Solution transformation of Cu₂O into CuInS₂ for solar water splitting

Though Cu₂O has demonstrated high performance as a photocathode for solar water splitting, its band gap is too large for efficient use as the bottom cell in tandem configurations. Accordingly, copper chalcopyrites have recently attracted much attention for solar water splitting due to their smaller and tunable band gaps. However, their fabrication is mainly based on vacuum evaporation, which is an expensive and energy consuming process. Here, we have developed a novel and low-cost solution fabrication method, and CuInS₂ was chosen as a model material due to its smaller band gap compared to Cu₂O and relatively simple composition. The nanostructured CuInS₂ electrodes were synthesized at low temperature in crystalline form by solvothermal treatment of electrochemically deposited Cu₂O films. Following the coating of overlayers and decoration with Pt catalyst, the as-fabricated CuInS₂ electrode demonstrated water splitting photocurrents of 3.5 mA cm^-2 under simulated solar illumination. To the best of our knowledge, this is the highest performance yet reported for a solution-processed copper chalcopyrite electrode for solar water splitting. Furthermore, the electrode showed good stability and had a broad incident photon-to-current efficiency (IPCE) response to wavelengths beyond 800 nm, consistent with the smaller bandgap of this material.