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

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 (E_cb) 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 V_oc 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 V_oc. Toward higher open circuit voltage, a design guide of organic dyes controlling the electron lifetime is discussed.