Diffusion barriers for CIGS solar cells on metallic substrates

Abstract Al 2 O 3 layers of 1–3 μm were deposited as diffusion barriers by RF sputtering from a ceramic target on metal foils of Ti, Kovar ® and Cr steel. Cu(In,Ga)Se 2 (CIGS)-based thin-film solar cells were deposited onto these substrates using a co-evaporation process for CIGS at T ⩾550 °C. CIGS solar cells of 0.25 cm 2 achieved efficiencies of approximately 10–11% without any Na doping. Without barriers, the cell efficiencies were limited to significantly lower values except for Ti substrates. The reduced efficiency values can be attributed mainly to a reduction in fill factors, and secondly, to reduced open-circuit voltages. The different solar cell efficiencies can be correlated with the amount of impurities entering the CIGS layer by diffusion from the substrates, as investigated by simultaneous secondary ion mass spectrometry and sputtered neutral mass spectrometry depth profiling. Without diffusion barriers, Fe and Cr concentrations of several hundred ppm were detected in CIGS layers on Cr steel. Fe, Ni, Co and Ti concentrations from Kovar ® and Ti substrates were much smaller, indicating a reduced diffusion. Using Al 2 O 3 barriers, the concentrations of Fe and Cr in CIGS are reduced proportionally to the barrier thickness by up to a factor of 100 when compared to systems without barriers.