Cs₄PbI₆‐Mediated Synthesis of Thermodynamically Stable FA_0.15Cs_0.85PbI₃ Perovskite Solar Cells

Abstract The stability issue is still one of the main limitations of the commercialization of perovskite photovoltaics. The mixed cation FA_xCs_1−xPbI₃ has shown great promise owing to its improved thermal and moisture stability. However, the study of FA_xCs_1−xPbI₃ is concentrated on formamidine (FA)‐rich perovskite, whereas cesium (Cs)‐rich FA_xCs_1−xPbI₃ perovskites are barely studied due to the inevitable phase separation when Cs > 30 mol%. Here, a Cs₄PbI₆‐mediated method is developed to synthesize Cs‐rich FA_xCs_1−xPbI₃ perovskites. It is demonstrated that Cs₄PbI₆ intermediate phase has a low Cs cation diffusion barrier and therefore offers a fast ion exchange with the preformed FA‐rich perovskite phase to finally form the Cs‐rich FA_xCs_1−xPbI₃ perovskite. The results indicate that ≈15% alloying with organic FA cations can sufficiently stabilize the perovskite phase with excellent phase and UV‐irradiation stability. The FA_0.15Cs_0.85PbI₃ perovskite solar cells achieve a champion power conversion efficiency of 17.5%, showing the great potential of Cs‐based perovskites for efficient and stable solar cells.