Exploring Polaronic, Excitonic Structures and Luminescence in Cs₄PbBr₆/CsPbBr₃

Among the important family of halide perovskites, one particular case of all-inorganic, 0-D Cs₄PbBr₆ and 3-D CsPbBr₃-based nanostructures and thin films is witnessing intense activity due to ultrafast luminescence with high quantum yield. To understand their emissive behavior, we use hybrid density functional calculations to first compare the ground-state electronic structure of the two prospective compounds. The dispersive band edges of CsPbBr₃ do not support self-trapped carriers, which agrees with reports of weak exciton binding energy and high photocurrent. The larger gap 0-D material Cs₄PbBr₆, however, reveals polaronic and excitonic features. We show that those lattice-coupled carriers are likely responsible for observed ultraviolet emission around ∼375 nm, reported in bulk Cs₄PbBr₆ and Cs₄PbBr₆/CsPbBr₃ composites. Ionization potential calculations and estimates of type-I band alignment support the notion of quantum confinement leading to fast, green emission from CsPbBr₃ nanostructures embedded in Cs₄PbBr₆.