Optical constants of Na-doped CuInS2 thin films

Optical properties of sodium-doped CuInS2 thin films grown by the single-source thermal evaporation method have been studied. The optical constants of the deposited films were obtained from the analysis of the experimental recorded transmission and reflectance spectral data over the wavelength range 300–1800 nm. It is observed that there is decrease in optical band gap with increase in sodium concentration. It has been found that the refractive index and extinction coefficient are dependent on the sodium concentration. The complex dielectric constants of CuInS2:Na films have been calculated in the investigated wavelength range. It was found that the refractive index dispersion data obeyed the single oscillator of the Wemple–Di Domenico model, from which the dispersion parameters and the high-frequency dielectric constant were determined. The electric free carrier susceptibility and the carrier concentration on the effective mass ratio were estimated according to the model of Spitzer and Fan.