Unravelling halide segregation in hybrid perovskites for Si tandem photovoltaics

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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.