A crystal capping layer for formation of black-phase FAPbI₃ perovskite in humid air

Black-phase formamidinium lead iodide (a-FAPbI₃) perovskites are the desired phase for photovoltaic applications, but water can trigger formation of photoinactive impurity phases such as d-FAPbI₃. We show that the classic solvent system for perovskite fabrication exacerbates this reproducibility challenge. The conventional coordinative solvent dimethyl sulfoxide (DMSO) promoted d-FAPbI₃ formation under high relative humidity (RH) conditions because of its hygroscopic nature. We introduced chlorine-containing organic molecules to form a capping layer that blocked moisture penetration while preserving DMSO-based complexes to regulate crystal growth. We report power conversion efficiencies of >24.5% for perovskite solar cells fabricated across an RH range of 20 to 60%, and 23.4% at 80% RH. The unencapsulated device retained 96% of its initial performance in air (with 40 to 60% RH) after 500-hour maximum power point operation.