Roles of Electrolytes on Charge Recombination in Dye-Sensitized TiO2Solar Cells (2): The Case of Solar Cells Using Cobalt Complex Redox Couples

Abstract

Dye-sensitized solar cells (DSC) were prepared from nanoporous TiO₂ electrodes with two different cobalt complex redox couples, propylene-1,2-bis(o-iminobenzylideneaminato)cobalt(II) {Co^II(abpn)} and tris(4,4‘-di-tert-buthyl-2,2‘-bipyridine)cobalt(II) diperchlorate {Co^II(dtb-bpy)₃(ClO₄)₂}. The performances of the DSCs were examined with varying the concentrations of the redox couples and Li cations in methoxyacetonitrile. Under 1 sun conditions, short-circuit currents (J_sc) increased with the increase of the redox couple concentration, and the maximum J_sc was found at the Li⁺ concentration of 100 mM. To rationalize the observed trends of J_sc, electron diffusion coefficients and lifetimes in the DSCs were measured. Electron diffusion coefficients in the DSCs using cobalt complexes were comparable to the previously reported values of nanoporous TiO₂. Electron lifetime was independent of the concentration of the redox couples when the concentration ratio of Co^II(L) and Co^III(L) was fixed. With the increase of Li⁺ concentration, the electron lifetime increased. These results were interpreted as due to their slow charge-transfer kinetics and the cationic nature of Co complex redox couples, in contrast to the anionic redox couple of I^-/I₃^-. The increase of the lifetimes with Li⁺ was interpreted with the decrease of the local concentration of Co^III near the surface of TiO₂. The addition of 4-tert-butylpyridine (tBP) with the presence of Li⁺ increased J_sc significantly. The observed increase of the electron lifetime by tBP could not explain the large increase of J_sc, implying that tBP facilitates the charge transfer from Co^II(L) to dye cation, with the association of the change of the reorganization energy between Co^II and Co^III.