In 2014 we observed white-light emission from layered lead-halide perovskites. When excited by UV light, these bulk solids emit across the entire visible spectrum, approximating sunlight. These hybrid phosphors have high color rendering indices (accurately representing illuminated colors) and tunable chromaticity coordinates, where halide substitution affords both 'warm' and 'cold' white light. They are promising as phosphors for solid-state lighting.
Most inorganic phosphors contain emissive dopants or surface sites. I will share our understanding of how a bulk material, with no obvious emissive sites, can emit every color of visible light. I will discuss the experiments that led us to propose that the broad emission stems from excited electron-hole pairs (or excitons) that couple strongly to the lattice, thereby forming transient lattice defects. Importantly, although we attribute the broad emission to these 'excited-state defects', it responds to systematic variation in the inorganic sublattice, allowing synthetic control over the emission. I will explain synthetic design rules we have uncovered for obtaining white light from layered perovskites. The understanding we have developed of perovskite white-light emitters can be applied to other low-dimensional systems. I will also describe other inorganic topologies that afford broadband emission with a large Stokes shift, which we attribute to self-trapping by analogy to the perovskite white-light emitters.