Modulating the Distribution of Formamidinium Iodide by Ultrahigh Humidity Treatment Strategy for High‐Quality Sequential Vapor Deposited Perovskite

Abstract The quality of two‐step processed perovskites is significantly influenced by the distribution of organic amine salts. Especially, modulating the distribution of organic amine salts remains a grand challenge for sequential vapor‐deposited perovskites due to the blocking effect of bottom compact PbI 2 . Herein, an ultrahigh humidity treatment strategy is developed to facilitate the diffusion of formamidinium iodide (FAI) from the top surface to the buried bottom interface on the sequential vapor‐deposited bilayer structure. Both experimental and theoretical investigations elucidate the mechanism that moisture helps to i) create FAI diffusion channels by inducing a phase transition from α ‐ to δ ‐phase in the perovskite, and ii) enhance the diffusivity of FAI by forming hydrogen bonds. This ultrahigh humidity treatment strategy enables the formation of a desired homogeneous and high‐quality α ‐phase after annealing. As a result, a champion efficiency of 22.0% is achieved and 97.5% of its initial performance is maintained after aging for 1050 h under ambient air with a relative humidity of up to 80%. This FAI diffusion strategy provides new insights into the reproducible, scalable, and high‐performance sequential vapor‐deposited perovskite solar cells.