Effect of solution-processed cesium carbonate on Cu(In,Ga)Se2 thin-film solar cells

Heavy alkali-metal treatments have been the most recent breakthrough in improving the efficiency of Cu(In,Ga)Se₂ (CIGS) solar cells. Alkali halides are generally evaporated onto the surface of the CIGS thin film by a vacuum process. Here, we report an alternative, low-cost solution process for the surface treatment of CIGS thin films using cesium carbonate (CsCO₃) as a new route to incorporate cesium (Cs) for improving solar cell performance. CIGS thin films were fabricated using pulsed hybrid reactive magnetron sputtering and the surface treatment was performed by spin-coating CsCO₃ solution on the surface of CIGS at room temperature, followed by vacuum annealing at 400°C. The surface chemistry of the CIGS thin film changed after the treatment and the efficiency of respective solar cells improved by more than 30%, mostly driven by an enhancement in open-circuit voltage. X-ray photoelectron spectroscopy revealed the depletion of copper and the presence of Cs on the surface of the CIGS thin film. Ultraviolet photoelectron spectroscopy showed the lowering of the valence band maximum by around 0.25 eV after the treatment, which plays a positive role in reducing interfacial recombination. High-resolution transmission electron microscopy indicates the presence of Cs and depletion of Cu at the grain boundaries of the CIGS thin film. These findings open a low-cost route for improving the performance of CIGS solar cells by surface modification using a solution process.