(a) Illustration of how emission from the self-trapped state can be used for white-light emission. Here, blue light from a light-emitting diode (LED) is absorbed by the phosphor, exciting it from the ground (green) to excited (blue) state. The excited-state carriers then form self-trapped excitons (STEs), giving rise to broad emission over the visible wavelength range that combines with the blue light from the LED to give white light. (b) Shift in the photoluminescence spectra of MA₃Bi₂X₉ QDs when the halide is changed from Br to I. Reproduced with permission from Leng et al., Angew. Chem. Int. Ed. 55, 15012 (2016). Copyright 2016 Wiley.