Emission Trend of Multiple Self-Trapped Excitons in Luminescent 1D Copper Halides

Low-dimensional metal halides are promising luminescent materials with efficient self-trapped exciton (STE) emission at room temperature; however, the understanding of optical behaviors and trends is still limited due to the complex excited-state energy landscape. In this Letter, first-principles calculations reveal multiple STEs in 1D CsCu₂X₃ (X = Cl, Br, I), which are bright visible-light-emitting materials. In this work, three types of STEs with distinct structures are identified, and their relative stabilities are halogen-dependent despite the same crystal structure shared by all three halides. The emission energy of each type of STE is found to be blue-shifted from CsCu₂Cl₃ to CsCu₂Br₃ to CsCu2I₃; the experimentally observed red shift of the emission peak is not due to the electronegativity trend of halogen atoms but originates from the emissions from three different types of STEs. The emitting STE is the lowest-energy STE in CsCu₂Br₃ and CsCu₂I₃ but a metastable one in CsCu₂Cl₃.