Performance improvement of semi-transparent ultra-thin CIGSe solar cell by transferring exfoliated WTe₂ multilayered-2D flakes to ITO substrate

Cu(In_1-x,Ga_x)Se₂ (CIGSe)-based solar cells have a high absorption coefficient (∼10⁵), by which they can be used as thin-film solar cells that have the potential to be applied in various fields. To employ such a CIGSe-based solar cell as a transparent type, the conventional Mo substrate should be replaced with a transparent conductive oxide (TCO) substrate as the back electrode, and indium tin oxide (ITO) is one of the candidate TCO materials. However, various problems need to be addressed because the utilization of ITO may hinder solar cell performance, one of which is the contact occurring at the interface between CIGSe and ITO. This can be solved by adding an appropriate material at the interface while preserving the inherent purpose fulfilled by using the ITO electrode. In this study, we attempt to improve the contact properties using semi-metallic WTe₂ flakes dispersed on the surface of the ITO substrate to form an electrical path between the CIGSe and the ITO substrate, which exhibit outstanding electrical conductivity among two-dimensional transition metal dichalcogenide materials. Accordingly, the effect of applying WTe₂ flakes at the ITO and CIGSe interfaces on the performance of semi-transparent ultra-thin (STUT) CIGSe solar cells is discussed.