Study of structural, optical and electrical properties of thermal evaporated undoped and Na doped CuInS₂ thin films

We have deposited undoped and Na doped CuInS₂ thin films by thermal evaporation on glass substrates followed by annealing in air atmosphere from 250 °C to 450 °C. Structural studies for annealed films show the presence of major peak (112) of the chalcopyrite phase for both undoped and Na doped CuInS₂. Peak's intensities increases with annealing temperature with the appearance of copper oxide CuO and Indium sulfide In₂S₃ as minor secondary phases. Crystallite sizes increases with annealing temperature. We have also calculated some optical constants such as band gap energies and absorption coefficients that give good results for photo-absorber in photovoltaic solar cells. The refractive index has been calculated by two methods and the results are close and follow the same variation. We have performed impedance spectroscopy studies at different temperatures from 420 °C to 470 °C. We found just one semi-circle in the impedance spectra starting from the origin and decreases progressively with increasing temperature. This behavior can be modeled by an equivalent circuit constituted of a resistor and a capacitor in parallel. Activation energies are estimated by two approaches with closes results and the total conductivity as function of temperature has been investigated.