Confronting the Air Instability of Cesium Tin Halide Perovskites by Metal Ion Incorporation

Tin halide perovskite's potential as a photovoltaic absorber has not been fully realized to date, largely due to its instability in ambient air. Here, we demonstrate by both experiments and simulations that the air instability of black-phase cesium tin iodide perovskite (γ-CsSnI₃) could be greatly lessened by a controlled incorporation of bismuth (Bi) ions into the crystal lattice. Hall effect measurements on films of γ-CsSnI₃ suggest the unwanted formation of a tin vacancy and p-type self-doping can be effectively suppressed by the Bi incorporation. Structural and optical results indicate that the Bi incorporation markedly enhances the air stability by impeding the direct conversion of γ-CsSnI₃ to zero-dimensional Cs₂SnI₆. By using a stochastic surface walking (SSW) method integrating neural network (NN) potential and density functional theory (DFT), it is revealed that the remarkable enhanced stability could be attributed to a combination of factors originating from lattice-contraction-induced strain, a suppressed tin vacancy, and an increased energy barrier for the transformation of γ-CsSnI₃ to Cs₂SnI₆. This study provides physical insights into the stabilization mechanism of tin perovskites by heterovalent B-site engineering, paving the way for realizing stable and efficient lead-free perovskite photovoltaics.