Effect of ZnO layer thickness on efficiency of Cu(In,Ga)Se ₂ thin-film solar cells

The effect of intrinsic ZnO(i-ZnO) layer thickness on the efficiency of Cu(In,Ga)Se ₂ (CIGS) thin film solar cells was investigated using ITO/ZnO/CdS/CIGS/Mo structures. CIGS thin films were deposited on Mo-coated soda-lime glass using co-evaporation. CdS buffer layers of about 50nm thickness were then grown by chemical bath deposition on the top of CIGS layer. Finally, the ZnO and ITO layers were deposited using rf-magnetron sputtering, resulting in solar cells with ITO/ZnO/CdS/CIGS/Mo structure. From the optical and electrical characteristics of the solar cells, we found a close relationship between the transmittance of the ZnO layer and the efficiency of the solar cells. Several characteristics improved for solar cells with a 50 nm thick ZnO layer relative to those with both 90 nm thick and no ZnO layer. Therefore, we conclude that the optimum ZnO thickness for CIGS-based solar cells is around 50 nm.