Structural regulation of electrochemically deposited copper layers for fabrication of thin film solar cells with a CuInS 2 photoabsorber

Electrodeposition of Cu layers on a Mo-coated glass (Mo/glass) substrate from CuSO 4 solutions containing citric acid and sodium citrate was studied for fabrication of CuInS 2 -based solar cells. When the deposition was performed using the CuSO 4 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 CuSO 4 solution containing sodium citrate. Electrochemical studies revealed that both organic additives worked as complexing agents to reduce the concentration of free Cu 2+ ions, but complexation abilities of these additives were different, leading to the structural difference in the resulting Cu layers. The CuInS 2 film derived from the former less-compact Cu layer had good adhesion between the CuInS 2 film and Mo/glass, whereas the CuInS 2 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 CuInS 2 film. As expected from these differences, the solar cell with an Al:ZnO/CdS/CuInS 2 /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.