Cs₄PbI₆‐Mediated Synthesis of Thermodynamically Stable FA0.15Cs0.85PbI₃ Perovskite Solar Cells

The stability issue is still one of the main limitations of the commercialization of perovskite photovoltaics. The mixed cation FA x Cs 1 −x PbI 3 has shown great promise owing to its improved thermal and moisture stability. However, the study of FA x Cs 1 −x PbI 3 is concentrated on formamidine (FA)‐rich perovskite, whereas cesium (Cs)‐rich FA x Cs 1 −x PbI 3 perovskites are barely studied due to the inevitable phase separation when Cs > 30 mol%. Here, a Cs 4 PbI 6 ‐mediated method is developed to synthesize Cs‐rich FA x Cs 1 −x PbI 3 perovskites. It is demonstrated that Cs 4 PbI 6 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 x Cs 1 −x PbI 3 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 0.15 Cs 0.85 PbI 3 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.