Tailoring the Cs/Br Ratio for Efficient and Stable Wide‐Bandgap Perovskite Solar Cells

Wide‐bandgap (WBG) perovskites are promising candidates for front cells in tandem devices. Taking advantage of composition engineering, most WBG perovskites are successfully obtained by heavy Br or Cs doping. However, the role of Cs/Br ratio in crystallization, phase homogeneity, and stability are fuzzy. Herein, three perovskites with a bandgap of 1.68 eV via tailoring the Cs/Br ratio are systematically studied, that are Cs 0.15 FA 0.85 PbI 2.5 Br 0.5 , Cs 0.3 FA 0.7 PbI 2.7 Br 0.3 , and Cs 0.4 FA 0.6 PbI 2.8 Br 0.2 . It is found that the Br‐rich precursor film undergoes ultrafast crystallization, forming a 3D structure with random crystal orientation, while Cs‐rich systems demonstrate a 2D‐dominated intermediate phase. After annealing, all the precursor films transform into (100)‐oriented perovskite films, and Cs 0.3 FA 0.7 PbI 2.7 Br 0.3 perovskite presents the highest crystallinity, lowest microstrain, and best‐phase homogeneity. As the Cs/Br ratio changes, both Cs and Br ions show the ability of arising phase segregation in the perovskite films. Increasing Cs content significantly improves the thermal stability, but heavy Cs content also sacrifices the air stability. Among the three systems, Cs 0.3 FA 0.7 PbI 2.7 Br 0.3 solar cells offer the highest efficiency of 20.17% with superior air, light, and thermal stability. The findings highlight the importance of rational composition design to achieve high‐quality WBG perovskite films for tandem applications.