In this paper, the vacuum thermal evaporation technique was used to investigate β-In2S3 doped with copper (Cu) as window layer thin film for solar cells. For various Cu doping levels, the characteristics of the thin films were studied utilizing structural, morphological, and optical characterization techniques, as well as dielectric testing. The research included X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), Raman spectroscopy, and energy-dispersive X-ray spectroscopy (EDS), Infrared analysis (FTIR), XPS, Ellipsometry and UV-NIR spectroscopy. The measurements demonstrated that the real and imaginary isolation constants were over the wavelength range and successfully estimated the Urbach energy (Eu) and band gap energy. Moreover, employing the Wemple-Didomenico model helped uncover the Cauchy coefficients’ refractive index and the dispersion and oscillation energies. Therefore, the results presented in this paper show that the use of Cu-In2S3 for perovskite cells offers a high-efficiency alternative in the solar energy field.