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 MAPb0.5Sn0.5I3 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.
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