Mechanism of Tin Oxidation and Stabilization by Lead Substitution in Tin Halide Perovskites

The recent development of efficient binary tin- and lead-based metal halide perovskite solar cells has enabled the development of all-perovskite tandem solar cells, which offer a unique opportunity to deliver high performance at low cost. Tin halide perovskites, however, are prone to oxidation, where the Sn 2+ cations oxidize to Sn 4+ upon air exposure. Here, we identify reaction products and elucidate the oxidation mechanism of both ASnI 3 and ASn 0.5 Pb 0.5 I 3 (where A can be made of methylammonium, formamidinium, cesium, or a combination of these) perovskites and find that substituting lead onto the B site fundamentally changes the oxidation mechanism of tin-based metal halide perovskites to make them more stable than would be expected by simply considering the decrease in tin content. This work provides guidelines for developing stable small band gap materials that could be used in all-perovskite tandems.