Vibronic Structure in Room Temperature Photoluminescence of the Halide Perovskite Cs 3 Bi 2 Br 9 .

We report a study on the optical properties of the layered polymorph of vacancy-ordered triple perovskite Cs3 Bi2 Br9 . The electronic structure, determined from density functional theory calculations, shows the top of the valence band and bottom of the conduction band minima are, unusually, dominated by Bi s and p states, respectively. This produces a sharp exciton peak in the absorption spectra with a binding energy that was approximated to be 940 meV, which is substantially stronger than values found in other halide perovskites and, instead, more closely reflects values seen in alkali halide crystals. This large binding energy is indicative of a strongly localized character and results in a highly structured emission at room temperature as the exciton couples to vibrations in the lattice.