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

Abstract Tin‐based halide perovskites (ASnX ₃ ) 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−x Cs _x SnBr ₃ and MA _1−x Cs _x SnI ₃ (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−x Cs _x SnBr ₃ , 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−x Cs _x SnI ₃ . Besides, the PL intensity of MA _1−x Cs _x SnBr ₃ 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.