Improved photoelectrochemical water splitting using ITO-enhanced In₂S₃−modified CuInS₂ photoelectrodes via low-temperature all-ink processing

This work reports a fully ink-based, low-temperature strategy (≤250 °C) for fabricating In₂S₃-modified CuInS₂ (CIS) photoelectrodes with enhanced interfacial properties for photoelectrochemical (PEC) water splitting. The In₂S₃ overlayer, deposited via spin-coating from a thiourea-based molecular ink, was optimized to improve charge separation and band alignment at the CIS surface. A two-cycle deposition yielded the best PEC performance, increasing the photocurrent density from 0.25 to 1.75 mA cm⁻² at 0 V_RHE and shifting the onset potential anodically from 0.5 V_RHE to 0.7 V_RHE. Further enhancement was achieved by incorporating a sputtered indium tin oxide (ITO) layer atop In₂S₃, which improved lateral electron conductivity and enabled more uniform Pt catalyst nucleation. This resulted in a photocurrent of 3.5 mA cm⁻², comparable to values obtained with electron scavengers. The improvements are attributed to reduced interfacial charge transfer resistance and more homogeneous catalyst distribution, as confirmed by electrochemical impedance spectroscopy (EIS) and SEM-EDX mapping. These findings highlight the critical role of surface/interface engineering and conductive top layer in developing scalable, high-performance PEC electrodes via low-temperature processing.