Dynamics of Electron Recombination and Transport in
Water-Splitting Dye-Sensitized Photoanodes
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Abstract
Water-splitting dye-sensitized photoelectrochemical
cells (WS-DSPECs)
use visible light to split water using molecular sensitizers and water
oxidation catalysts codeposited onto mesoporous TiO<sub>2</sub> 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 IrO<sub>2</sub> 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