Charge Transfer Dynamics in Aqueous Dye-Sensitized Photoelectrochemical Cells: Implications for Water Splitting Efficiency

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) utilize molecular species for light-harvesting and water oxidation in order to store solar energy as hydrogen fuel. To engineer these devices for better performance, research has centered around suppressing charge recombination at the semiconductor-sensitizer interface and developing better catalysts for water oxidation. Yet it remains quantitatively unknown how much DSPECs can benefit from these improvements. We use a simplified photoanode process to model the charge transport dynamics in DSPECs under surface reaction-limiting conditions. By combining intensity-modulated photocurrent spectroscopy (IMPS) and numerical simulations, we explore in detail how electron transport and recombination rates as well as the sensitizer regeneration rate affect the steady-state photocurrent and the charge carrier concentration distribution. Numerical simulations confirm that fast electron diffusion in the semiconductor, a slow interfacial charge recombination rate, and rapid catalysis of water oxidation can improve the incident-photon-to-current-efficiency of DSPECs. The benefit, however, is largely compromised by the low charge injection efficiency, a problem that has not yet been fully appreciated. These simulations indicate that the best-known water oxidation catalysts are already adequate and that improvements in light harvesting and injection yields are the most important challenges for designing higher-performance WS-DSPECs.