We have investigated the optical and electrical properties of the CdS films under a variety of the post-deposition annealing treatments.CdCl2-treatment results in the decrease of the shallow defect density, the increase of the bandgap, the photoconductivity response, as well as the optical transmission in the wavelength range between 500 and 600 nm of the CdS films. While non-CdCl2-treatment (annealed withoutCdCl2) leads to the opposite results. Also, after coated with the CdTe films and annealed,CdCl2-treated CdS films of the annealed CdS/CdTe films have higher photoconductivity response than the as-deposited CdS films of the annealed CdS/CdTe films. The role ofCdCl2in the post-treatment and the influence of theCdCl2treatment on the CdTe/CdS cell performance is analyzed. ©1999 American Institute of Physics.