Bismuth Stabilizes the α-Phase of Formamidinium Lead Iodide Perovskite Single Crystals
Soumya Kundu, Dongyang Zhang, Abdelrahman M. Askar, Erin G. Moloney, Michael M. Adachi, Ayesha Nadeem, Shahram Moradi, Vishal Yeddu, Ahmed L. Abdelhady, Oleksandr Voznyy, Makhsud I. Saidaminov
Abstract
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.