Passivating Perovskites in Air Via an Alternating Cation Interlayer Phase Formed by Benzylamine Vapor Fumigation

2D perovskites are widely employed to improve efficiency and stability of perovskite solar cells (PSCs), but the processes are rarely accomplished in air due to the difficulty of controlling the formation processes. An ultra-thin 2D capping layer with an alternating cation interlayer (ACI) structure is in situ formed by fumigating 3D perovskites with benzylamine vapor. The whole process is finished in air within 10 s regardless of the humidity, after which both defects and tensile strain are reduced and the interfacial energy band gets benignly aligned to a type-I heterojunction, avoiding direct charge recombination at the interface. Theoretical analysis reveals that the ACI phase is thermodynamically more stable than an analogous Ruddlesden-Popper phase. The strategy can passivate various perovskites, including methylammonium (or multi-cation) lead (or lead/tin mixed) halide perovskites, prepared via either vapor or solution approaches, increasing the absolute power conversion efficiency by up to 2.5%. It can uniformly passivate PSCs without area limitation, and provide a repeatable methodology for passivating perovskites.