There is currently a pressing global need to reduce emissions of carbon dioxide, and at the same time satisfy the world's growing desire for cheap electricity. Solar cells, which directly convert the Sun's radiation into electricity, offer a realistic method of generating electricity sustainably, on a large scale and at costs similar to and even lower than more polluting conventional forms of power generation (coal, gas, nuclear). Over the past few years a new class of solar cells based on metal-halide perovskite semiconductors has emerged. Power conversion efficiencies for these materials have increased at an unprecedented rate for a new photovoltaics material and now exceed 20%. An intense worldwide research effort into these materials is now underway; however nearly all research is focussed on solution processed perovskites, and most highly efficient solar cells are small area devices not suited to large area deployment. In this project we will build on our early lead in the area of vapour deposited perovskites to develop highly efficient large area perovskite solar cells. Our evaporation technique offers superior film uniformity over large areas and is highly reproducible as compared with more common solution processing methods. Using the vapour deposition route we will develop all-perovskite tandem and multi-junction solar cells to further improve the efficiency for these remarkable devices. We utilise the recently funded EPSRC "National thin-film cluster facility for advanced functional materials" to adapt our advances in perovskite materials and device technologies to current industrial thin-film production methods.

A major global challenge of the present epoch is transforming our energy system to become clean, secure and efficient. A major challenge for the UK is ensuring industrial leadership in low-carbon energy technologies, which will dominate the future energy market, and "securing the economic benefits of the transition to a low-carbon economy". In this prosperity partnership, we have uniquely combined pioneering academic and industrial leaders in perovskite photovoltaics and will develop the underlying materials, science and technology, which will allow us to develop the next generation of multi-junction perovskite solar cells. The ambition of the project is to go well beyond the state-of-the-art, and therefore deliver over 37% efficient triple junction perovskite solar cells. This will be possible through a combined effort of new materials development, fundamental investigations, thin-film device engineering and interface modification, and significant effort on understanding and improving materials and device stability. The major technical outputs of the project will be to deliver technology at three different stages, for beyond project downstream development and manufacturing.

Climate change and energy security are some of the greatest challenges to be faced by mankind over the coming century. Renewable sources of energy and increases in energy efficiency are key solutions that will allow the world to maintain and enhance its current level of prosperity. Photovoltaic cells, in particular, allow large-scale, sustainable generation of electricity: the solar energy incident on the surface of the earth in one hour is enough to provide the whole world's current annual energy requirements. In addition, light-emitting diodes for solid-state lighting can significantly reduce the power demand for lighting, but still require further improvements in cost per given quality of light. Further advances in these fields rely crucially on the discovery and development of new semiconducting materials that can efficiently turn light into electricity, and vice versa. The relatively recent use of hybrid metal halide perovskite semiconductors in photovoltaic and light-emitting devices has been particularly exciting here. These materials now deliver solar cells with power conversion efficiencies exceeding 25% for single-junction thin-film cells (close to the thermodynamic limit of 30%), and efficient light-emitting diodes. However, some issues remain with this current class of ABX3 metal halide perovskites, including toxicity of lead which is incorporated in the highest performing materials, as well as long-term material stability, and stable band-gap tunability, required for higher efficiency tandem solar cells and colour-tunable light emission. Therefore, the discovery of a new catalogue of semiconductors which overcome such issues would be extremely exciting at this point. This research programme will enable the discovery of new semiconductors within the broader class of metal-halide compositions (beyond the now well-established group of ABX3 perovskites) which is still unexplored to a surprising extent. New materials discovery will be enabled by a closely-knit feedback loop based on the complementary and world-leading expertise portfolios of the four co-investigators, encompassing computational modelling and prediction, materials synthesis, thin-film fabrication and passivation and combinatorial spectroscopic characterization. These activities will evolve in three well-defined strands, focusing on computational design, materials synthesis and processing, and experimental assessment of critical material properties. These strands will be carried out in parallel, will be exceptionally well interlinked, and evolve as part of a feedback loop in which any new finding in one strand will feed highly useful information into the other two strands. This co-ordinated effort will allow us to turn discovery of new semiconductors from the current slow, trial-and-error, needle-in-a-haystack search into a rapid, targeted and systematic exploration of a vast group of potential candidate materials. Such directed discovery will unearth a new library of high-performance materials, given that the currently available materials are likely to be just the tip of the iceberg of actually available, but as yet undiscovered semiconductors.

Substantial manufacturing-cost reductions in mainstream silicon-wafer (c-Si) based solar cell technologies have recently been achieved mainly due to savings through economy of scale. Hence a recent forecast for the future large-scale use of photovoltaics predicts that solar energy will contribute nearly a third of newly-installed electricity generation capacity worldwide between now and 2030. To reach this goal however and to assure a widespread deployment of PV, the cost for PV-generated energy still needs to be further reduced. A large fraction of the cost of solar power is not the modules themselves, but the fixed costs of frames, inverters, installation and land, which is termed the balance of systems (BOS). The BOS is not reducing in price as fast as the module costs, hence the only sure means to continue the downward drive in the cost of PV is to enhance the absolute efficiency of the modules, without overtly increasing their cost. The key aim of this project is to realise highly efficient hybrid tandem solar cells with high stability. The specific target is to achieve a power conversion efficiency of over 25% when integrating a wide band gap perovskite solar cell with a crystalline silicon solar cell. A solar cell is composed of a light absorbing photoactive material as the main component which generates electrical current. But this layer is contacted by multiple further materials to ensure efficient charge extraction and high voltage generation in the solar cell. Our philosophy is to undertake an extremely focussed project, employ as many existing proven materials as possible, apart from the perovskite absorber layer, and integrate them judiciously within the perovskite-Silicon tandem solar cells. This will minimise the risk, and maximise the possibility of delivering an entirely stable tandem solar cell. In the process of doing so, and throughout the investigations, we will create highly efficient bifacial perovskite solar cells (which can receive light illumination from both sides) and enhance our understanding of the fundamental mechanisms occurring at the junctions between the perovskite and the charge collection layers. The project is extremely timely, since the perovskite solar cells are already at the appropriate efficiency to enhance existing PV in a tandem configuration, provided effective integration into a tandem structure can be achieved. In addition, much progress on the overall stability of the perovskite solar cells and large area processing has already been achieved, making it highly likely that the output of this project will be transferred directly into a commercial product.

Renewable energy sources offer exciting opportunities to address challenges caused by energy security and climate change. Photovoltaic (PV) cells in particular can enable sustainable generation of electricity on a large scale: the solar energy incident on the surface of the earth in one hour is enough to provide the whole world's current yearly energy requirements. As an exciting newcomer to the PV landscape, organic-inorganic metal halide perovskites now show certified power conversion efficiencies for single-junctions thin film solar cells in excess of 22%. The best performing single-junction cells are currently all based on lead iodide perovskites with A-PbI3 formula, where the cation A is typically methylammonium (MA), formamidinium (FA), Caesium (Cs) or a mixture thereof. Many analysts in the renewable energy sector believe that the most effective commercialisation of these novel perovskites is in combination with existing, well-established silicon technology. Here, a perovskite thin-film cell is combined with a silicon cell in a 2- or 4-terminal tandem cell, boosting efficiency at small additional cost. For optimised tandem architectures, the photocurrents created by each cell need to be balanced, which requires a perovskite with band gap near 1.75eV, significantly above the typical bandgap of ~1.5eV displayed by the established A-PbI3 materials. To date, the only high-performance perovskite thin-film materials ideally matched for tandem applications with silicon are based on the A-Pb(Br_x I_(1-x))3 system, which allows band gap tunability from ~1.5 to ~2.2eV when the bromide content is varied between x=0 (iodide only) and x=1 (bromide only). However, the mixed halide perovskites are affected by an instability whose origin mystifies researchers. When illuminated with visible light, the material segregates spontaneously into iodide-rich and bromide-rich domains. This effect is transient, and recovers in the dark over the timescale of minutes. For photovoltaic applications, the potential voltage shifts and charge trapping associated with this effect are highly detrimental to the aim of stable PV operation. Recent research at Oxford and in the international research community has shown that materials can sometimes be stabilized through choice of A-cation and enhanced crystallinity. However, photo-stability was found to depend sensitively on processing conditions, with instability recurring when protocols or environmental conditions were varied. These incipient studies suggest that the photo-induced halide segregation is not as such intrinsic and therefore can be remedied, but a global picture of how this can be done remains elusive. Our programme will identify the causes underlying this effect and pioneer new materials that are photo-stable over projected solar cell life spans. We will achieve these aims through a novel programme that brings together a team of world-leading investigators with complementary skills in photovoltaic materials and devices, advanced spectroscopy and high-resolution electron microscopy, and in-situ crystal structure analysis. The outcomes of this programme will enable the development of long-term photo-stable, fully optimized materials for use in tandem cells with established silicon photovoltaic technology.