Halogen-independent optoelectronic properties in copper halides: A case study of K2CuX3 (X=Cl, Br)

The ternary copper halides have garnered significant attention as luminescent and nontoxic alternatives to lead halide perovskites for optoelectronic applications. These materials exhibit broadband emission, resulting in a visible light emission with an exceptionally high photoluminescence efficiency. In this study, taking the 1D K2CuX3 (X = Cl, Br) as typical examples, first-principles calculations were employed to elucidate the physical mechanisms underlying the efficient luminescent and halogen-independent optical properties. Our findings demonstrate (i) K2CuX3 has a direct band gap and a large transition matrix, which makes it more likely to undergo radiative recombination; (ii) the excellent defect tolerance opens up more possibilities for thermal relaxation and recombination; (iii) excitonic properties analysis revealed that the broadband emissions originate from self-trapped excitons, which is localized within a single [CuX4] tetrahedron. More important, due to the dominate contributions of Cu orbitals at the band edge in K2CuBr3 and K2CuCl3, the calculated STE emission energies (2.959 eV for K2CuBr3 and 2.953 eV for K2CuCl3) is strikingly similar. These defect tolerances and self-trapped excitons enable K2CuX3 to exhibit halogen-independent optoelectronic properties, which align with experimental observations. This study offers crucial insights into structural-emission relationships in copper halide materials.