Below is a list of DPhil projects for 2022 in semiconductor materials, devices and nanostructures; please address all informal enquiries to the named supervisor.

Optoelectronic properties of hybrid halide perovskite semiconductors

Marina Filip

Hybrid organic-inorganic perovskites have recently emerged as highly promising materials for a variety of optoelectronic devices, including solar cells and LEDs. They can be easily synthesized in a variety of chemical compositions, structures and dimensionalities through inexpensive solution processing methods, allowing for a broad tunability of their optoelectronic properties. However, while the materials space of hybrid organic-inorganic perovskites is continually expanding, their optoelectronic properties and structure-property relationships are not yet fully understood. In this project, we will study the electronic and optical properties of these novel materials from an atomistic perspective, using first-principles computational modeling methods, such as density functional theory (DFT), the GW method, and the Bethe-Salpeter equation (BSE).

First principles, or ab initio methods, are implemented in massively parallel computational packages which are highly optimised to operate efficiently on large supercomputing architectures, and tackle complex many-body problems in functional materials. As part of this project, we will use ab initio methods to understand many-body phenomena in complex semiconductors and insulators, and develop computational frameworks for calculating the optoelectronic properties of these materials accurately and efficiently.

Eligible candidates should have a Bachelor or Master degree in Physics, or related subjects, such as Chemistry or Materials Science. It is essential to have a background in Quantum Mechanics and Solid State Physics, as evidenced by your transcripts. Programming skills, experience with electronic structure codes, as well as any prior research experience are desirable but not essential.

High throughput computational design of novel semiconductors for optoelectronic applications

Marina Filip

In the past decade, a tremendous mobilisation of efforts in the Materials Science community has been directed towards the discovery of novel functional materials, to enable the development of new technologies. The task of designing a material that has never before been observed in real life requires the ability to predict how a certain arrangement of distinct chemical elements in the periodic table will react to form a material, and what physical properties this material might have. First principles computational materials modelling techniques are ideally suited for this task, because they rely on an atomistic perspective to predict materials properties. In this project we will use first principles methods to understand the fundamental physical properties of semiconductors and insulators, and design automated strategies to screen through a great variety of chemical compositions and structures in the search for semiconductors which are chemically stable, and have useful properties for optoelectronics. We will initially focus our attention on the heterogeneous and chemically diverse family of perovskites [PNAS, 115, 21, 5397-5402 (2018)]. First principles, or ab initio methods, are implemented in massively parallel computational packages which are highly optimised to operate efficiently on large supercomputing architectures. We will use and develop upon these techniques, as well as use materials databases, such as the Materials Project, to computationally design and discover new semiconductors and insulators.

Eligible candidates should have a Bachelor or Master degree in Physics, or related subjects, such as Chemistry or Materials Science. It is essential to have a background in Quantum Mechanics and Solid State Physics, as evidenced by your transcripts. Programming skills, experience with electronic structure codes, as well as any prior research experience are desirable but not essential.

Charge generation dynamics in novel materials for solar cells

Laura Herz

Metal halide pervoskites have emerged as an extremely promising photovoltaic (PV) technology due to their rapidly increasing power conversion efficiencies (PCEs) and low processing costs. Surprisingly, many of the fundamental mechanisms that underpin the remarkable performance of these materials are still poorly understood. Factors that influence the efficient operation of perovskite solar cells include electron-phonon coupling, charge-carrier mobility and recombination, light emission and re-absorption, and ion migration.

During this project we will advance the efficiencies of perovskite solar cells by gaining an understanding of fundamental photon-to-charge conversion processes using a combination of ultra-fast optical techniques, e.g. photoluminescence upconversion and THz pump-probe spectroscopy. These studies feed directly into collaborative efforts aimed at addressing remaining challenges in the in the creation of commercially available perovskite solar cells e.g. stability, band-gap tunability, lead-free perovskites, trap-free materials, material morphology control and alternative device structures. The project will be part of active collaboration with researchers working on solar cell fabrication within Oxford and the UK.

Transitions from quantum confined to fully delocalised electronic states in semiconductor nanocrystal assemblies

Laura Herz

The last decade has seen rapid progress in the fabrication and assembly of nanocrystals into thin layers of semiconducting material. Such systems may allow facile deposition of high-quality inorganic semiconductor layers through simple and scalable protocols such as ink-jet printing. However, these procedures raise fundamental questions on the nature of charge transport through such layers. While in sufficiently small nanocrystals, quantum confinement leads to the formation of discrete electronic layers that may exhibit "atom-like", energetically discrete states, increasingly electronic coupling between nanocrystals may induce the formation of mini-bands or bulk-like continuum states.

In this project, we will explore such transitions between fundamentally different regimes of electronic coupling and charge transport. We will spectroscopically investigate nanocrystal networks made of established lead chalcogenide inorganic semiconductors, but also explore more recently developed metal halide perovskite colloid materials. These studies will be interesting not only from a fundamental point of view, but also allow for development of such systems in light-emitting photovoltaics or transistor devices.

Unveiling electron motion at surfaces and interfaces on ultrashort length and ultrafast time scales

Michael Johnston

Over the course of the project we will develop and implement a new instrument based on our recent advances in terahertz photoncs. The instrument will enable us to gain a deep understand nanoscale charge dynamics in metal halide perovskite semiconductors and semiconductor nanowires. The advances made will contribute to our active existing research programme in developing efficient multijunction solar cells.

Terahertz photonics with semiconductor nanostructures

Michael Johnston

In this project you will develop novel photonic devices to enable a powerful form of ultrafast femtosecond spectroscopy at terahertz frequencies. The photonic devices will be based on ‘nanowires’, which are single crystals of semiconductors, with diameters of only tens of nanometre, but lengths of many microns. Owing to their geometry these nanowires have properties ideal for new and novel device applications. In particular the large surface area to volume ratio of nanowires allows these single crystals be grown in crystal structures that are not possible in the bulk forms of the materials, and allows for unusual light-matter interactions. The large surface area also makes nanowires ideal for applications such as chemical sensing and catalytic conversion.

During your DPhil the novel properties of nanowires will be exploited for spectroscopy at terahertz frequencies. The terahertz region of the electromagnetic spectrum encompasses a wide range of frequencies from the upper bound of microwave band to the lower bound of infrared light. The spectral region contains a wealth of spectroscopic information for a wide range of physical systems, with THz photons covering the characteristic energy scales of phonon, plasmons and excitons in semiconductors, and the correlations in solids that lead to phenomena such as superconductivity and magnetism.

In this project you will develop THz detectors and modulations based on nanowires, and implement them in state-of-the-art THz spectroscopy systems. You will also have the opportunity to exploit these new devices for investigating charge carrier dynamics in other novel semiconductors.

Ultrafast terahertz polarimetry

Michael Johnston

Single cycles of electromagnetic radiation are the ultimate tools for investigating light-matter interactions. According to the Heisenberg uncertainty principle (or indeed Fourier theory) a pulse of light very well localised in time will have a very broad frequency spectrum, thus single-cycle pulses are great tools for time-resolved spectroscopy. The spectral range of interest for many phenomena in Condensed Matter Physics is the terahertz frequency range, which corresponds to photon energies of ~1meV-20 meV. This is the energy range of the spectral features of charge transport in semiconductors, as well as the energies of quasiparticles associated with correlations, such as superconductor cooper-pair binding energies, phonon energies and exciton binding energies.

In this project you will utilise our recent development of cross-nanowire THz detectors (Science, 368:510--513, 2020) to examine the polarisation response of THz metamaterials, and the physics of semiconductors and magnetioc thin films via the THz Hall effect and Inverse Spin-Hall Effect respectively. You will also have the opportunity develop new THz devices as part of this project.

Vapour deposition of perovskite solar cells

Michael Johnston

Metal halide Perovskite (MHP) solar cells have emerged as promising semiconductor devices for next generation photovoltaics. Remarkably, the power conversion efficiency of single-junction solar cells has reached >25%. Efficient tandem solar cells based on perovskites have also recently been achieved. To date most research into MHP solar cells has focussed on solution processing, however this technique is challenging for multi-junction devices. This project will focus developing on highly efficient multijunction solar cells, using a vapour co-deposition technique.

The project will involve designing evaporation chamber components optimised of metal halide perovskite deposition, and devising new layer growth methodologies. The candidate will also gain experience in solar cell characterisation and a range of spectroscopy techniques.

Understanding the fundamental efficiency limits of organic solar cells

Moritz Riede

The first wave of products using organic semiconductors has very successfully entered the market: organic light emitting diodes (OLED) are used in the displays of many mobile phones and TVs, featuring brilliant colours etc.. Key to their commercial success are vacuum processing of small organic molecules under precise control into well-defined multilayer stacks and the use of molecular doping, i.e. the modification of a semiconductor's properties by a controlled addition of "impurities". Both concepts are much less used in organic solar cells (OSC), but it can be applied here with similar benefits, enabling a inexpensive, efficient, light-weight and flexible renewable energy source made from earth abundant non-toxic raw materials. Best proof of this is that the current technology leader, the start-up Heliatek, is using both concepts.

There has been a significant improvement of power conversion efficiencies (PCSs) over the past years, enabled by novel materials, in particular non-fullerene acceptors, and record OSC achieved PCEs of ~17% by now. Previously unthinkable PCEs of >20% are within reach, which would get close to the main competing technology of silicon solar cells (~26%). For this, some loss mechanisms have to be overcome. The main loss mechanism are high energy losses at the open circuit voltage, of which much remains unclear. The goal of this DPhil is to get experimental access to the exact location in the OSC, i.e. the donor-acceptor interface, where these losses take place, characterise them and subsequently find ways to modify and control these processes. Only then will it be possible to significantly reduce these losses and to demonstrate PCEs exceeding 20% for OSC, i.e. a technology that in principle can be scaled to the required terawatt.

For the preparation of the samples we rely on vacuum processes, similar to those that are used for the production of OLEDs. To arrive at a better understanding of the loss processes, an extensive range of experimental methods to characterise the optical and electronic behaviour of our samples will be used. In this research, we will be collaborating with other groups in Oxford and with international experts from eg Canada, Germany and Switzerland.

In-situ microstructural characterisation of organic solar cells

Moritz Riede

The solar cells investigated in this project have the potential to allow us to harness the power of the sun clean and at cost lower than coal everywhere we are and go, even in the UK. They can be made, for example, flexible such they can be rolled up like a newspaper, and could become much cheaper than existing solar cell technologies. To achieve this, our solar cells are not based on silicon, the material of most solar cells currently sold, but on organic semiconductors.

The electrical and optical properties of organic solar cells (OSCs) and as result the performance critically depend on the molecular arrangement of the organic semiconductors and the domains they form in the thin organic layers used to absorb the light. This initial microstructure forms during the deposition process, can be tuned post-deposition and can evolve during the operation of the OSC.
The goal of this project is to work with and expand the capabilities of our vacuum deposition system we have recently installed at I07 (X-ray scattering) at Diamond (see DOI: https://dx.doi.org/10.1063/1.4989761), as well as our a currently developed setup at ISIS (neutron scattering). Combining these techniques along with extensive optoelectronic characterisation in Oxford allows to tackle questions about initial film formation, the effect of post-deposition treatments as well as the evolution of microstructure over time. Achieving and maintaining a favourable microstructure is crucial for many processes in OSCs. Thus, we expect that the results will improve our understanding of the OSC device physics and lead to improved device performance, which will be critical on the route towards commercialisation.

The work will be carried out in a well established collaboration between Diamond, ISIS and Oxford University, bringing together expertise in microstructural characterisation of these organic films, device fabrication and OSC device physics.

All-perovskite multi-junction solar cells

Henry Snaith

Multi-junction perovskite solar cells promise to deliver much higher efficiency than existing PV technologies. However, many challenges exist in terms of development of absorber materials with the appropriate band gaps, developing the correct device structure with multiple layers of different semiconductor materials, and understanding optoelectronic processes occurring in the materials and at the interfaces. This PhD student will work on broad challenges associated with improving the efficiency and stability of these solar cells, and make use of the new EPSRC National Cluster Facility for Advanced Functional Materials.

Enhancing the long term stability and optoelectronic quality of metal halide perovskite semiconductors and optoelectronic devices

Henry Snaith

The project will focus upon understanding and controlling the crystallisation of metal halide perovskites in order to deliver improved optoelectronic quality of the materials. This may include fewer defects, higher charge carrier mobility, longer chare carrier lifetime and ultimately improved performance in optoelectronic devices. The materials developed will be investigated by a range of microscopy and spectroscopic techniques and integrated into functional electronic devices including solar cells and light emitting diodes. The overall aim of the project is to deliver improved functionality and enhanced longevity of the materials and devices, whilst also expanding our understanding of the basic principles which govern both optoelectronic operation and degradation under environmental stressing conditions.

Perovskite light-emitting diodes

Henry Snaith

As well as being recognised as outstanding materials for solar energy conversion, metal halide perovskites can be tuned to be highly emissive, and emit light in the required visible emission channels for displays. There is much effort on developing perovskites as phosphors, where they absorb blue light from a conventional GaN LED, and reemit green of red light as desired. However, there it is also possible to create highly efficient LEDs, where charge carrier injection in a diode structure results in the emission of blue, green or red light. The efficiency of these LEDs have already matched commercial OLEDs. However, the long-term operational stability requires orders of magnitude improvement in order to match the tens to hundreds of thousands of hours required. This project will focus upon understanding instabilities in perovskite LEDs and devising novel routes to enhance the long-term stability. Research approaches can include materials chemistry, device physics and advanced spectroscopies, dependent upon the capability and desires of the preferred candidate.

We intend to secure a CASE studentship for this project, in partnership with Helio Display Materials.