Improving V_oc With Indium and Alkali Fluorides in Cu(In,Ga)Se₂ Solar Cells Deposited at Low Temperature on Polyimide

Flexible, lightweight, Cu(In,Ga)Se ₂ photovoltaic technology is becoming more and more popular, thanks to the very high efficiencies already achieved (>20%) and to the possible employment of roll-to-roll deposition techniques, offering new application opportunities. In this paper, we aim to improve the understanding and performances of recently discovered new front surface engineering approaches. We show that the improvement of Voc generally observed using heavy alkali (below Na) can be boosted by adding indium during the alkali postdeposition treatments (PDTs). The obtained modification of the absorber using NaF/KF + In PDT leads to an improvement of V _oc by 20 mV with respect to NaF/KF PDT and by almost 50 mV with respect to NaF-PDT only. The electrical performances of solar cells prepared with different PDTs are analyzed and discussed in light of structural and optical characterization data, supported by physical modeling. We show that KF PDTs modify the absorber over a thin region close to the surface, and also deep in the bulk of the film. Our results with KF show a record efficiency of 18.1% without antireflecting coating (ARC), a value that is close to the highest reported value of 20.4% (with ARC).