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.
Wuerz et al. (Mon,) studied this question.