First-Principles Investigation on the Stability, Electronic Structure, and Exciton Self-Trapping Mechanism of 0D and 1D Cs3Cu2Cl5

Copper(I) halides with broadband emission from self-trapped excitons (STEs) are promising luminescent materials due to their structural diversities and intriguing photophysical properties. Despite great efforts invested in these materials, an in-depth understanding of their structural variations and the corresponding STE emission mechanisms is still lacking. Here, first-principles calculations reveal two distinct STEs in 0D and 1D polymorphs of Cs3Cu2Cl5, which feature the same chemical composition but different crystal structures. The electronic band structure analysis reveals that the strong Cu–Cl 4s–3p coupling along the 1D [Cu2Cl5]3– chain brings a large band dispersion, resulting in an increase of optical anisotropy in the 1D structure. The STE in 0D Cs3Cu2Cl5 involves the formation of one Cu–Cu bond and the breakage of two Cu–Cl bonds within an isolated [Cu2Cl5]3– unit. In contrast, the exciton self-trapping in the 1D structure is found to be associated with the formation of one Cu–Cu bond accompanied by two Cu–Cl bonds in the [Cu2Cl5]3– chain. Despite significant structural differences in 0D and 1D Cs3Cu2Cl5, quantitative characterizations of the configuration coordinate diagrams in the two polymorphs lead to similar STE emission energies (2.38 and 2.48 eV for 0D and 1D, respectively), which theoretically explains similar green-light emission experimentally observed in both compounds. This study provides important insights for understanding complex relations between the structural dimensionality, building units, and emission properties in low-dimensional metal halides.