Sodium control in Ultrathin Cu(In,Ga)Se2 solar cells on transparent back contact for efficiencies beyond 12

Ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells on transparent conductive oxide (TCO) back contacts combine advantages of ultrathin cells for reducing material consumption of rare indium and gallium and TCO-transparency benefited applications in tandems, bifacial configurations etc. However, their efficiencies are still limited and the back barrier potential is a primary reason from an electrical perspective. In this work, we explore the effects of Nadoping by post deposition treatment (PDT) on the performance of ultrathin CIGSe solar cells on ITO (Sn:In2O3)-coated Na-free glass substrates. Na doping enhances not only the open circuit-voltage (Voc) by increasing the doping level, but also the fill factor (FF) by switching the Schottky contact to an Ohmic contact at the CIGSe/ITO interface, which we propose is due to the increased recombination at the back interface. The optimum performance is achieved at a NaF dose of 2 mg with a top efficiency of 12.9%, which exhibits an enhancement by nearly 48% relative compared to the references without Na doping. To our best knowledge, this is the highest efficiency achieved for ultrathin cells (