Interstitial Engineering toward Stable Tin Halide Perovskite Solar Cells

Sn‐based halide perovskites are the most promising alternatives for developing Pb‐free perovskite solar cell materials. However, the stability of Sn halide perovskites is the biggest concern for future developments. The phase stability and the doping‐level control should be resolved for Sn perovskites to compete with Pb‐based analogs. Herein, interstitial engineering is used to enhance the stability of Sn‐based halide perovskites using alkali metals through ab initio calculations and controlled experiments. This study reveals that alkali metal interstitials can promote the performance of Sn perovskites by controlling their phase stability, suppressing free carrier density, and locking lattice vibration. K+ shows the most promising behavior among alkali–metal cations in terms of phase stabilization and defect formation energy. The roles of small alkali metals on the stability of Sn perovskites are investigated by theoretical calculations and controlled experiments. K+ incorporation can enhance the Sn‐based perovskites by reducing structural instability and unintentional hole doping.