Controlling the Formation Process of Methylammonium‐Free Halide Perovskite Films for a Homogeneous Incorporation of Alkali Metal Cations Beneficial to Solar Cell Performance

Abstract Incorporating multiple cations of the 1A alkali metal column of the periodic table (K+/Rb+/Cs+) to prepare perovskite films is promising for boosting photovoltaic properties but requires a uniform distribution. The effects of NH4Cl additives and alkali metal cations (K+/Rb+/Cs+) on the one‐step formation process of methylammonium‐free, formamidinium‐based, iodide perovskite films are analyzed in a step‐by‐step manner. NH4Cl improves the solubility of PbI2 in solution by forming an intermediate and then favors the perovskite phase formation. Moreover, during the annealing process, this additive is shown to increase grain size, to improve crystallinity and to suppress PbI2 formation. K at low concentration is always homogeneously distributed across the film thickness. On the other hand, Cs is more concentrated at the surface and Rb in the depths of pristine films. With NH4Cl additives, these two alkali metals are more homogeneously distributed because NH4Cl slows down the movement of Cs+ and Rb+, it changes the growth direction of the perovskite film, making the overall crystallization quality improved and the distribution more uniform. It results in perovskite films with large monolithic grains. Combined with a perovskite film surface treatment with n‐propylammonium iodide, a high stabilized power conversion efficiency of 22.04% is reached.