. Bredas,
Proc. Natl. Acad. Sci. U.S.A., 99 (2002), p. 10982–10987. [journal | article | SI ]

The energy-transfer processes taking place in conjugated polymers are investigated by means of ultrafast spectroscopy and correlated quantum-chemical calculations applied to polyindenofluorenes end-capped with a perylene derivative. Comparison between the time-integrated luminescence and transient absorption spectra measured in solution and in films allows disentangling of the contributions arising from intrachain and from interchain energy-migration phenomena. Intrachain processes dominate in solution where photoexcitation of the polyindenofluorene units induces a rather slow energy transfer to the perylene end moieties. In films, close contacts between chains favors interchain transport of the excited singlet species (from the conjugated bridge of one chain to the perylene unit of a neighboring one); this process is characterized by a 1-order-of-magnitude increase in transfer rate with respect to solution. This description is supported fully by the results of quantum-chemical calculations that go beyond the usual point-dipole model approximation and account for geometric relaxation phenomena in the excited state before energy migration. The calculations indicate a two-step mechanism for intrachain energy transfer with hopping along the conjugated chains as the rate-limiting step; the higher efficiency of the interchain transfer process is mainly due to larger electronic coupling matrix elements between closely lying chains.

Time-resolved photoluminescence cross-correlation measurements on InAs quantum dots,
L. M. Herz, R. T. Phillips, E. C. Le Ru, and R. Murray,
Phys. Stat. Sol. A, 190 (2002), p. 565–569. [journal | article | SI ]

We have studied recombination and relaxation dynamics in InAs quantum dots by means of photoluminescence cross-correlation techniques with sub-picosecond time-resolution. We have determined the relative radiative efficiencies for carrier recombination from different energy levels of the quantum dots and show that the radiative efficiency decreases with increasing transition energy, indicating that carrier access to non-radiative recombination centres increases for higher-energy states inside the dots. Cross-correlation measurements with both excitation beams of the same circular polarization and with the beams having opposite circular polarization are shown to give direct insight into the spin relaxation dynamics in the quantum dots and the wetting layer.

Effects of aggregation on the excitation transfer in perylene- end-capped polyindenofluorene studied by time-resolved photoluminescence spectroscopy,
L. M. Herz, C. Silva, R. H. Friend, R. T. Phillips, S. Setayesh, S. Becker, D. Marsitsky, and K. Müllen,
Phys. Rev. B, 6419 (2001), p. 195203. [journal | article | SI ]

We have investigated the excitation transfer in a system comprising poly(6,6',12,12'-tetra-2-ethylhexyl-2,8-indenofluorene) (PIFTEH) chains end-capped with perylene dye molecules, using femtosecond time-resolved photoluminescence (PL) spectroscopy as well as polarized photoluminescence measurements. The transfer of excitons from isolated PIFTEH chains to perylene molecules is completed within the first 30-40 ps after excitation, and we extract a Forster radius R₀ =(1.8+/-0.3) nm from the time-resolved PL transients. We have modelled the polarization anisotropy for a guest-host system subject to Förster interactions via a Monte Carlo simulation and find that the emission from acceptors becomes unpolarized at sufficiently large acceptor concentrations, permitting an accurate determination of the Forster radius from time- integrated photoluminescence anisotropy measurements. While spectral overlap calculations predict a large efficiency for the transfer of excitations to the perylene molecules from sites where the PIFTEH chains aggregate, no transfer is observed experimentally, which we attribute to chain packing effects within the sample prohibiting sufficiently close contact between PIFTEH aggregates and perylene molecules.

Exciton migration to chain aggregates in conjugated polymers: influence of side-chain substitution,
L. M. Herz, C. Silva, R. T. Phillips, S. Setayesh, and K. Müllen,
Chem. Phys. Lett., 347 (2001), p. 318–324. [journal | article | SI ]

We have performed time-resolved photoluminescence (PL) measurements to study the transfer of excitons from noninteracting polymer chains to chain aggregates for thin films of polyindenofluorene with different attached side chains. The transfer time is shown to be fast (approximate to 35 ps) and side-chain independent indicating that the substitution of different side chains does not affect the local concentration of aggregate sites within aggregate-containing domains, but rather the extent to which such domains are formed. We find the aggregate emission efficiency to be relatively high (greater than or equal to 20%) while still lower than the efficiency of emission from non-interacting chains.

Effects of interchain interactions, polarization anisotropy, and photo-oxidation on the ultrafast photoluminescence decay from a polyfluorene,
L. M. Herz and R. T. Phillips,
Phys. Rev. B, 61 (2000), p. 13691–13697. [journal | article | SI ]

The ultrafast dynamics of excitons and excimers in an aligned film of poly(dioctylfluorene) (F8) have been studied by femtosecond time-resolved photoluminescence spectroscopy. The results indicate that the excimer formed in F8 involves only a small change in intermolecular separation upon excitation and has a limited, but nonzero, mobility. From changes in the photoluminescence decay upon photooxidation of F8 we deduce that the exciton diffusion constant is larger than the excimer diffusion constant by a factor of 21+/-2, thereby explaining why the excitonic luminescence is more strongly affected by the presence of quenching sites than the excimer luminescence. The decay of excitonic photoluminescence exhibits a significant polarization anisotropy, consistent with migration of excitons between regions with different orientation of the polymer chains. In contrast, excimer migration between such domains is inhibited since the excimer experiences the domain borders as potential barriers.

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