Effect of different cadmium salts on the properties of chemical-bath-deposited CdS thin films and Cu(InGa)Se2 solar cells

Thin films of cadmium sulfide (CdS) have been prepared on soda-lime glass, Mo-coated glass, and Cu(InGa)Se2 (CIGS) thin films by a chemical bath deposition (CBD) technique using three different cadmium (Cd) sources: cadmium sulfate (CdSO4), cadmium acetate (CdAc2), and cadmium chloride (CdCl2). Among the three cadmium salts, CdCl2, which forms the most stable Cd-anion complex, produced the thinnest CdS films, presumably due to the slow release of Cd ions. Considering the different growth rates of the three Cd salts, CdS films with a thickness of approximately 70 nm were deposited on the CIGS absorber for the subsequent fabrication of devices with a glass/Mo/CIGS/CdS/i-ZnO/ZnO:Al structure. The CIGS/CdS samples were annealed at temperatures in the range of 100-200 °C. It was found that the optimum annealing temperature for the best cell performance was around 150 °C and that the short circuit current density (JSC) was mainly improved due to the enhanced light absorption after thermal annealing. However, annealing at 200 °C resulted in the damage of the CIGS/CdS junction, as evidenced by the reduction of the open-circuit voltage and fill factor without a significant change in JSC. At the optimum annealing temperature (150 °C), CdCl2 yielded the highest CIGS cell efficiency (9.96%), followed by CdAc2 (9.60%) and CdSO4 (9.09%).