Enhanced free-exciton luminescence in Cs2SnBr6−I : A first-principles study

A2BX6-type tin halide perovskites have received extensive attention as environment-friendly lead-free luminescent materials. However, the limited knowledge of the partial anion substitution effects on their luminescent properties hinders their further development in this field. Therefore, we investigated such effects using Cs2SnBr6 (CSB) double perovskite as an example in this work. We first found that CSB remains stable upon partial anion substitution of bromine by iodine. Next, we explored the electronic origin of the tunable band gaps upon the presence of both Jahn-Teller-like or non-Jahn-Teller-like distortions in [SnBr6−xIx]2− (x = 1–3) octahedrons. As a further stage, we performed hybrid HSE06 functional calculations considering spin–orbit coupling (SOC) to obtain more accurate band gaps. Importantly, all substituted materials except Cs2SnBr4I2 (structure 2a) show direct band gap characteristics and enhanced light emissions. Finally, we identified the free exciton (FE) emission mechanism is more favorable than self-trapped exciton (STE) emission in these materials owning to their high electronic dimensionality, small effective mass, non-Jahn-Teller-like distortion, small exciton binding energy, small energy separation, small self-trapping depth, low detrapping barrier, and non-local deformation in the presence of heavy halogen. We believe our findings of Cs2SnBr6−xIx would promote the design of tin halide perovskites with better luminescent properties.