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.