Chemical Origin of Exciton Self-Trapping in Cs ₃ Cu ₂ X ₅ Cesium Copper Halides

Copper halides Cs₃Cu₂X₅ (X = Cl, Br, I) are promising materials for optoelectronic applications due to their high photoluminescence efficiency, stability, and large Stokes shifts. Based on previous investigations of the excitation in specific halides, we uncover a universal chemical bonding origin for the Stokes shift in these materials across the entire X = Cl, Br, I series using density functional theory calculations. Upon excitation, one [Cu₂X₅]^3- anion undergoes sizable local distortions, driven by Cu–X and Cu–Cu bond formation. These structural changes coincide with the formation of a self-trapped exciton, where, particularly, the hole is strongly localized on one anion. These structural changes are also found to lead to a robust energy minimum with little dependence on the initial distortion method. Analysis of the electronic structure and bonding reveals reduced antibonding interactions and enhanced bonding character in the excited state, thereby stabilizing the distorted geometry. Our results establish a direct link between orbital-specific hole localization and bond formation. They provide a fundamental understanding of the excitation mechanism in Cs₃Cu₂X₅ and offer design principles to tune optical properties in 0D copper halides.