Structurally Reinforced All‐Inorganic CsPbI 2 Br Perovskite by Nonionic Polymer via Coordination and Hydrogen Bonds

The all‐inorganic CsPbI 2 Br perovskite with superior thermal durability faces challenges of low‐phase stability and high moisture sensitivity. Herein, a nonionic additive of polyethyleneimine (PEI) with multiple amino groups is introduced to form hydrogen bond with I − /Br − ions and coordinate with Pb 2+ /Cs + ions simultaneously. The strong interplay between PEI and CsPbI 2 Br achieves a well‐controlled grain size, reduced defects, and reinforced phase structure of CsPbI 2 Br film, which boosts the power conversion efficiency (PCE) of perovskite solar cells to 15.48%. The hydrophobic long alkyl chain of PEI greatly improves the humidity resistance, retaining 81.9% of initial PCE of zjr unsealed device under 20 ± 5% relative humidity (RH) for 500 h. Remarkably, a PCE of 13.37% is achieved by the device based on CsPbI 2 Br–PEI film processed under ambient condition (≈22% RH, ≈25 °C).