Band Gap Engineering Based on X-Site Ion Tuning in Copper Organic–Inorganic Hybrid Perovskite

This paper reports two copper-based organic–inorganic hybrid perovskite single crystals, (2A5MP)2CuX4 (X = Cl, Br), synthesized by a halogen-regulated method. Through comprehensive characterization (single-crystal XRD, powder XRD, FTIR, Raman spectroscopy, TGA, and UV–vis DRS), we elucidated crystal structures, intermolecular forces, functional groups, bond configurations, optical properties, and thermal stability. Both compounds crystallized in the monoclinic space group C2/c. (2A5MP)2CuCl4 has an optical band gap of 2.47 eV and is stable up to 172 °C, while (2A5MP)2CuBr4 has a band gap of 1.66 eV with a stability of up to 185 °C. When Br– occupies the X site, (2A5MP)2CuBr4 exhibits an optical band gap (1.66 eV) close to the optimal value for solar cell absorbers. Furthermore, replacing Cl– with Br– enhances thermal stability by 13 °C. Therefore, (2A5MP)2CuBr4 demonstrates significant potential as a light-absorbing layer for solar cells.