A-Site Effect on the Oxidation Process of Sn-Halide Perovskite: First-Principles Calculations

Tin-halide perovskite solar cells (Sn-PSCs) are promising candidates as an alternative to toxic lead-halide PSCs. However, Sn²⁺ is easily oxidized to Sn⁴⁺, so Sn-PSCs are unstable in air. Here, we use first-principles density functional theory calculations to elucidate the oxidation process of Sn²⁺ at the surface of ASnBr₃ [A = Cs or CH₃NH₃ (MA)]. Regardless of the A-site cation, adsorption of O₂ leads to the formation of SnO₂, which creates a Sn vacancy at the surface. The A-site cation determines whether the created vacancies are stabilized in the bulk or at the surface. For CsSnBr₃, the Sn vacancy is stabilized at the surface, so further oxidation is limited. For MASnBr₃, the Sn vacancy moves into bulk region, so additional Sn is supplied to the surface; as a result, a continuous oxidation process can occur. The stabilization of Sn vacancy is closely related to the polarization that the A-site cation causes in the system.