Dynamics of electron recombination and transport in water-splitting dye-sensitized photoanodes

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) use visible light to split water using molecular sensitizers and water oxidation catalysts codeposited onto mesoporous TiO₂ electrodes. Despite a high quantum yield of charge injection and low requirement for the catalytic turnover rate, the quantum yield of water splitting in WS-DSPECs is typically low (<1%). Here we examine the charge separation and recombination processes in WS-DSPECs photoanodes functionalized with varying amounts of IrO₂ nanoparticle catalyst. Charge extraction and transient open-circuit voltage decay measurements provide insight into the relationship between light intensity, conduction band electron density, open-circuit photovoltage, and recombination time scale. We correlate these results with electrochemical impedance spectroscopy and present the first complete equivalent circuit model for a WS-DSPEC. The data show quantitatively that recombination of photoinjected electrons with oxidized sensitizer molecules and scavenging by the water oxidation catalyst limit the concentration of conduction band electrons and by extension the photocurrent of WS-DSPECs.