Thin films of CdTe/CdS were prepared using the thermal evaporation technique with pressure 2 × 10−5 mbar on substrate from glass. The thickness of CdCl2 layer around 150 nm was deposited making as-deposited CdTe/CdS films. The films covered by cadmium chloride (CdCl2) layer were annealed in atmosphere of (O2 + Ar) at different temperatures of 400 °C, 450 °C and 480 °C for 15 min. The effect of CdCl2 as a top layer on morphological, structural and optical properties was studied, and the results were compared with as-deposited films and also with (O2 + Ar) annealing process. The refractive index in whole wavelength and film thickness was estimated by Cauchy dispersion relation and Swanepoel's envelope method, respectively. Furthermore, many optical parameters such as transmittance, absorbance, reflectance, and optical band gap, Urbach energy, film thickness, electronic polarizability