Epitaxially grown tin perovskites

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This project explores epitaxially grown tin (Sn) perovskites as alternatives to lead (Pb) based materials. Importantly, Sn perovskites have a narrow bandgap that is in the ideal range for a single-junction solar cell, which will also enable all-perovskite multijunction solar cells. In addition, they are more environmentally friendly. However, so far power conversion efficiencies of Sn perovskites are falling short to their lead counterparts. One of the most critical obstacles to overcome is the tendency of Sn2+ to oxidize to Sn4+. Here, we will employ epitaxial growth methods to avoid solvents that promote chemical reactions leading to oxidation of Sn and we will characterize these Sn perovskites down to the atomic level.Specifically, we will experimentally evidence the atomic structure of Sn perovskite surfaces such as CH3NH3SnI3 to localize Sn4+ defects and understand interface phenomena that readily occur in perovskite solar cells. Furthermore, we will add dopants and adsorbates, such as environmental gas molecules to the surfaces, to fundamentally study the specific interactions that occur on the atomic scale with respect to performance enhancements, degradation, and oxidation. To bridge the size gap to the application, we will use surface techniques on the micrometer scale probing large-scale inhomogeneities, grain boundaries, workfunctions, contact potential differences and surface photovoltages. A full understanding on a device level necessitates the fabrication of Sn-based perovskite solar cells using state of the art solution-processing as a benchmark. Then we will apply the optimized architectures and use the knowledge from the nanoscopic and microscopic characterizations as well as epitaxially grown Sn perovskite absorbers to fabricate novel Sn based solar cells that are highly efficient and long-term stable. We believe that the correlation between atomic, micro- and macroscale on the same type of samples will be particularly fruitful to gain a thorough understanding of Sn perovskites.