Multiple Self-Trapped Emissions in the Lead-Free Halide CsCuI.

Low-dimensional copper halides with high luminance have attracted increasing interest as heavy-metal-free light emitters. However, the optical mechanisms underpinning their excellent luminescence remain underexplored. Here, we report multiple self-trapped emissions in CsCuI. Power-dependent photoluminescence spectra reveal the appearance of multiple self-trapped emission peaks with increasing excitation power, and this emission behavior is explored across a temperature range of 80-420 K. The zero-dimensional structure and soft crystal lattice contribute to the multiple self-trapped emissions in CsCuI: this explains the origin of the broad emission and the luminescence mechanism in CsCuI and will assist in improving our understanding of the optical properties of other metal halides. We incorporate the CsCuI in light-emitting diodes that achieve a peak luminance of 140 cd/m and an external quantum efficiency of 0.27%.