CGS-Thin Films Solar Cells on Transparent Back Contact

The goal of this work is to fabricate and study wide gap CuGaSe2 solar cells to be used as top cell of a mechanically stacked tandem device. CuGaSe2 (CGS) absorber thin films are prepared by co-evaporation in a three stage process onto Mo/soda-lime glass substrates and onto different transparent conductive oxides (TCOs): SnO2:F (FTO) and In2O3:Sn (ITO). Laser Light Scattering (LLS) is used as an in-situ process control. A deterioration of the transparency of the TCOs is observed after CGS deposition, especially for the ITO. The cell performance is lower for devices on TCOs due to the increased resistivity of the transparent back contact which leads to a decrease in fill factor (FF). The Cu-content during the process appears to be a critical parameter in order to control the optical transmission as well as the efficiency of the device. Quantum efficiency measurements also reveal a smaller minority carrier collection for the solar cells grown on TCOs. A total area efficiency of 4.2 % for a device on FTO is achieved.