Development of High-Efficiency Cd-Free Cu(In,Ga)Se₂ Solar Cells Using Chemically Deposited ZnS Film

Abstract

A ZnS film covered completely the CIGS surface without pinholes. The thickness of the ZnS film could be controlled according to the cycle number of the CBD process. As the thickness of the ZnS film increased, the J_SC value of a CIGS solar cell with the ZnS film decreased sharply due to the increase in the series resistance of the cell. The QE spectrum of the CIGS solar cell with a 50 nm-thick ZnS film consequently showed very small values through the whole wavelength range. On the other hand, the V_OC and FF values of the CIGS solar cell with a 27 nm-thick ZnS film were significantly influenced by the sputtering power for deposition of the ZnO:Al film. By decreasing the sputtering power during deposition of the ZnO:Al film, the V_OC and FF values of the CIGS solar cells with a 27 nm-thick ZnS film were sharply improved due to a reduction of sputtering damage at the ZnS surface and ZnS/CIGS interface. Additionally, the J_SC value of the CIGS solar cell with a 27 nm-thick ZnS film was improved by annealing the ZnS film. As a result of an annealing process, the QE value in a wavelength range of 400–700 nm increased. Finally, we achieved a conversion efficiency of 14.21 % for a CIGS solar cell with a ZnS film by optimizing the fabrication conditions.