Effects of Electron Trapping and Protonation on the Efficiency of Water-Splitting Dye-Sensitized Solar Cells

摘要

Water-splitting dye-sensitized photoelectrochemical (WS-DSPECs) cells employ molecular sensitizers to absorb light and transport holes across the TiO_2 surface to colloidal or molecular water oxidation catalysts. As hole diffusion occurs along the surface, electrons are transported through the mesoporous TiO_2 film. In this paper we report the effects of electron trapping and protonation in the TiO_2 film on the dynamics of electron and hole transport in WS-DSPECs. When the sensitizer bis(2,2'-bipyridine)(4,4'-diphosphonato-2,2'-bipyridine)-ruthenium(Ⅱ) is adsorbed from aqueous acid instead of from ethanol, there is more rapid hole transfer between photo-oxidized sensitizer molecules that are adsorbed from strong acid. However, the photocurrent and open-circuit photovoltage are dramatically lower with sensitizers adsorbed from acid because intercalated protons charge-compensate electron traps in the TiO_2 film. Kinetic modeling of the photocurrent shows that electron trapping is responsible for the rapid electrode polarization that is observed in all WS-DSPECs. Electrochemical impedance spectroscopy suggests that proton intercalation also plays an important role in the slow degradation of WS-DSPECs, which generate protons at the anode as water is oxidized to oxygen.