Iodide electron transfer kinetics in dye-sensitized nanocrystalline TiO2 films

Electron transfer kinetics plays a key role in determining the energy conversion efficiency of dye-sensitized photoelectrochemical solar cells. Photoinduced charge separation in such cells results in oxidation of the sensitizer dye. The resulting dye cation may be rereduced by recombination with injected electrons or by electron transfer from iodide ions in the redox electrolyte, often referred to as the regeneration reaction. In this paper, we employ transient absorption spectroscopy to investigate the kinetic competition between these two pathways in Ru(dcbPY)(2)(NCS)(2)-sensitized nanocrystalline film TiO2 electrodes immersed in a propylene carbonate electrolyte. The experiments monitored both the dye cation decay kinetics and the yield of product species, assigned to I-2(-) radicals generated by electron transfer from iodide ions to the dye cation. The kinetic competition between the recombination and the regeneration processes is found to be dependent upon both the iodide concentration and the electrical bias applied to the dye-sensitized electrode. Similar regeneration kinetics were observed when Zn-tetra-p-carboxy-phenyl-porphyrin was used as sensitizer dye. In contrast to the recombination reaction, the rate of the dye cation regeneration reaction by iodide is found to be independent of applied bias. At high iodide concentrations, the cation regeneration reaction is sufficiently fast to compete successfully with the recombination reaction for all biases studied. When an intermediate iodide concentration is used, the acceleration of the recombination kinetics at negative biases results in a reduction in the dye cation lifetime and a loss of I-2(-) yield. This bias dependence is found to be in good agreement with a numerical modeling of the data employing a continuous time random walk model for the charge recombination dynamics, assuming the iodide reaction to be first-order, in iodide concentration. We conclude by discussing the implications of these observations for solar cell function.