Growth and annealing of CuIn0.75Ga0.25Se2 thin films

Abstract

CuIn0.75Ga0.25Se2 (CIGS), a promising material for solar cell applications, was produced in the form of thin films by the evaporation of pre-reacted polycrystalline CIGS material onto glass substrates. The deposits were characterized using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. They were found to have a strong preferred orientation with the (112) plane parallel to the substrate. Results from energy dispersive analysis with X-ray (EDAX) and Rutherford backscattering spectroscopy (RBS) indicated that the as-grown films were slightly deficient in selenium, otherwise the composition was comparable with the starting polycrystalline material. Electrical measurements revealed both n- and p-type conductivities with surface resistivity values in the range of 10-1-103 Omega cm. A two-stage post-deposition heat treatment of the films was performed to improve the composition and crystal structure and to optimize the electrical properties. It was observed that the first annealing stage produced an excellent improvement in the composition of the films. An increase in the film grain size (to >2 mu m) was observed when the films were subsequently annealed in a 9:1 mixture of N2:H2. Structural and compositional characteristics are used to explain observed changes in electrical properties.