High‐throughput compositional mapping of triple‐cation tin–lead perovskites for high‐efficiency solar cells

Abstract Mixed tin–lead perovskites suffer from structural instability and rapid tin oxidation; thus, the investigation of their optimal composition ranges is important to address these inherent weaknesses. The critical role of triple cations in mixed Sn–Pb iodides is studied by performing a wide range of compositional screenings over mechanochemically synthesized bulk and solution‐processed thin films. A ternary phase map of FA (Sn_0.6Pb_0.4)I₃, MA(Sn_0.6Pb_0.4)I₃, and Cs(Sn_0.6Pb_0.4)I₃ is formed, and a promising composition window of (FA_0.6−xMA_0.4Cs_x)Sn_0.6Pb_0.4I₃ (0 ≤ x ≤ 0.1) is demonstrated through phase, photoluminescence, and stability evaluations. Solar cell performance and chemical stability across the targeted compositional space are investigated, and FA_0.55MA_0.4Cs_0.05Sn_0.6Pb_0.4I₃ with strain‐relaxed lattices, reduced defect densities, and improved oxidation stability is demonstrated. The inverted perovskite solar cells with the optimal composition demonstrate a power conversion efficiency of over 22% with an open‐circuit voltage of 0.867 V, which corresponds to voltage loss of 0.363 V, promising for the development of narrow‐bandgap perovskite solar cells.