Sequential Introduction of Cations Deriving Large‐Grain CsxFA1−xPbI3 Thin Film for Planar Hybrid Solar Cells: Insight into Phase‐Segregation and Thermal‐Healing Behavior

Composition engineering of perovskite materials has been demonstrated to be important for high‐performance solar cells. Recently, the energy favorable hybridization of formamidinium (FA) and cesium (Cs) in three dimension lead halide perovskites has been attracting increasing attention due to its potential benefit on durability. Herein, we reported a simple and effective method to produce phase‐pure CsxFA1‐xPbI3 thin film via sequential introduction of cations, in which the FA cation was introduced by interdiffusion annealing in the presence of N‐methylimidazole (NMI). NMI was employed as an additive to slow down the crystallization and thus drive the formation of CsxFA1‐xPbI3 with micrometer grain size, which probably facilitate the charge dissociation and transportation in photovoltaic devices. More importantly, composition dependent phase‐segregation has been revealed and investigated for the first time during the phase‐pure mixed‐cation perovskites CsxFA1‐xPbI3. The present findings demonstrated that suppressing phase‐segregation of mixed‐cation perovskites by meticulous composition engineering is significant for further development of efficient photovoltaics. It also suggested that phase‐pure Cs0.15FA0.85PbI3 may be a promising candidate with superior phase‐durability, which performed an efficiency over 16% in planar perovskite solar cells.