Structure, morphology and properties of thinned Cu(In, Ga)Se2 films and solar cells

Cu(In, Ga)Se2 (CIGS) thin films with different thicknesses were deposited by the three-stage coevaporation process at a constant substrate temperature. The structure, morphology and optical and electrical properties of films and solar cell performance were studied. It was found that the degree of (1 1 2) preferred orientation becomes strong with reducing thickness. Owing to the reaction of Cu2−xSe with In and Ga supplied in the third stage, a lot of cavities existed in the upper part of the films which were thicker than 1 µm. On reducing the thickness, the band gap increased a little and the absorption coefficient decreased in the region of low photon energies. However, thinning the CIGS film had little influence on the electrical properties of the films which were thinner than 1.2 µm. The solar cells with thinned absorber layers disclosed that there were no thickness-related losses unless the CIGS absorber was thinner than 1 µm. The efficiency of the solar cell with a thickness of 1.26 µm was 13.21%, while it was only 9.28% for the absorber of 0.57 µm in this study, and the reasons are discussed. (paper)