Cs₄ PbI₆ -Mediated Synthesis of Thermodynamically Stable FA_0.15 Cs_0.85 PbI₃ 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} PbI₃ has shown great promise owing to its improved thermal and moisture stability. However, the study of FA_x Cs_{1 -x} PbI₃ is concentrated on formamidine (FA)-rich perovskite, whereas cesium (Cs)-rich FA_x Cs_{1 -x} PbI₃ perovskites are barely studied due to the inevitable phase separation when Cs > 30 mol%. Here, a Cs₄ PbI₆ -mediated method is developed to synthesize Cs-rich FA_x Cs_{1 -x} PbI₃ perovskites. It is demonstrated that Cs₄ PbI₆ 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} PbI₃ 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 PbI₃ 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.