Cs4 PbI6 -Mediated Synthesis of Thermodynamically Stable FA0.15 Cs0.85 PbI3 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.