Assessment of Indium-Free Transparent Conductive Oxide Back Contacts for High-Efficiency Ultra-Thin Cu (In,Ga)Se₂ Solar Cells Down to 250 nm

This work examines the feasibility and performance impact of replacing the usual molybdenum back contact with indium-free transparent conductive oxides (TCOs) like fluorine-doped tin oxide (SnO₂:F) and aluminum-doped zinc oxide (ZnO:Al) for ultra-thin Cu (In,Ga)Se₂ (CIGS) solar cells (250–450 nm). Motivated by indium scarcity and cost reduction, these TCOs are evaluated for their figure of merit, stability under Se atmosphere, Na diffusion permeability, and band alignment with CIGS absorbers. Using simulations, prototype fabrication, and comprehensive characterizations, the compatibility of these TCOs with CIGS absorbers is assessed. Solar cells with thicknesses of 450 and 250 nm are fabricated. Their performance was compared under both rear and front illumination, as well as with the use of reflectors. A record efficiency of 8.6% with front illumination is achieved for a 250-nm CIGS absorber using a gold back reflector with SnO₂:F, single-step CIGS deposition, and no heavy alkalines doping. The best rear-illuminated efficiencies are obtained with ZnO:Al back contacts, reaching 6% for a 250-nm CIGS, with only a 9% loss in J_sc compared to front illumination, confirming a lower surface recombination rate at the ZnO:Al/CIGS interface compared to Mo/CIGS or SnO₂:F/CIGS interfaces.