Role of Electrolytes on Charge Recombination in Dye-Sensitized TiO2 Solar Cell (1): The Case of Solar Cells Using the I-/I3- Redox Couple

Abstract

Performance of dye-sensitized solar cells (DSCs) was investigated depending on the compositions of the electrolyte, i.e., the electrolyte with a different cation such as Li⁺, tetra-n-butylammonium (TBA⁺), or 1,2-dimethyl-3-propylimidazolium (DMPIm⁺) in various concentrations, with and without 4-tert-butylpyridine (tBP), and with various concentrations of the I^-/I₃^- redox couple. Current−voltage characteristics, electron lifetime, and electron diffusion coefficient were measured to clarify the effects of the constituents in the electrolyte on the charge recombination kinetics in the DSCs. Shorter lifetimes were found for the DSCs employing adsorptive cations of Li⁺ and DMPIm⁺ than for a less-adsorptive cation of TBA⁺. On the other hand, the lifetimes were not influenced by the concentrations of the cations in the solutions. Under light irradiation, open-circuit voltages of DSCs decreased in the order of TBA⁺> DMPIm⁺ > Li⁺, and also decreased with the increase of [Li⁺]. The decreases of open-circuit voltage (V_oc) were attributed to the positive shift of the TiO₂ conduction band potential (CBP) by the surface adsorption of DMPIm⁺ and Li⁺. These results suggest that the difference of the free energies between that of the electrons in the TiO₂ and of I₃^- has little influence on the electron lifetimes in the DSCs. The shorter lifetime with the adsorptive cations was interpreted with the thickness of the electrical double layer formed by the cations, and the concentration of I₃^- in the layer, i.e., TBA⁺ formed thicker double layer resulting in lower concentration of I₃^- on the surface of the TiO₂. The addition of 4-tert-butylpyridine (tBP) in the presence of Li⁺ or TBA⁺ showed no significant influence on the lifetime. The increase of V_oc by the addition of tBP into the electrolyte containing Li⁺ and the I^-/I₃^- redox couple was mainly attributed to the shift of the CBP back to the negative potential by reducing the amount of adsorbed Li cations.