Investigation of Grain Growth in Chalcopyrite CuInS2 Photoelectrodes Synthesized under Wet Chemical Conditions for Bias-Free Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) cells offer a promising method for producing green hydrogen through the splitting of water using solar energy. However, the cost-effective synthesis of highly crystalline p-type semiconductor materials for PEC cells remains a significant challenge for industrial applications. Herein, a CuInS₂ photoelectrode is fabricated using a scalable and economical wet chemical spin-coating technique. To enhance the crystallinity and photoelectrochemical activity of the photoelectrode, the grain size is precisely controlled by adjusting the atomic ratio, thickness, morphology, and Ag doping. Evaluating a novel growth mechanism of CuInS₂ from Cu–In–O reveals that Ag doping significantly promotes grain growth. Consequently, the CuInS₂ photocathode achieves one of the highest photoelectrochemical activities (−9.8 mA cm⁻² at 0 V_RHE) reported for CuInS₂ photoelectrodes synthesized via wet chemical methods. Bias-free water splitting is achieved using a CuInS₂-based photoelectrode in a photovoltaic–PEC cell configuration. These results highlight the potential of CuInS₂, prepared through wet chemical methods, for cost-effective photoelectrochemical water splitting.