In this study, unmodified and graphene (G)-modified TiO2–CuO mixed oxide thin films were synthesized via spray pyrolysis, incorporating varying graphene content (y = 2, 4, 6, and 8 at. %). The modified samples were subjected to photocatalytic (Rhodamine B (RhB), Malachite green (MG), Methylene Blue, and Methyl Orange) and photovoltaic performance evaluations. X-ray diffraction (XRD) confirmed the formation of well-crystalline thin films, while X-ray photoelectron spectroscopy (XPS) (XPS) provided insights into the electronic states of Cu, Ti, C, and O elements. After graphene modification, the Cu 2p and Ti 2p spectra exhibited a negative and positive shift, respectively, indicating Cu reduction and Ti oxidation. Optical absorption analysis revealed an increase in band gap energy with higher graphene concentrations, reaching 1.78 eV at 6 at. % graphene content. The as-prepared samples were tested for photocatalytic degradation of organic dyes in polluted water, including Rhodamine B (RhB), Malachite Green (MG), Methylene Blue (MB), and Methyl Orange (MO). The film dropped at 8 at. % graphene demonstrated remarkable photocatalytic efficiency, achieving degradation rates of 90 %, 85 %, 96 %, and 87 % for RhB, MG, MB, and MO, respectively, within 2 h of solar illumination. Furthermore, the application of G-TiO2-CuO as a secondary absorber layer in CZTS solar cells was optimized using Silvaco TCAD software, resulting in an efficiency enhancement from 10.25 % to 15.31 %. These findings highlight the crucial role of graphene modification in enhancing the physical properties of semiconductor materials, making them promising candidates for advanced optoelectronic applications.