Interfacial Electron-Transfer Kinetics in Metal-Free Organic Dye-Sensitized Solar Cells: Combined Effects of Molecular Structure of Dyes and Electrolytes

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Abstract
Electron diffusion coefficient, lifetime, and density in the TiO2 electrode of dye-sensitized TiO2 solar cells (DSCs) employing I−/I3− redox couples were measured with eight different metal-free organic dyes and three Ru complex dyes. At matched electron density, all DSCs using organic dyes (ODSCs) showed shorter electron lifetime with comparable or larger diffusion coefficients in comparison to the DSCs using the Ru dyes (RuDSC). The shorter lifetime was attributed partially to the slower dye cation reduction rate of the organic dyes by I−, faster electron diffusion coefficient in the TiO2, and mostly higher I3− concentration in the vicinity of the TiO2 surface. Whereas a slight shift of the conduction band edge potential (Ecb) of the TiO2 was seen with a few organic dyes, no correlation was found with the dipole moment of the adsorbed dyes. This implies that the adsorbed dyes interact with cations in the electrolyte, so the direction of the dipole is altered or simply screened. The increase of [I3−] in the vicinity of the TiO2 surface was interpreted with partial charge distribution of the dyes. Under one-sun conditions, less electron density due to shorter electron lifetime was found to be the main reason for the lower values of Voc for all ODSCs in comparison to that of RuDSCs. Among the organic dyes, having larger molecular size and alkyl chains showed longer electron lifetime, and thus higher Voc. Toward higher open circuit voltage, a design guide of organic dyes controlling the electron lifetime is discussed.