Current Understanding of Band-Edge Properties of Halide Perovskites: Urbach Tail, Rashba Splitting, and Exciton Binding Energy.

The band-edge structure of halide perovskites, derived from the hybridization of atomic orbitals, plays a fundamental role in determining their optical and electronic properties. Several important concepts have been frequently discussed to describe the influence of band-edge structure on their optoelectronic properties, including Urbach tail, Rashba splitting, and exciton binding energy. In this Perspective, we provide a fundamental understanding of these concepts, with the focus on their dependence on composition, structure, or dimensionality. Subsequently, the implications for material optimization and device fabrication are discussed. Furthermore, we highlight the Rashba effect on the exciton fine structure in perovskite nanocrystals (PNCs), which explains the unique emissive properties. Finally, we discuss the potential influence of band-edge properties on the light emission process. We hope that this Perspective can inspire the investigation of band-edge properties of halide perovskites for light-emitting diodes, lasers, and spin electronics.