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 (Sn0.6Pb0.4)I3, MA(Sn0.6Pb0.4)I3, and Cs(Sn0.6Pb0.4)I3 is formed, and a promising composition window of (FA0.6−xMA0.4Csx)Sn0.6Pb0.4I3 (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 FA0.55MA0.4Cs0.05Sn0.6Pb0.4I3 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.
