(a) E–k diagram of a general material with a trap state. E is energy and k the wavevector. (b) General configuration coordinate diagram for a material that forms a self-trapped exciton (STE). Q is the configuration coordinate. Qualitative description of the dependence of the energy barrier for self-trapping on the dimensionless localization parameter λ for structurally 3D, 2D, and 1D materials. Here, FC refers to free carriers, LC refers to localized carriers, and g_ac denotes a dimensionless electron–phonon coupling parameter for acoustic phonons. Adapted with permission from Hoye et al., Adv. Energy Mater. 2100499 (2021). Copyright 2021 Author(s), licensed under a Creative Commons Attribution (CC BY) license.