Control of Structural and Electrical Properties of Indium Tin Oxide (ITO)/Cu(In,Ga)Se₂ Interface for Transparent Back-Contact Applications

Development of transparent-conducting oxide (TCO) back contact for Cu(In,Ga)Se₂ (CIGS) absorber is crucial for bifacial CIGS photovoltaics. However, inherent GaO_x formation at the TCO/CIGS interface has hampered the photocarrier extraction. Here, by controlling the Na doping scheme, we show that the hole transporting properties at the indium-tin oxide (ITO)/CIGS back contact can be substantially improved, regardless of the GaO_x formation. Na incorporation from the glass substrate during the GaO_x forming phase created defective states at the interface, which allowed efficient hole extraction from CIGS, while post Na treatment after GaO_x formation did not play such a role. Furthermore, we discovered that an almost GaO_x-free interface could be made by reducing the underlying ITO film thickness, which revealed that ITO/CIGS junction is inherently Schottky. In the GaO_x-free condition, post-Na treatment could eliminate the Schottky barrier and create ohmic junction due to generation of conducting paths at the interface, which is supported by our photoluminescence analysis.