Transparent back-junction control in Cu(In,Ga)Se2 absorber for high-efficiency, color-neutral, and semitransparent solar module

Abstract

Transparent oxide back contact of Cu(In,Ga)Se2 (CIGS) solar cells is crucial for semitransparent or bifacial CIGS photovoltaics. Parasitic GaOx resistive layer formed at the back junction, however, has been the bottleneck for transparent photovoltaic applications. Here, we show that a complementary control of the back junction enabled a high efficiency, semitransparent CIGS module. By employing a thin Ag precursor layer prior to CIGS evaporation, the back-junction barrier could be eliminated. Atomic probe tomography measurement suggests that an appreciable amount of Ag doping in the GaOx may form hole-tunneling channels at the back junction. We also found that potassium fluoride (KF) post-deposition treatment (PDT) increased the back-junction resistance and the bulk CIGS resistance compared to the case with Na PDT. The KF-induced increased resistances turned out to be beneficial for suppressing the parasitic shunt current along the P1-scribing region in our CIGS solar module. Interestingly, J？V rollover caused by the KF-induced back-junction barrier completely vanished after see-through laser scribing of the module. We attribute the barrier-lowering to a laser-induced local heating at the back junction. The complementary engineering strategy introduced here would allow functional oxide back contact in CIGS photovoltaic modules for both high efficiency and semi-transparency. ？ 2021 John Wiley & Sons, Ltd.