This project explores epitaxially grown tin (Sn) perovskites asalternatives to lead (Pb) based materials. Importantly, Sn perovskiteshave a narrow bandgap that is in the ideal range for a single-junctionsolar cell, which will also enable all-perovskite multijunction solarcells. In addition, they are more environmentally friendly. However, sofar power conversion efficiencies of Sn perovskites are falling short totheir lead counterparts. One of the most critical obstacles to overcomeis the tendency of Sn2+ to oxidize to Sn4+. Here, we will employepitaxial growth methods to avoid solvents that promote chemicalreactions leading to oxidation of Sn and we will characterize these Snperovskites down to the atomic level. Specifically, we willexperimentally evidence the atomic structure of Sn perovskitesurfaces such as CH3NH3SnI3 to localize Sn4+ defects andunderstand interface phenomena that readily occur in perovskite solarcells. Furthermore, we will add dopants and adsorbates, such asenvironmental gas molecules to the surfaces, to fundamentally studythe specific interactions that occur on the atomic scale with respect toperformance enhancements, degradation, and oxidation. To bridgethe size gap to the application, we will use surface techniques on themicrometer scale probing large-scale inhomogeneities, grainboundaries, workfunctions, contact potential differences and surfacephotovoltages. A full understanding on a device level necessitates thefabrication of Sn-based perovskite solar cells using state of the artsolution-processing as a benchmark. Then we will apply the optimizedarchitectures and use the knowledge from the nanoscopic andmicroscopic characterizations as well as epitaxially grown Snperovskite absorbers to fabricate novel Sn based solar cells that arehighly efficient and long-term stable. We believe that the correlationbetween atomic, micro- and macroscale on the same type of sampleswill be particularly fruitful to gain a thorough understanding of Snperovskites.