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 FAx Cs1-x PbI3 has shown great promise owing to its improved thermal and moisture stability. However, the study of FAx Cs1-x PbI3 is concentrated on formamidine (FA)-rich perovskite, whereas cesium (Cs)-rich FAx Cs1-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 FAx Cs1-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 FAx Cs1-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 FA0.15 Cs0.85 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.