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University of Oxford, Department of Physics

Research in the Semiconductors Group


Research in the Herz group explores the fundamental science and applications of semiconducting materials and nanostructures ranging from organic molecules and solids, III-V inorganic semiconductors & nanostructures, to hybrid systems such as sensitized metal oxides and organic-inorganic perovskites. Current work focuses on common themes such as molecular self-assembly, energy and charge transfer, bio-mimetic light-harvesting, nanoscale electronic phenomena and interfacial effects. The group has leading expertize in femtosecond spectroscopic techniques, and applies this to unravel the ultrafast dynamics of excitations in a diverse range of semiconductor and nanostructures. Close collaborations exist with device physicists, theoretical researchers, synthetic chemists and materials scientists in order to advance the development of these novel materials for applications, such as light-emitting devices and next-generation photovoltaic cells.

Research Themes:

Metal halide perovskites

New organic-inorganic metal halide perovskite materials emerged recently as active materials in photovoltaic cells exceeding power conversion efficiencies of 26%. Our group has been at the forefront of developing an understanding of the fundamental processes that underpin the outstanding performance of these materials. We investigate a wide range of topics in this area, including the fundamentals of electron-phonon coupling, charge-carrier mobility and recombination, light emission, photon reabsorption, atomic microstructure and ion migration. These studies feed directly into collaborative efforts aimed at addressing remaining challenges in the creation of commercially available perovskite solar cells, e.g. stability, band-gap tunability, halide segregation, lead-free perovskites, trap-free materials, material morphology control and alternative device structures.

See e.g.:
Adv. Mater. 35 (2023), p. 2210834.
Advanced Energy Materials, 12 (2022), p. 2200847.
Nature Communications, 12 (2021), p. 6955
ACS Energy Letters, 6 (2021), p. 799–808.
Science 370 (2020), p. eabb5940.
Nature Communications 9 (2018), p. 293.
Nature Energy 3 (2018), p. 855
Energy Environ. Sci. 10 (2017), p. 361.
Nano Lett. 17 (2017), p. 5782.
Science 351 (2016), p. 151.
Science 354 (2016), p. 861
Nature Communications 7 (2016), p. 11755
Adv. Mater. 26 (2014), p. 1584.
Science 342 (2013), p. 341.
Reviews by our group:
Nature Energy, 7 (2022), p. 794–807.
ACS Energy Letters, 6 (2021), p. 2413.
Energy and Environmental Science 13 (2020), p.2024.
JPC Letters 10 (2018), p.6853.
ACS Energy Letters 2 (2017), p.1539.
Annu. Rev. Phys. Chem. 67 (2016), p.65.
Acc. Chem. Res. 49 (2016), p. 146.

Low-dimensional lead halide perovskite nanostructures

Low-dimensional halide perovskites, including nanocrystals and two-dimensional layered structures, offer a multitude of device applications, including efficient light-emitting devices and photovoltaic cells. We explore the fascinating physics displayed by such systems of lower dimensionality, examining e.g. how electronic quantum confinement affects exciton formation dynamics, charge-carrier transfer and transport modes, and optoelectronic properties.

See e.g.:
Adv. Func. Mater. 33 (2023), p. 2300363.
ACS Energy Letters 8 (2023), p. 2543.
Adv. Func. Mater. 30 (2020), p. 1909904.
Nature Materials 19 (2020), p. 1201.
Nano Lett. 19 (2019), p. 3953.

Chalcogenide, bisimide and halide semiconductors

The stellar performance of lead halide perovskites has inspired the search for new, related semiconductors with nontoxic inorganic ingredients and high long-term stability. Our group investigates a wide range of new chalcogenide, metal halide, and bisimide semiconductors in this space, including Cs2AgBiBr6 Cs2AgSbBr6 and their alloys Cs2AgSbxBi1-xBr6, AgBiS2 and AgBiNa2 nanocrystals, Ag3SI, BiOI, (4FPEA)4AgBiX8 (X = Cl, Br, I) layered materials, and alloys in the CuI-AgI-BiI3 phase space, such as Cu2AgBiI6. We investigate factors currently limiting the photovoltaic performance of these materials, unravel the origin and impact of ultrafast charge-carrier localisation effects, electronic and structural dimensionality, alloying and cation disorder, and bandstructure tuning.

See e.g.:
Advanced Materials, 35 (2023), p. 2305009.
J. Phys. Chem. Lett. 14 (2023), p. 10340.
J. Phys. Chem. Lett. 14 (2023), p. 6620.
ACS Energy Letters 8 (2023), p. 1485
Advanced Optical Materials 10 (2022), p. 2200354.
Nature Communications 13 (2022), p. 4960.
Adv. Func. Mater. 32 (2022), p. 2108392.
J. Phys. Chem. Lett. 12 (2021), p. 3352.
ACS Energy Letters 6 (2021), p. 1729.
Advanced Materials, 33 (2021), p. 2007057.

Bio-mimetic porphyrin nanorings and nanostructures

Natural evolution very early on solved the problem of how to capture light efficiently and use it to initiate the primary electron transfer reactions of photosynthesis in a surprisingly efficient manner. We explore synthetic molecules that mimic such biological light-harvesting complexes, but may ultimately allow high-efficiency conversion of solar light into energy in the form of electrical current, rather than biological mass. We examine the dynamics of photoexcitations in Zn-porphyrin nanorings and nanostructures, revealing electronic delocalization effects and the ultrafast transfer of energy in multiple chromophoresforming part of nanostructured complexes.

See e.g.:
Nature Chemistry, 14 (2022), p. 1436.
J. Am. Chem. Soc. 141 (2019), p. 7965.
J. Am. Chem. Soc. 140 (2018), p. 5352.
ACS Nano 10 (2016), p.5933.
J. Phys. Chem. Lett. 7 (2016), p. 332.
J. Am. Chem. Soc. 137 (2015), p. 14256.
Chemical Science 6 (2015), p. 181.
J. Phys. Chem. Lett. 5 (2014), p. 4356.
J. Am. Chem. Soc. 136 (2014), p. 8217.

Organic semiconductors, organic photovoltaics (OPV) and Luminesecent Solar Concentrators (LSC)

Organic pi-conjugated molecular materials offer another solution towards efficient light-harveting and light-emitting devices. The Herz group investigates ultrafast excitation processes in a wide variety of organic (carbon-based) semiconductors including conjugated polymers, small molecules, and carbon nanotubes. We explore blend materials for OPV comprising electron and hole-transporting materials at whose interface charges may dissociate, charge-extraction layers for metal-halide perovskite solar cells, and LSCs which are cheap plastic lightguides containing fluorophores that absorb light and channel it to the edge where a highly efficient PV cell is placed. The Herz Group explores central aspects such as energy transfer, polarization switching, electronic delocalization and lattice relaxation in organic semiconductors, and the impact of morphology and intermediate charge-transfer states on charge generation in blends of electron and hole transporters.

See e.g.:
Nature Communications, 11 (2020), p. 5525.
J. Phys. Chem. Lett. 10 (2019), p. 1729.
ACS Appl. Mater. Interfaces, 11 (2019), p. 21543
Nature Communications, 8 (2017), p. 15953.
J. Phys. Chem. Lett., 6 (2015), p. 1170.
Proc. Natl. Acad. Sci. U.S.A., 112 (2015), p. 7656.
Adv. Optical Mater., 2 (2014), p. 687.
J. Phys. Chem. C, 118 (2014), p. 17351.
ACS Nano, 6 (2012), p. 6058.
Nano Lett., 11 (2011), p. 66.
J. Phys. Chem. Lett., 1 (2010), p. 2788.
Phys. Rev. B, 78 (2008), p. 115321

Dye-sensitized solar cells

Dye-sensitized solar cells (DSCs) contain a monolayer of light-absorbing dye (the sensitizer) adsorbed to the surface of a mesoporous electron-transporting metal oxide, usually fabricated from sintered nanoparticles. Light absorption promotes an electron to the dye excited state, with subsequent electron transfer into, and transport through the metal-oxide to the collection point. The dye is also in contact with hole-transporter which regenerates the oxidized dye molecules and completes the circuit. Using ultrafast spectroscopic techniques we are able to probe individually the processes leading from light absorption to photocurrent generation. From such measurements we assess how performance is affected by certain changes in material system design, e.g. electrode composition and nanostructure, pore filing, light-soaking effects or inclusion of interlayers and additives.

See e.g.:
J. Phys. Chem. C, 119 (2015), p. 9159.
Adv. Func. Mater.,24 (2014), p. 668.
J. Phys. Chem. C, 117 (2013), p. 668.
Energy Environm. Sci., 5 (2012), p. 9566.
ACS Nano, 5 (2011), p. 5158.

Inorganic semiconductor nanowires

Inorganic semiconductor nanowires show tremendous potential as ultrafast switching devices and ultrafast optoelectronic devices such as fast photodetectors or modulators for use in very high bandwidth communications systems. We have investigated the ultrafast charge carrier dynamics in such nanowires using a variety of all-optical non-contact techniques, such as photoluminescence up-conversion and optical-pump terahertz-probe transient spectroscopy. We explore a variety of scientific aspects, such as surface plasmons, doping, surface defects, overcoating with organic semiconductors.

See e.g.:
Science 368 (2020), p.510.
Nano Lett. 18 (2018), p.3703.
ACS Nano 10 (2016), p.4219
Nano Lett. 15 (2015), p.1336
Nano Lett. 14 (2014), p.5989
Nano Lett. 13 (2013), p.4280
Nano Lett. 12 (2012), p.6293
Nano Lett. 12 (2012), p.5325
Nano Lett. 12 (2012), p.4600.

Supramolecular self-assembled nanostructures

Self-assembly has fast become a staple tool used in the control of complex supramolecular systems with applications in biological sensing. When governed by molecular recognition, self-assembly can produce systems from solution with specific architectures and properties that are determined by intermolecular interactions such as hydrogen bonding, hydrophobic, hydrophilic, and pi-pi interactions. We investigate how the arrangement of chromophores in such supramolecular architectures affects the transfer of photoexcitations in a variety of systems, as for example example DNA-templated assemblies of pi-stacked conjugated chromophores and nanoparticles, and supramolecular polymers.

ACS Nano, 7 (2013), p. 408.
ACS Nano, 6 (2012), p. 4777.
J. Phys. Chem. C, 115 (2011), p. 10550.
Chem. Commun., 47 (2011), p. 884.
J. Am. Chem. Soc., 131 (2009), p. 17696.

Current Research Support

* H. J. Snaith, L. M. Herz, S. Islam, M. B. Johnston, M. Rosseinsky, M. Filip, I. McCulloch, N. Noel, Advanced Device Concepts for Next-Generation Photovoltaics, EPSRC grant EP/X038777/1 (1 Oct 2023 – 30 Sept 2028)

* L. M. Herz, M. B. Johnston, N. Noel, Exploring intrinsic quantum confinement in metal halide perovskites, Leverhulme Trust Research Grant RPG-2022-272 (1 Aug 2023 – 1 Apr 2027)

* M. B. Johnston, H. Kraus, L. M. Herz, Ultrafast Terahertz Polarimetry Enabled by Semiconductor Nanowire Sensors , EPSRC grant EP/W018489/1 (1 Sept 2022 – 31 Aug 2026)

* L. M. Herz, H. J. Snaith, M. B. Johnston, M.R. Filip, Metal halide semiconductors: materials discovery beyond ABX3 perovskites , EPSRC grant EP/V010840/1 (1 March 2021 – 31 Aug 2024)

* H.J.S. Snaith, C. Case and L.M. Herz, All-perovskite multi-junction solar cells, EPSRC grant EP/S004947/1/1 (1 Nov 2018 - 31 July 2024)