Adaptation of the surface-near Ga content in co-evaporated Cu(In,Ga)Se2 for CdS versus Zn(S,O)-based buffer layers

In this work, we show that in order to optimize the efficiency of Cu(In1 -x,Gax)Se2 (CIGS) solar cells with Cd-free Zn(S,O)-based buffer layers, the Ga concentration in the CIGS absorber layer towards the hetero-interface has to be adapted. We varied the In and Ga deposition rates in the last stage of our 3-stage co-evaporation process, leading to different compositional ratios xf = [Ga] / ([Ga] + [In]) between 0.15 and 0.6 in the top 400 nm of the absorber layer. All absorber layers were then completed with both CdS and Zn(S,O) buffer layers by chemical bath deposition. While cells with our standard grading of xf ≈ 0.4 in the front region result in a best performance of 15% with a CdS buffer, similar efficiencies with a Zn(S,O) buffer layer are only obtained when the Ga content near the hetero-interface is reduced down to x ≈ 0.25. The maximum efficiency for the CdS buffer layer coincides with the maximum open circuit voltage (Voc) and fill factor (FF). Interestingly, for the Zn(S,O) buffer layer, this is not the case: the Voc increases steadily for higher Ga ratios, while the FF is fairly constant for 0.25 < x < 0.5 and decreases drastically for more extreme values. The findings are explained by differences in the conduction band offsets which result from the conduction band shift close to the surface due to Ga content variations. The results illustrate the importance of the absorber layer adaptation for different buffer layers and are an important step on the way to Cd-free buffer layers.