KBF4 Additive for Alleviating Microstrain, Improving Crystallinity, and Passivating Defects in Inverted Perovskite Solar Cells

Abstract Triple‐cation mixed perovskites have attracted much attention recently owing to their prominent optoelectronic properties and good stability for perovskite solar cells. However, the introduction of those cations with different sizes in the perovskite materials will drive the perovskite lattice away from ideal cubic structure and lead to microstrain in the resultant films. Herein,a small amount of KBF 4 as an additive to elevate the quality of triple‐cation mixed perovskite thin films is introduced. It is found that KBF 4 can enhance the crystallinity and alleviate microstrain of the perovskite thin films. Moreover, KBF 4 can passivate defects in perovskite grains, leading to much longer carrier lifetimes. Consequently, the resultant devices show improved fill factor, enhanced device efficiency, and better device stability. Under optimum fabrication conditions, triple‐cation mixed perovskite solar cells with an inverted structure show power conversion efficiency over 23% as well as excellent stability under different conditions.