Excited-State Dopant–Host Energy-Level Alignment: Toward a Better Understanding of the Photoluminescence Behaviors of Doped Phosphors

Luminescent materials, also known as phosphors, have been widely used for applications such as emissive displays, fluorescent lamps, light-emitting diodes, and X-ray scintillation detectors. The energy-level diagram of a phosphor is extremely important for understanding its photoluminescence behavior. Here, we demonstrate through a combined density functional theory and experimental study that excited-state energy-level alignment accounts for the photoluminescence behaviors much better than ground-state energy-level alignment. An efficient doped phosphor should exhibit a type I excited-state dopant-host energy-level alignment, regardless of whether its ground-state alignment is type I. A type II excited-state dopant-host energy-level alignment implies that exciton dissociation, resulting in photoluminescence quenching. Our results provide not only a better understanding of the photoluminescence behaviors of the reported phosphors but also critical guidance for designing prospective luminescent materials.