Study on the thickness effect of wide-bandgap CuGaSe2 thin films for applications with tandem solar cells

For application on the top cell of a tandem structure, chalcopyrite CuGaSe₂ (CGS) thin films were prepared on as-deposited 200-nm-thick indium-tin-oxide (ITO) thin films grown by using radiofrequency (RF) magnetron sputtering. CGS thin films with a wide bandgap of 1.63 eV for use in high open-circuit-voltage solar cells were deposited by using a three-stage co-evaporation process with Cu, Ga, and Se elemental sources with compositional ratios of Cu/Ga = 0.88 and Se/(Cu + Ga) = 0.98. In this study, we examined the effect of the thickness of the top-cell CGS thin films on the tandem cell performance, which is a key factor for improving the cell’s efficiency for optimum light absorption. The film thickness was varied from 0.5 μm to 2 μm in intervals of 0.5 μm by controlling the process time to confirm the optical and the electrical properties of solar cells. Based on our experimental results as a function of the CGS film’s thickness, we achieved a solar cell efficiency of 5.77% with a 1.5-μm-thick CGS thin film in the cell structure fabricated as Al/ZnO: Al/i-ZnO/CdS/CGS/ITO/SLG.