Charge Recombination with Fractional Reaction Orders in Water-Splitting Dye-Sensitized Photoelectrochemical Cells

In water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs), charge recombination competes with catalytic water oxidation to determine the overall efficiency of the system. The kinetics of these processes have been difficult to understand because transient absorbance (TA) experiments typically show nearly complete charge recombination on the submillisecond time scale; in contrast, electrochemical measurements such as open circuit photovoltage decay suggest a charge recombination time scale that is 2-3 orders of magnitude longer. Here we explore these processes with dye-sensitized nanocrystalline TiO2 and TiO2/Ta2O5 core-shell photoanodes in aqueous electrolytes using TA spectroscopy, intensity-modulated photovoltage spectroscopy (IMVS), and photoelectrochemical impedance spectroscopy (PEIS). The fast recombination rates measured by TA result from strong laser excitation that leads to high electron occupancy in TiO2, whereas IMVS modulates the concentration of charge-separated states near solar irradiance levels. The recombination processes measured by electrochemical methods such as IMVS, PEIS, and transient photovoltage are the discharging of injected electrons in TiO2, as evidenced by the close agreement between the nearly first-order recombination rates probed by IMVS and the RC time constants derived from PEIS data. However, IMVS measurements at variable probe light intensity reveal that the reaction orders for the recombination of injected electrons with oxidized sensitizer molecules are far from unity. This kinetic analysis is relevant to understanding steady-state recombination rates in full WS-DSPECs in which molecular and nanoparticle catalysts are used to oxidize water.