Bandgap Engineering Based on A-site Ions Tuning in Tin Halide Perovskite

Tin-based halide perovskites (ASnX₃) have garnered substantial interest due to their unique photoelectric properties and environmentally friendly features. The A-site ions tuning strategy has been proven to promote material performance. However, there is a lack of systematic research on the optical properties, lattice structure variation, and band structure evolution in tin-based perovskites when the A-site ions tune from organic to inorganic. Herein, MA_1-xCs_xSnBr₃ and MA_1-xCs_xSnI₃ (0≤x≤1) flakes are synthesized through a one-pot reaction method. By controlling the Cs ratio, a tunable photoluminescence (PL) emission covering a wide range of 560-685 nm can be observed in MA_1-xCs_xSnBr₃, with bandgap tuned from 1.8 to 2.15 eV, while the PL ranges from 900 to 950 nm with the bandgap 1.2-1.3 eV for MA_1-xCs_xSnI₃. Besides, the PL intensity of MA_1-xCs_xSnBr₃ significantly enhances with the increasing Cs ratio. First-principles calculations reveal that the octahedron shrinks gradually as the Cs ratio increases. It increases the orbital overlap between Sn and Br and causes a symmetry variation, thus decreasing the bandgap and increasing emission intensity. This work reveals the photophysical mechanism of improved optical properties and bandgap variation in tin-based perovskites, paving the way for their future applications.