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 photovoltaic-generated electricity, and also, the semitransparent solar cell 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 _x layer is inserted between Cu(In,Ga)Se ₂ (CIGSe) absorber and tin-doped indium oxide back-contact to enhance the hole collection at the back electrode. A WO _x interlayer with a thickness of 6 nm is found to be optimum because it causes a ∼38% relative increase in the fill factor of a ∼450 nm thick CIGSe-based device compared to the reference device without a WO _x interlayer. While fixing the thickness of CIGSe, increasing the WO _x interlayer thickness to ≥6 nm results in decreases of solar cell parameters primarily because of the emergence of a GaO _x interfacial layer at the CIGSe/WO _x junction.