Highly Efficient Broadband Green Emission of (TPA)CuCl2 Single Crystals: Understanding the Formation of Self-Trapped States

Recently, low-dimensional metal halides (LDMHs) have attracted tremendous attention due to their fascinating optoelectronic properties. Here, we report an organic–inorganic hybrid Cu(I)-based metal halide of (TPA)CuCl2 (TPA+ = tetrapropylammonium cation), where the isolated [CuCl2]− units are surrounded by TPA+, thus forming a zero-dimensional block. Moreover, the as-synthesized compound shows a broad green emission with a photoluminescence (PL) quantum efficiency of 91.8% and a large Stokes shift of 230 nm, stemming from the self-trapped exciton (STE) transition. Variable-temperature Raman spectra reveal that there is a strong anharmonic electron–phonon coupling in (TPA)CuCl2, which provides clear evidence for the formation of STEs, and this is also supported by the temperature-dependent PL spectra of (TPA)CuCl2. Moreover, it was found that TPA+ not only works as a ligand but also participates in the STE emission. Our results provide a clear statement that the formation of the STE is related not only to the low-frequency phonon of [CuCl2]− clusters but also to the mid-infra vibration of organic molecules, which promotes the understanding of the emission mechanism of LDMHs.