This is an extension of the Fellowship: 'Non-equilibrium electron-ion dynamics in thin metal-oxide films' (EP/K003151/1). The development of low-cost high-efficiency solar cell devices would allow us to make more use of the vast amount of free and clean energy available in sunlight. Materials which absorb light to generate energetic electrons in solar cells are known as solar absorbers. In current consumer level solar cells the solar absorber is crystalline silicon. Silicon based cells exhibit high efficiencies (~25%) but are relatively expensive to produce. For example, it currently takes about 14 years of operation for a typical 4 kW domestic installation to break even (e.g. see http://www.theecoexperts.co.uk/are-solar-pv-panels-good-investment). Driven by the desire to reduce cost there has been a continued focus on the development of new high-efficiency solar absorber materials that are less expensive to manufacture than silicon to form the basis of next generation solar cell technologies. A general trend in materials development has been the progression from silicon towards more complex binary, ternary and quaternary compound semiconductors, which offer a wider compositional and structural parameter space within which desired properties can be optimised. Highly performing examples include CuInGaSe2, CdTe, Cu2ZnSn(S,Se)4 (CZTS) and lead-halide perovskites (e.g. CH3NH3PbI3, MAPI). Unlike silicon these emerging materials often contain relevantly high concentrations of point defects since they are almost always non-stoichiometric. They are also usually polycrystalline and grain boundaries (together with associated point defects) are known to affect material performance by contributing to non-radiative electron-hole recombination and reduction of open circuit voltage (both effects that reduce efficiency). While predictive computational materials screening approaches have proved invaluable in helping to identify promising solar absorber materials there are currently no screening approaches that consider the properties of grain boundary defects. This proposal aims to fill this critical gap in the materials modelling toolbox by developing systematic approaches to screen materials against the thermodynamic and electronic properties of grain boundaries. These approaches will be applied to identify optimal compositions and dopants for CdTe, lead-halide perovskites and CZTS materials to help optimise performance and accelerate innovation. We will work closely with experimental collaborators and our industrial partner (Dyesol) to validate theoretical models and test predictions in order to deliver improvement in solar cell performance. The computational screening approaches we develop will also be made available to the wider materials modelling community and will find application in many other areas where the electronic properties of grain boundaries impact on material performance (including thermoelectrics, batteries, photoelectrochemical cells, varistors, transparent conducting oxides and dielectrics to name a few).

My vision is to enable reliable large-scale manufacturing of novel advanced organic or hybrid organic/inorganic materials which have complex three-dimensional structure. An advanced material is one with new properties that allows companies to develop novel high-value products to meet market needs, and in doing so generate growth and high-technology exports. Cutting-edge manufacturing is key to wealth creation in the UK. The UK cannot compete in the low technology (commodity) materials sector: these are now manufactured in countries with low cost labour markets. To manufacture an advanced material, we have to understand its structure in detail. This means being able to observe and measure it over many length scales (nanometres to millimetres), and then use that information to understand its physical characteristics. Once we have understood how to create a material in the laboratory setting, the next challenge is to scale-up processing capability. Often the manufacturing process itself has a big impact on the microscopic structure of the material, and hence its physical properties. This leads to a development cycle. To maintain desirable properties, process variables are changed, informed by predictive modelling and re-examination of the microscopic structure. The aim is to identify process steps that critically impact on the product output capacity and reliability. This project will work directly with industrial partners to use novel ways of discern microscopic structure so as to inform the product development cycle. The industrial partners are both large UK firms with interests in the energy sector: one working on developing polymer components for energy storage; the other working on up scaling process technologies for new types of low cost solar cells. For both materials systems, application performance success hinges on complex hierarchical structures. Scientists and engineers have realised that is often not only the material itself, but the way different structural arrangements, each at a different scale, interact with one another. As well as studying materials of immediate commercial application, this project also aims to harvest the information contained in very similar natural materials which also have complex hierarchical structures (spider silk in particular). Prior development of this class of polymers has been hampered by the absence of measurement instruments and methods capable of accurately observing their composition and complex structure. I aim to refine a new type of electron microscopy that I have developed in order to measure, from the scale of nanometres to millimetres, soft-matter properties that define their electrical and structural performance. This will be tailored to the particular needs of my industrial collaborators, but the technique will also have much wider application. For example, I will also use my method to try to unlock the exact structural mechanisms that are found in the natural material silk - which has extraordinary properties as yet it is not understood how to retain these in the man-made equivalent. With the support of a visiting civil engineering expert who has developed scalable mechanical models for complex hierarchical structures, I aim to build a scalable model that will help to predict the link between process parameter variation and resulting materials properties. This will be informed using my new characterisation method. Finally, in the light of the results from the research, I hope to pool the knowledge gained from both the industrial and academic partners to formulate a more general understanding of the development cycle for these technologically and economically important class of materials.

High surface area nanoporous films formed by sintering metal oxide nanoparticles are highly stable, non-toxic and inexpensive to produce on an industrial scale. They find a wide range of applications in gas sensing and catalysis where high surface area is essential to maximise the interaction of molecules with the film. They also find applications as charge transport layers in third generation solar cells, e.g. dye- or perovskite-sensitised cells, where efficient photoinjection of electrons and holes is ensured by coating nanoporous films with a light absorbing material. For solar cells, as well as for other important applications of nanoporous films such as electrodes in fuel cells and photoelectrochemical cells, good charge carrier mobility is also an essential requirement. Unfortunately, despite their numerous advantages, the electronic mobility of nanoporous oxide films is in general very poor. For example, the mobilities of nanoporous TiO2, ZnO and SnO2 films have been shown to be between two and four orders of magnitude smaller than those of corresponding single crystals. This low mobility is a key factor limiting the efficiency of (photo-)electrochemical and photovoltaic applications and is usually attributed to increased charge carrier trapping at surfaces and at interfaces between nanoparticles. Since charge trapping is associated with ions near surfaces we hypothesise that it should be possible to eliminate these traps by suitable chemical modification of the surfaces of nanoparticles prior to sintering into a film. This approach would retain the advantages of nanoporous films in terms of high surface area, non-toxicity and processability while improving mobility. Such modifications have been attempted previously, but due to the lack of understanding on the origin of charge trapping or the effects of surface modification, success has been limited. Here, we propose to combine the predictive power of first principles theoretical modelling with structural, spectroscopic and photophysical materials characterisation, in order to quantify the factors responsible for charge trapping at surface and interfaces in nanoporous oxide films at an atomistic level. Once validated and refined on unmodified films, theoretical methods will be used to assess modification strategies to reduce charge-trapping. In particular, we will consider the incorporation/substitution of anions and cations near the surface of oxide nanoparticles to eliminate the problematic trapping sites. The ability to theoretically screen various possible modification routes (i.e. different cations and anions) is a key advantage of our proposed approach. Application, testing and optimisation of such strategies may offer a new paradigm for knowledge-led design of solar oxide materials. We aim to demonstrate the effectiveness of our approach by increasing the mobility of nanostructured TiO2 and ZrO2 to deliver an improvement in the efficiency of perovskite-sensitised solar cells, which are emerging as an attractive third generation photovoltaic technology. The size of the third generation photovoltaic market is predicted to grow to $38bn by 2022, making this an area with significant potential for economic impact. Improving the mobility of nanoporous oxides could bring the efficiency of these devices from their current level (about 20%) to closer to the theoretical maximum of about 30%. An increase in overall efficiency from 20% to only 23% percent would increase the total power output by 15%, which when coupled with lower manufacturing costs would make the technology very attractive. We will work with leading manufacturers of nano-TiO2 (Cristal) and perovskite-sensitised solar cells (Dyesol Limited) to test the performance of our modified films. More generally, the ability to tailor the electronic properties of interfaces in nanoporous films by controlled modification should find applications in other technologies including sensing, catalysis and electronics.

Photovoltaic (PV) devices convert sunlight directly into electricity and form an increasingly important part of the global renewable energy landscape. Today's PVs are based on conventional semiconductors which are energy-intensive to produce and restricted to rigid flat plate designs. The next generation of PVs will be based on very thin films of semiconductors that can be processed from solution at low temperature, which opens the door to exceptionally low cost manufacturing processes and new application areas not available to today's rigid flat plate PVs, particularly in the areas of transportation and buildings integration. The emerging generation of thin film PVs also offer exceptional carbon dioxide mitigation potential because they are expected to return the energy used in their fabrication within weeks of installation. However, this potential can only be achieved if the electrode that allows light into these devices is low cost and flexible, and at present no electrode technology meets both the cost constraint and technical specifications needed. This proposal seeks to address this complex and inherently interdisciplinary challenge using three new and distinct approaches based on the use of nano-structured films of metal less than 100 metal atoms in thickness. The first approach focuses on the development of a low cost, large area method for the fabrication of metal film electrodes with a dense array of holes through which light can pass unhindered. The second approach seeks to determine design rules for a new type of 'light-catching' electrode that interacts strongly with the incoming light, trapping and concentrating it at the interface with the semiconductor layer inside the device responsible for converting the light into electricity. The final approach is based on combining ultra-thin metal films with ultra-thin films of transparent semiconductor materials to achieve double layer electrodes with exceptional properties resulting from spontaneous intermixing of the two thin solid films. The UK is a global leader in the development of next generation PVs with a growing number of companies now focused on bringing them to market, and so the outputs of the proposed programme of research has strong potential to directly increase the economic competitiveness of the UK in this young sector and would help to address the now time critical challenge of climate change due to global warming.