Luminescence of Polybromide Defects in Cs4PbBr6

Cs<sub>4</sub>PbBr<sub>6</sub> is a member of the halide perovskite family that is built from isolated (zero-dimensional) PbBr<sub>6</sub><sup>4-</sup> octahedra with Cs<sup>+</sup> counter ions. The material exhibits anomalous optoelectronic properties: optical absorption and weak emission in the deep ultraviolet (310 - 375 nm) with efficient luminescence in the green region (540 nm). Several hypotheses have been proposed to explain the giant Stokes shift including: (i) CsPbBr<sub>3</sub> phase impurities; (ii) self-trapped exciton; (iii) defect emission. We show -- within the modern first-principles theory of defects -- that many of the low energy point defects in Cs<sub>4</sub>PbBr<sub>6</sub> lead to the formation of polybromide (Br<sub>3</sub>) species that exist in a range of charge states. We further demonstrate from excited-state calculations that tribromide moieties are photoresponsive and can contribute to the observed green luminescence. Photoactivity of polyhalide molecules is expected to be present in other halide perovskite-related compounds.