The present study was conducted to investigate the optical constants of copper (Cu)-doped zinc oxide (ZnO) films annealed at different temperatures. The absorption coefficient of the films increased by changing the annealing temperature. The lattice dielectric constant eL, concentration of the free-charge carriers, plasma frequency, Spitzer–Fan model, and the waste heat of electrical energy in the films were analyzed using the refractive index n and extinction coefficient k spectra. The results of the study showed that changing the annealing temperature values significantly influences the refractive index and extinction coefficients of CZO films. The increase in the electrical susceptibility vc in annealed films can be due to the increased concentration of free-charge carriers in these films. The maximum electrical energy loss (tan d) in the films as a function of photon energy occurred at 400 C. The as-deposited films showed minimum volume and surface energy loss functions in their range of optical band gaps. The variations in the phase and group velocity of the films with post-annealing temperature were consistent with the variations in the reduction of the density of freecharge carriers for these films. Films annealed at 600 C had the maximum root mean square roughness of about 5.62 nm. Films annealed at 400 C had the maximum extinction coefficients k such that they increased for all ranges of wavelength. vc had the maximum values in the films annealed at 600 C, for all ranges of wavelength, and they increased by increasing the wavelength. The increase in vc of the films annealed at 600 C can be attributed to the increase in the density of freecharge carriers in these films.
