Reduction of red-kink effect in Cu(In,Ga)Se2 thin-film bottom solar cells for tandem devices by sodium induced passivation of defects in CdS buffer layer

Abstract

When using Cu(In,Ga)Se2 (CIGS) thin-film solar cells as bottom cells in tandem devices, they are not illuminated with blue light. In this case, the red-kink effect, a distortion in the current–density voltage (JV) curve under illumination with red-light can be very pronounced and limit the efficiency of the bottom cell. JV measurements with different edge transmission filters revealed that the reason for the red-kink effect is located in the CdS buffer layer. The red-kink effect is very pronounced in sodium-free CIGS solar cells and can be strongly reduced by doping of CIGS with sodium (Na) or white light soaking. We observe a connection between the red-kink effect and the grain size of the CIGS layer. The red-kink effect in Na-free fine-grained CIGS is observed to be higher than in samples with coarse-grained CIGS. In contrast, after Na doping of the CIGS layer, the red-kink effect is more strongly reduced in fine-grained CIGS layers, which can be explained by better Na diffusion out of CIGS grain boundaries into the CdS layer in the case of fine-grained CIGS. The JV curves could be simulated by acceptor-like defects in the CdS layer, which lead to compensated CdS. These defects can either be passivated by Na or by illumination with blue light, leading to a decrease in the red-kink effect. Hence, a sufficient supply of Na into the CdS layer is necessary to passivate these defects in the CdS layer and to produce high efficient CIGS bottom cells for tandem applications.