Influence of Cbd-Deposited CdS on the Carrier Collection in Cigs-Based Solar Cells

CdS films were deposited with chemical bath deposition (CBD) onto Cu(In,Ga)Se₂/Mo/soda-lime glass structures. The basic ingredients for the CBD were ammonia (NH₄OH), cadmium acetate (CdAc), and thiourea CS(NH₂)₂. Two recipes with very low thiourea concentration were compared to the baseline recipe. From quantum efficiency measurements we observe an increased carrier collection for longer wavelengths for the modified buffer recipes as compared to baseline. The higher carrier collection is explained by an increase in the depletion region width. A lower bandgap is observed for the modified buffer. The solar cells with modified buffer layers have lower voltages and fill factors than the solar cells with our baseline buffer, leading to a lower efficiency for the modified cell structures in spite of slightly higher current densities. A tentative explanation is given assuming an inverted CIGS layer at the CIGS/buffer interface, possibly with Cd as a donor element