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.