Understanding the electronic structure landscape in wide band gap metal halide perovskites

To mitigate the worst impacts of climate change there is an immediate global need for clean, secure, and efficient energy generation. Solar energy generation through photovoltaics has the potential to produce the electricity required for a growing population which is cheap and produces significantly lower carbon emissions than conventional power generation technologies. Metal halide perovskites (MHPs) have emerged in recent years as an exciting new photovoltaic technology both on their own and in combination with exciting solar technologies in tandem photovoltaics. The latter, wherein two or more solar cells are coupled to overcome thermodynamic limits, will produce photovoltaics with power conversion efficiencies > 30 %. As the implementation of photovoltaics hinges on their cost and this is directly related to their power conversion efficiencies, tandem photovoltaics offer considerable potential for lowering the cost of photovoltaic implementation.

Wide band gap MHPs (> 1.7 eV) are essential for perovskite tandem photovoltaics however they have undergone less investigation than their narrow band gap counterparts resulting in their performance being constrained far below the theoretical limit. There is a lack of understanding of the electronic structure of these materials which translates directly into significant voltage losses in photovoltaic devices. This has led to qualitative selection of charge transport layers in device design and an inability to fine tune the energy level alignment at these interfaces. The difficulty in understanding these systems is due, in large part, to the chemical complexity of these systems. The highest performing wide band gap MHPs are mixed cation, mixed halide systems and their complexity makes studying their electronic structure incredibly challenging.

To overcome these challenges this project will build a holistic overview of the electronic structure of wide band gap MHPs, using experimental model systems and ab-initio materials modelling to create theoretical models which can be used to understand experimental measurements of device relevant systems. Single crystals of MHPs will be used as model systems and studied using photoelectron and photophysical spectroscopies to probe their electronic structure. High resolution atomic force microscopy techniques will be adapted to non-destructively image the structure of the crystals and provide information on the structural defects present on the surfaces. Together these measurements will facilitate the development of models which will provide new insights into the fundamental electronic structure of MHPs. These models will be applied to photovoltaic device architectures to determine the origin of voltage losses and a quantitative series of recommendations for overcoming these losses will be produced. Understanding the electronic structure of MHP materials is critical in ensuring the successful commercialisation of technologies based on these exciting new semiconductors. This research will not only facilitate the continued development of photovoltaics based on these materials but will contribute to our fundamental understanding of this new class of semiconductor materials.