In this work, CdS and Cu doped CdS films (at the Cu percentages of 1, 3 and 5) have been deposited onto glass substrates at 350 + 5 degrees C by ultrasonic spray pyrolysis technique and their application potential for photovoltaic solar cells have been investigated. Optical properties and thicknesses of the films have been investigated by spectroscopic ellipsometry (SE). Ellipsometric angle psi was used as the source point for optical characterizations. The optical constants (n and k) and the thicknesses of the films have been fitted according to Cauchy model. Also, optical properties of the produced films have been analyzed by transmittance and reflectance spectra. Refractive index (n), extinction coefficient (k) and reflectance (R) spectra have been taken by spectroscopic ellipsometer, while transmittance spectra have been taken by UV/vis spectrophotometer. The optical method has been used to determine the band gap type and value of the films. Mott-Schottky (M-S) measurements have been made to determine the conductivity type and carrier concentration of the films. Samples showed n-type conductivity and carrier concentration of undoped CdS sample was found to be 1.19 x 10(19) cm(-3). Also, it was concluded that Cu doping has an acceptor effect in CdS samples. From the results of these investigations, the application potential of CdS: Cu films for photovoltaic solar cells as window layer was searched. (C) 2010 Elsevier B. V. All rights reserved.