Stabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State Alloys

Goldschmidt tolerance factor (t) is an empirical index for predicting stable crystal structures of perovskite materials. A t value between 0.8 and 1.0 is favorable for cubic perovskite structure, and larger (>1) or smaller (<0.8) values of tolerance factor usually result in nonperovskite structures. CH(NH₂)₂PbI₃ (FAPbI₃) can exist in the perovskite α-phase (black phase) with good photovoltaic properties. However, it has a large tolerance factor and is more stable in the hexagonal δ_H-phase (yellow phase), with δ_H-to-α phase-transition temperature higher than room temperature. On the other hand, CsPbI₃ is stabilized to an orthorhombic structure (δ_O-phase) at room temperature due to its small tolerance factor. We find that, by alloying FAPbI₃ with CsPbI₃, the effective tolerance factor can be tuned, and the stability of the photoactive α-phase of the mixed solid-state perovskite alloys FA_1-xCs_xPbI₃ is enhanced, which is in agreement with our first-principles calculations. Thin films of the FA_0.85Cs_0.15PbI₃ perovskite alloy demonstrate much improved stability in a high-humidity environment; this contrasts significantly with the pure FAPbI₃ film for which the α-to-δ_H phase transition (associated with yellowing appearance) is accelerated by humidity environment. Due to phase stabilization, the FA_0.85Cs_0.15PbI₃ solid-state alloy showed better solar cell performance and device stability than its FAPbI₃ counterparts. Our studies suggest that tuning the tolerance factor through solid-state alloying can be a general strategy to stabilize the desired perovskite structure for solar cell applications.