Performance and Uniformity Improvement in Ultrathin Cu(In,Ga)Se 2 Solar Cells with a WO x Nanointerlayer at the Absorber/Transparent Back-Contact Interface.

Thinning CIGSe absorber layer to less than 500 nm is desirable for reducing the cost per unit watt of PV-generated electricity and also the semitransparent solar cells based on such a thin absorber can be used in bifacial and superstrate configurations if the back electrode is transparent. In this study, a WO layer is inserted between Cu(In,Ga)Se (CIGSe) absorber and tin-doped indium oxide (ITO) back-contact to enhance the hole collection at the back electrode. A WO interlayer with a thickness of 6 nm is found to be optimum because it causes a ~38% relative increase in the fill factor (FF) of a ~450 nm thick CIGSe-based device compared to the reference device without a WO interlayer. While fixing the thickness of CIGSe, increasing the WO interlayer thickness ≥6 nm results in decreases of solar cell parameters primarily due to the emergence of a GaO interfacial layer at the CIGSe/WO junction.