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 CuInS 2 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 CuInS 2 from Cu–In–O reveals that Ag doping significantly promotes grain growth. Consequently, the CuInS 2 photocathode achieves one of the highest photoelectrochemical activities (−9.8 mA cm −2 at 0 V RHE ) reported for CuInS 2 photoelectrodes synthesized via wet chemical methods. Bias‐free water splitting is achieved using a CuInS 2 ‐based photoelectrode in a photovoltaic–PEC cell configuration. These results highlight the potential of CuInS 2 , prepared through wet chemical methods, for cost‐effective photoelectrochemical water splitting.