Efficiency enhancement in photoelectrochemical water splitting: Defect passivation and boosted charge transfer kinetics of zinc oxide nanostructures via chalcopyrite/chalcogenide mix sensitization

ZnO thin films in nanorod (NR) and nanoflower (NF) morphologies were used as photoelectrode scaffolds for efficient visible-light-driven photoelectrochemical (PEC) water splitting process, where their decoration with copper indium gallium sulfide (CIGS) and indium sulfide (${\mathrm{In}}_{2}{\mathrm{S}}_{3}$) layers resulted in significant PEC performance enhancement. ZnO NF/CIGS/${\mathrm{In}}_{2}{\mathrm{S}}_{3}$ photoelectrodes exhibited a remarkably high PEC efficiency ($\ensuremath{\sim}6.0%$ applied bias photon-to-current efficiency, 83% incident photon-to-current efficiency) due to the negligible dark current, while ZnO NR/CIGS/${\mathrm{In}}_{2}{\mathrm{S}}_{3}$ generated a photocurrent density of $30.0\phantom{\rule{0.28em}{0ex}}\mathrm{mA}.\mathrm{c}{\mathrm{m}}^{--2}$ at 0.4 V (vs Ag/AgCl), being one of the highest performances reported in the literature for copper-based chalcopyrite photoelectrodes so far. The interfacial photoelectrode-electrolyte charge transport dynamics, investigated via intensity-modulated photocurrent spectroscopy, exhibited a sevenfold increase in charge transfer efficiencies with a significant drop in surface recombination kinetics for ZnO NF after CIGS/${\mathrm{In}}_{2}{\mathrm{S}}_{3}$ decoration. The obtained results show consistency with numerically modeled electric field distribution profiles and electron paramagnetic resonance results of ZnO NF, rationalizing the enhanced charge transfer rates for decorated samples and confirming the defect passivating nature of CIGS/${\mathrm{In}}_{2}{\mathrm{S}}_{3}$.