Influence of Cation on Charge Recombination in Dye-Sensitized TiO2 Electrodes

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Abstract
The reaction of a dye cation recombining with an electron in TiO2, in the presence of Li+, Ca2+, and TBA+ cations, was studied with laser-induced transient absorption measurements. The active cations, Li+ and Ca2+, shorten the dye cation lifetime on sensitized TiO2 but not ZnO electrodes. By combining the absorbance measurements of the dye cation with simultaneous measurements of the current transient, the contribution of the recombination reaction to the current is identified. Furthermore, classical porous electrode theory is used to quantify the behavior of the heterogeneous electrode, and in doing so, the processes contributing to photoinduced current are identified as Helmholtz layer charging, porous electrode charging, recombination reactions, and surface diffusion of the active cations. The rate of charge recombination is proportional to the concentration of initially deposited active cations. The effect of water on the recombination rate and the current is also observed.