Metal halide semiconductors: materials discovery beyond ABX3 perovskites

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.