Bismuth Stabilizes theα-Phase ofFormamidinium Lead Iodide Perovskite SingleCrystals

alpha-FAPbI3(FA = formamidinium) perovskite offersan optimal bandgap for single-junction solar cells but converts intoa more thermodynamically stable photoinactive delta-polymorph atroom temperature. FA- or I-site substitutional alloying stabilizes alpha-FAPbI3; however, it leads to compositional segregation inoperational devices. Here, we stabilize alpha-FAPbI3single crystalsthrough Pb-site doping with a heterovalent metal-bismuth (Bi).We show that undoped alpha-FAPbI3has an alpha-to delta-phase half-lifetransition of <0.15 h, while the optimum concentration of Biextendsitby4ordersofmagnitude.Differential scanningcalorimetry (DSC) reveals that Bi has effectively decreased the delta-to alpha-phase onset transition temperature. Density functional theory(DFT) calculations suggest a relatively clean gap, supportingpreviousfindings on the improved photovoltaic performance of Bi-doped alpha-FAPbI3-based solar cells.