Phase-Engineered α/δ-FAPbI₃ Heterojunctions for Valleytronics, Band Alignment, and Stability

Cubic FAPbI3 perovskite exhibits exceptional optoelectronic properties, yet the α-to-δ phase transition mechanism remains incompletely understood. In this work, we construct α-FAPbI3 (111)/δ-FAPbI3 (0001) heterojunctions and employ density functional theory to investigate the impact of δ-phase formation on thermodynamic stability, along with electronic and optical properties. Our results demonstrate that a higher δ-phase content significantly enhances stability, with the α-to-δ phase transition occurring at a low energy barrier. Tuning the α/δ phase ratio induces a transition from type-I to type-II band alignment. Moreover, the hexagonal structures of α/δ-FAPbI3 heterojunctions exhibit spin-valley coupling, indicating their potential as valleytronic materials─an effect rarely reported in halide perovskites. This allows for valley-selective excitation of spin-polarized carriers using left- or right-handed circularly polarized light. Additionally, the difference in work function between the two phases drives the spontaneous flow of electrons from α-FAPbI3 to δ-FAPbI3 and holes in the opposite direction. Careful design of the α/δ phase ratio allows for effective tuning of the optical properties of FAPbI3. These findings highlight the crucial role of α/δ phase engineering in tailoring the electronic and optical properties of FAPbI3, offering a theoretical foundation for the design of high-performance perovskite-based optoelectronic devices.