Phase diagram and stability of mixed-cation lead iodide perovskites: A theory and experiment combined study

Alloying structurally similar perovskites to form mixed-cation lead iodide perovskites, e. g. , CsₗFA (₁-ₗ) PbI₃, MAₗFA (₁-ₗ) PbI₃, and CsₗMAₘFA (₁-ₗ-ₘ) PbI₃, could improve the performance of perovskite-based solar cells and light-emitting diodes. However, a phase diagram of them and a clear understanding of the underlying atomic-scale mechanism are still lacking. Using ab initio calculations combined with high-throughput experimentation, we demonstrate the phase diagram of mixed-cation lead iodide perovskites. Only a small proportion of monovalent cations (Cs^+/Rb^+/MA^+) could be incorporated into the FAPbI₃/MAPbI₃ matrix; otherwise it will be separated into -CsPbI₃, -RbPbI₃, MAI, etc. The smaller the radius of doping cations, the harder it is to incorporate them into a perovskite lattice and the easier it is to stabilize the perovskite phase. In FAPbI₃-based multication perovskites, moreover, over 10 mol % alloying is needed to convert phase to phase at room temperature. The combined upper and lower limits for doping concentration restrict the appropriate alloying ratio to a narrow window. We further plot the relative energy diagram for triple-cation perovskite CsₗMAₘFA (₁-ₗ-ₘ) PbI₃, which reveals the ideal doping ratio for uniform stable alloying. This theory-experiment-combined study provides a clear microscopic picture of phase stability and segregation for mixed-cation perovskite solids.