Assessment of Transparent Conductive Oxides as Back Contacts for Inline-Fabricated Cu(In,Ga)Se2 Solar Cells

The integration of transparent back contacts (TBCs) into Cu(In,Ga)Se₂ (CIGS) and (Ag,Cu)(In,Ga)Se₂ thin-film solar cells presents significant opportunities for enhancing device performance and enabling applications such as bifacial and tandem solar cells. However, the implementation of these TBCs in industry-relevant inline deposition processes poses several technological challenges, in particular regarding their thermal and chemical stability during the CIGS process at elevated temperatures while at the same time maintaining high electrical conductivity and high optical transparency. We investigate various transparent conductive oxides as TBCs candidates and the use of a thin Mo protection layer to address these challenges, focusing on optimizing CIGS process parameters to ensure seamless integration within existing industrial frameworks. Our results demonstrate that under appropriate CIGS growth conditions and TBC selection, TBCs with high transparency and low free carrier absorption in the near-infrared wavelength region can be successfully incorporated into an in-line CIGS deposition process. We establish a laser-scribing process for cell definition that replaces mechanical scribing, which often showed poor cell definition, and photolithography, which is a slow multi-step process. Indium tin oxide requires a thin Mo protection layer to remain functional after CIGS deposition. Zirconium-doped indium oxide (InZrO), however, does not require a protection/sacrificial layer and remains optical transparent and conductive, withstanding our CIGS deposition conditions under which other TBCs already degrade. For CIGS cells with a band gap of 1.1 eV, 14.2 % efficiency and high back contact transparency can be demonstrated with InZrO. Our analysis paves the way for the industrial implementation of advanced photovoltaic CIGS devices with improved efficiency and new functionalities.