Barrierless Exciton Self‐Trapping and Emission Mechanism in Low‐Dimensional Copper Halides

Low‐dimensional copper halides having nontoxic elements are attracting increasing attention for their peculiar emission properties. Self‐trapped excitons (STEs) account for their high photoluminescence quantum yields (PLQYs) with emission that can stretch across the entire visible spectrum. However, intrinsic factors that influence the formation or loss of the emissive species in low‐dimensional copper halides remain elusive. Here, a comprehensive study on the STE formation dynamics of one‐dimensional CsCu2I3 and zero‐dimensional Cs3Cu2I5 is presented. It is found from STE kinetic analysis that a slower STE formation demonstrated by the 1D structure is not hindered by a potential barrier, but instead related to the number of phonons released in the self‐trapping process. It is further revealed that in 1D CsCu2I3, the non‐radiative recombination of STEs mainly occurs via the intersection between the STE state and the ground state in the configuration coordinate diagram, placing an intrinsic limit on the PLQY at room temperature. These findings show that the STE formation is affected by both the self‐trapping depth and the phonon energy as opposed to a potential barrier in low‐dimensional copper halides. The better understanding of STE formation and recombination processes provide basis for improving design and performance for broadband light emitting devices.