Department of Physics, University of Oxford Homepage

University of Oxford, Department of Physics

Semiconductors Group

Current Research

Nanostructured molecular and inorganic semiconducting materials are a relatively recent innovation, yet they have already proven to be crucial ingredients to an increasing number of applications. Such materials offer large functional areas per material volume, making them ideal for efficient light-harvesting in photovoltaic cells, energy storage in batteries or fuel cells, and for chemical or biological sensing. In addition, the emergence of such nanostructures has allowed the exploration of new fundamental science, as for example artificial light harvesting and photocurrent generation at interfaces, supramolecular self-assembly and an understanding of material dynamics. Our group is particularly interested in increasing our understanding of how properties on the nanoscale affect the generation and mobility of charge carriers and the diffusivity of photoexcitations. Our experiments are largely based on a range of femtosecond spectroscopic methods to follow the dynamics of photoexcitations in a material. These techniques provide non-contact analytical tools to examine how molecular or hybrid materials may be optimized for implementation in solar cells, light-emitting devices or transistors.

A brief description of some of our recent work is given below; more details may be obtained from our publications. Information on our laboratory infrastructure and a listing of current research support may be found at the end of this page.

Examples of recent research

Hybrid metal halide perovskite solar cells

New organic-inorganic metal halide perovskite materials emerged recently as active materials in photovoltaic cells exceeding power conversion efficiencies of 16%. Our group has unravelled the ultrafast dynamics governing the charge carrier motion and recombination in these materials high-lighting the role of non-Langevin charge recombination underpinning the exceptional photovoltaic performance of these materials. We have further investigated the processes leading to emission from these materials and found their homogeneous phonon-broadening makes the material suitable for sub-picosecond light amplification.

Bio-mimetic porphyrin nanorings

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. Synthetic molecules that mimic such biological light-harvesting complexes should allow high-efficiency conversion of solar light into energy in the form of electrical current, rather than biological mass. We have recently shown that energy delocalisation is extremely rapid (300fs – similar to natural systems) in alkyne-linked Zn-porphyrin octamers that were curved into a rigid ring by use of an octadentate template co-ordinating with Zn. Our current work explores fully cyclic alkyne-linked porphyrin chromophores that directly mimic natural light harvesting systems. We are particularly interested in establishing how large we can make such biomimetic chromophores and still maintain fully delocalized excited states.

Dye-sensitized solar cells

Organic-inorganic hybrid cells based on dye-sensitised metal-oxide particles have recently entered commercial production. Such 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 can assess how performance is affected by certain changes in material system design, e.g. electrode composition and nanostructure, light-soaking effects or inclusion of interlayers and additives.

Organic photovoltaics (OPV) and Luminsecent Solar Concentrators (LSC)

Fully organic pi-conjugated molecular materials offer another solution towards cheap light-harveting devices. They may be utilized in organic photovoltaics which employ blends of electron and hole-transporting materials at whose interface charges may dissociate, as a result of suitable energy level alignment (type-II heterojunction). Charge transfer at such interfaces occur on femtosecond timescales; as a result, ultrafast spectroscopy has been an important activity for OPV development. The Herz Group explores central aspects such the importance of intermediate charge-transfer states at the interface between electron and hole transporters, the impact of morphology on charge generation and exploring a wide range of possible ingredients, including carbon nanotubes as electron transporting materials. In addition, the group explores molecules for Luminescent Solar Concentrators (LSC) which are cheap plastic lightguides containing fluorophores that absorb light and channel it to the edge where a highly efficient PV cell is placed to convert the light into electrical current. The Herz Group currently investigates ultrafast polarization switching and energy transfer in coupled molecular assemblies and dyads for LSCs. Our aims here are also to improve our current understanding of competing dynamic energy-transfer processes, e.g. through-bond, distributed dipole coupling and vibrationally activated mechanisms that permit polarization switching leading to better-designed systems with low light losses from the waveguide.

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 demonstrated that the peak conductivity of charge carriers injected into GaAs nanowires can be as high as 1/3 of the conductivity in the bulk. However, we find that the large surface area to volume ratio for these wires leads to an ultrafast conductivity decay as charges are trapped at surface defect traps. In further work we demonstrate that such defect states can be passivated effectively e.g. through overcoating with higher-bandgap organic semiconductors, or by surface delta-doping from an organic polymer overcoat.

Supramolecular self-assembled nanostructures

Self-assembly has fast become a staple tool used in the control of complex supramolecular systems. 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 energy within the system. For example, we recently demonstrated that efficient energy transfer, in excess of that expected from simple Foerster calculations, is feasible even for long DNA-templated assemblies of pi-stacked conjugated chromophores. In another recent study, we have investigated the effects of phase segregation on the excitation transfer in nanoparticles designed for biological sensing applications.

Exciton delocalization in organic semiconductors

The extent to which electronic delocalization occurs along and between conjugated polymer chains in the solid film is still a matter of debate. In order to investigate excitonic delocalization effects in molecular materials we employ polarization-dependent femtosecond photoluminescence spectroscopy, which can reveal ultrafast changes in the nature of the emitting oscillator dipole. For example, we have recently shown that the formation of an aggregate excited state in a model conjugated polymer (poly-3-hexylthiophene) is mediated by vibrational relaxation from a low-symmetry to a high-symmetry ordered state for the ensemble (see Figure). We are also exploring whether such delocalization is likely to aid to charge separation in materials blends for photovoltaics.

Laboratory Infrastructure

Figure: Top, left: Pump-probe set-up with regenerative amplifier system. Top, right: Photoluminescence upconversion set-up. Bottom, left: Group Photo. Bottom, right: Sample preparation area.

Current Research Support

* L.M. Herz, M.B. Johnston and H.J.S. Snaith, Organometal halide photovoltaic cells: tailoring fundamental light conversion pathways, EPSRC grant EP/L024667/1 (1 April 2014 - 30 Sept 2017)

* M.B. Johnston, L.M. Herz and H.J.S. Snaith, Perovskite Heterostructures by Vapour Deposition, EPSRC grant EP/P006329/1/1 (1 Sept 2016 - 29 Feb 2020)

* M.B. Johnston and L.M. Herz III-V Semiconductor Nanowires: Attaining Control over Doping and Heterointerfaces, EPSRC grant EP/M017095/1 (1 June 2015 - 30 Nov 2018)

* H.J. Snaith, L.M. Herz, M.B. Johnston, M. Riede, P. Radaelli, A. Watt, C. Grovenor, I.A. Walmsley, J.M. Kim, D.D.C. Bradley, R.H. Friend, and J. Walls A National Thin-Film Cluster Facility for Advanced Functional Materials, EPSRC grant EP/M022900/1 (1 June 2015 - 31 May 2020)

* P Stavrinou, N. Stingelin, M. Heeney, S.C. Dunn and L.M. Herz, EPSRC Centre for Doctoral Training in Plastic Electronic Materials, EPSRC grant EP/L016702/1 (1 April 2014 - 30 Sept 2022)

* N. Stingelin (Imperial College London), L.M. Herz (University of Oxford), G. Frey (Technion), A. Köhler (University of Bayreuth), C. Draxl (Humboldt Universität Berlin), R. Janssen (TU Eindhoven), J. Wilson (Holst Center), W. Kowalski (InnovationLab), N. Banerji (Universite de Fribourg), S. Hayes (University of Cyprus), H. Bolink (Universitat de Valencia), Interfaces in opto-electronic thin film multilayer devices (INFORM), Horizon 2020 ITN Network (Call: H2020-MSCA-ITN-2015, Topic: MSCA-ITN-2015-ETN, Action: MSCA-ITN-ETN, Proposal Number: 675867) (1 Sept 2015 – 31 Aug 2019)

* L.M. Herz (with L. Schmidt-Mende, University of Konstanz), Perovskite nanowire films for semi-transparent photovoltaics, Royal Society International Exchange Scheme (1 July 2015 – 30 June 2017)