Abstract
Tin alloying is a promising strategy to reduce lead content in metal halide perovskites solar cells and to modulate the perovskite band gap. Mixed tin-lead perovskites have shown photovoltaic efficiencies approaching those of lead perovskites and improved long-term stability compared to that of pure tin perovskites. We here demonstrate that the recent success of mixed perovskites lies in a composition-dependent struggle between tin and iodine chemistry at the material's surface. Tin oxidation, which plagues tin perovskite-based devices with low efficiency and thermodynamic instability, is hindered in mixed MAPb(0.5)Sn(0.5)I(3) by the competition with oxidation of iodine-related defects, the latter being generally favored by both thermodynamics and kinetics. Tin oxidation can be promoted, however, under Sn-poor conditions. When Sn is alloyed to Pb in low concentrations, it acts as a dopant and Sn(IV) is promptly formed on the perovskite surface.