From Large to Small Polarons in Lead, Tin and Mixed Lead-Tin Halide Perovskites

The origin of the long carrier-lifetime in Lead halide perovskites is still under debate and, among different hypothesis, the formation of large polarons preventing the recombination of charge couples is one of the most fascinating. In this work, using state of the art ab initio calculations, we report a systematic study of the polaron formation process in metal halides perovskites focusing on the influence of the chemical composition of the perovskite on the polaron properties. We examine variations in A-site cations (FA, MA, Cs and Cs:MA), B-site cations (Pb, Sn and Pb:Sn) and X-site anions (Br, I). Our study confirms the stronger structural distortions occur for Cs than for MA and FA, with the effect of different A-site cations being almost additive. For the same A-cation, bromide features stronger distortions than iodide perovskites. The pure Sn phase has an almost double polaron stabilization energy than the pure Pb phase. Surprisingly, the trend of polaron stabilization energy is non monotonic in mixed Sn:Pb perovskites, with a maximum for small Sn percentages. Polaron formation is found to be promoted by bond asymmetry, ranging from small to large polarons in mixed Sn:Pb perovskites depending on the relative Sn percentage.