Polaron and Exciton in a Lead-Free Vacancy-Ordered Cs₂SnBr₆ Double Perovskite

Electron–phonon and electron–hole interactions have played a central role in the transport and optoelectronic properties of lead-free vacancy-ordered double perovskites (VODPs). In this report, state-of-the-art first-principle calculations were employed to study the polaron and exciton in Cs2SnBr6. Electron polaron and hole polaron were found to be small (1–2 lattice units) with a large formation energy of >4 eV. The Sn–Br bond length within the [SnBr6] octahedra was elongated (shrunk) by 3.5% (0.5%) as one electron (hole) was added. Furthermore, the exciton with a binding energy of 250 meV and a size of ∼5 lattice units was identified. Our study provides a key picture to understand the intrinsic optoelectronic properties of VODP Cs2SnBr6, which is vital to designing efficient light emission materials in the future.