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(NH 2 ) 2 PbI 3 (FAPbI 3 ) 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, CsPbI 3 is stabilized to an orthorhombic structure (δ O -phase) at room temperature due to its small tolerance factor. We find that, by alloying FAPbI 3 with CsPbI 3, the effective tolerance factor can be tuned, and the stability of the photoactive α-phase of the mixed solid-state perovskite alloys FA 1– x Cs x PbI 3 is enhanced, which is in agreement with our first-principles calculations. Thin films of the FA 0.85 Cs 0.15 PbI 3 perovskite alloy demonstrate much improved stability in a high-humidity environment; this contrasts significantly with the pure FAPbI 3 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 0.85 Cs 0.15 PbI 3 solid-state alloy showed better solar cell performance and device stability than its FAPbI 3 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.