Structural regulation of electrochemically deposited copper layers for fabrication of thin film solar cells with a CuInS2 photoabsorber

Abstract

Electrodeposition of Cu layers on a Mo-coated glass (Mo/glass) substrate from CuSO4 solutions containing citric acid and sodium citrate was studied for fabrication of CuInS2-based solar cells. When the deposition was performed using the CuSO4 solution containing citric acid, a Cu layer consisting of less-compact roundish grains with a rugged surface morphology was obtained. On the other hand, a compact Cu layer composed of large angular grains was formed from the CuSO4 solution containing sodium citrate. Electrochemical studies revealed that both organic additives worked as complexing agents to reduce the concentration of free Cu2+ ions, but complexation abilities of these additives were different, leading to the structural difference in the resulting Cu layers. The CuInS2 film derived from the former less-compact Cu layer had good adhesion between the CuInS2 film and Mo/glass, whereas the CuInS2 film obtained from the latter compact Cu layer was partially exfoliated because of large volume expansion stresses during the transformation of Cu and In stacked layers into the CuInS2 film. As expected from these differences, the solar cell with an Al:ZnO/CdS/CuInS2/Mo/glass structure derived from the former less-compact Cu layer showed better properties than those of the solar cell derived from the latter compact Cu layer.