Efficient and Stable Wide‐Bandgap Perovskite Solar Cells Derived from a Thermodynamic Phase‐Pure Intermediate

Wide‐bandgap perovskites based on alloying cesium and formamidinium lead mixed halides (Cs x FA 1– x Pb(I y Br 1– y ) 3 ) have received great attention due to their potential application in high‐efficiency tandem solar cells. However, the fast crystallization of Cs x FA 1– x Pb(I y Br 1– y ) 3 perovskite films results in a high trap density and hinders the further enhancement of the photovoltaic performance. Herein, an intermediate engineering is developed to retard the fast crystallization of Cs 0.17 FA 0.83 PbI 1.8 Br 1.2 wide‐bandgap perovskite by adding FACl in the precursor solution. The introduction of the FACl additive leads to the formation of a thermodynamic phase‐pure intermediate which facilitates the further crystallization of a high‐quality perovskite thin film at elevated temperature and thus enhances the ultimate device performance. The champion device achieves an efficiency over 19% and exhibits 83.8% retention after 50 days aging without encapsulation.