Synergy of Hydrophobic Surface Capping and Lattice Contraction for Stable and High‐Efficiency Inorganic CsPbI2Br Perovskite Solar Cells

CsPbI 2 Br has been recognized as a promising material for photovoltaic applications due to its excellent optoelectronic properties and compositional stability. Unfortunately, its desired perovskite phase is not stable in humid environments as it is spontaneously transformed into a yellow non‐perovskite phase. Herein, we present our strategy to use phenylethylammonium chlorine (PEACl) treatment to significantly improve the moisture‐resistance of the CsPbI 2 Br film without compromising its high solar cell efficiency. It is found that: 1) small‐sized hydrophobic aromatic group PEA + forms in the edge‐on orientation on the CsPbI 2 Br surface and 2) smaller halide Cl − is doped into the CsPbI 2 Br lattice during post‐annealing, leading to a smaller lattice structure with beneficial crystallization quality. Compared with the reference sample without the PEACl treatment, the present device achieves a comparable power‐conversion efficiency of 14.05% and much improved moisture resistance.