Properties of Cu(In,Ga)Se2 Thin Film Solar Cells Fabricated by Single CIGS Targets with Various Compositions

Abstract

Among all thin film-type solar cells, the highest conversion efficiency of ~20% has been obtained from CIGS thin films deposited by a three-stage co-evaporation method. Since this method is very complicated and thus inappropriate for a mass production of CIGS thin films, we tried to develop a simple technology utilizing the sputter method in this study. Various CIGS sputter targets composed of Cux(In0.7Ga0.3)Sey (x=0.85~1, y=2~2.5) were synthesized by planetary ball milling. Employing these targets, CIGS films could be successfully produced by RF magnetron sputtering. Compositional and structural analyses of CIGS films were performed by electron probe micro analyzer, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The CIGS thin film fabricated by using the single Cux(In0.7Ga0.3)Sey(x=1, y=2) target showed densely packed morphology. XRD patterns revealed that CIGS films consisted of chalcopyrite structure without secondary phases. These CIGS thin films were selenized with Se powder in a tube furnace. Cu-Se related secondary phases were observed after selenization. Meanwhile, the CIGS thin films fabricated by employing the Cu-poor and Se-excess single CIGS targets showed porous microstructure and chalcopyrite structure including (In,Ga)2Se3 secondary-phase. After selenization, secondary phase was eliminated and CIGS grains are more densely packed compared with CIGS films before selenization. The CIGS solar cells with the structure of Al/AZO/ i-ZnO/CdS/CIGS/Mo/glass was fabricated to measure the conversion efficiency. Up to now, a conversion efficiency of ~3.29% has been achieved from the CIGS solar cell fabricated by using the single CIGS target, and further improvement is under progress. Details on the effects of the target composition and selenization conditions on the conversion efficiency of CIGS solar cells will be presented for a discussion.