Anomalous luminescence blue-shift trends induced by the intensity of the self-trapping effect in Cs3Cu2X5 and CsCu2X3 (X = Cl, Br, I).

Modulating the halogen composition enables effective tuning of the band gap and emission color in lead halide perovskites. Nevertheless, recent experimental observations have revealed that ternary copper halides (TCHs) exhibit an entirely opposite behavior. Here, we unravel the microscopic origin of the anomalous photoluminescence (PL) blue-shift from Cl to Br to I in zero-dimensional (0D) Cs3Cu2X5 and one-dimensional (1D) CsCu2X3 (X = Cl, Br, I) via first-principles calculations. Several key findings were obtained: (i) both compounds exhibit a weak dependence on halogen substitution as the dominant contribution of Cu orbitals at the band edges. (ii) The self-trapped exciton (STE)-dominated luminescence mechanism endows them with superior optoelectronic performance. (iii) The STE binding energy decreases remarkably from Cl to Br to I due to the self-trapping effect governed by Cu-X bond strength, which induces the anomalous PL blue-shift in both Cs3Cu2X5 and CsCu2X3. This work unveils the fundamental mechanism of the anomalous luminescence trend and a clear correlation among exciton behavior and optical properties in low-dimensional TCHs.