"The structural and optical properties of CdS window layer at different thicknesses in the range of (100-300 nm) deposited on pre-cleaned glass substrates (CdS/glasses) via the thermal evaporation process are studied in the current work. The structural analysis is done using Rietveld refinement and atomic pressure microscope techniques. The films of CdS/glass showcase a wurtzite behavior. XRD and AFM tests are confirmed that the structural parameters improve as the thickness of the CdS-layer increases. The optical constants (the refractive index n, the extinction coefficient, k and the bandgap energy values are estimated from spectroscopic ellipsometry (SE) via the construction of an optical model. The refractive index of the CdS/glass films increases with the increase of CdS-layer thickness. This, in turn, is due to the rise of the size of the crystal in the thin layers. It is also found that as the thickness of the CdS-layer rises, so does the overall behavior of the extinction coefficient. Additionally, the bandgap energy of the direct transition decreases from 2.45 eV (d=100 nm) to 2.25 eV (d=300 nm). The dark and illumination I-V photovoltaic characteristics of fabricated devices are explored by depositing a p-CdTe thin layer (500 nm) over varying thicknesses of CdS thin films (100- 300 nm) prepared on glass substrates (2 mm) and thus, the Ni/n-CdS/i-AgSe/p-CdTe/Pt heterojunction has been successfully fabricated with an AgSe buffer layer deposited directly on the p-CdTe absorber laryer. The n-CdS window layer is deposited on i-AgSe buffer layer. In dark conditions, based on the dependence of the forward and reverse current-voltage, the essential behavior related to the fabricated diode has been determined. As well, in the illumination case, the open-circuit voltage, the short-circuit current, the fill factor, the power conversion efficiency, (PCE), photoresponsivity, quantum efficiency, dependence of generated photocurrent on the light intensity, dependence of the generated photocurrent on wavelength (λ)for the studied solar cell are computed and discussed."