Bismuth Stabilizes the α‑Phase of Formamidinium Lead Iodide Perovskite Single Crystals

α-FAPbI₃ (FA = formamidinium) perovskite offers an optimal bandgap for single-junction solar cells but converts into a more thermodynamically stable photoinactive δ-polymorph at room temperature. FA- or I-site substitutional alloying stabilizes α-FAPbI₃; however, it leads to compositional segregation in operational devices. Here, we stabilize α-FAPbI₃ single crystals through Pb-site doping with a heterovalent metal–bismuth (Bi). We show that undoped α-FAPbI₃ has an α- to δ-phase half-life transition of <0.15 h, while the optimum concentration of Bi extends it by 4 orders of magnitude. Differential scanning calorimetry (DSC) reveals that Bi has effectively decreased the δ- to α-phase onset transition temperature. Density functional theory (DFT) calculations suggest a relatively clean gap, supporting previous findings on the improved photovoltaic performance of Bi-doped α-FAPbI₃-based solar cells.