The Case of Solar Cells Using Cobalt Complex Redox Couples:Roles of Electrolytes on Charge Recombination in Dye-Sensitized TiO_2 Solar Cells (2)

摘要

Dye-sensitized solar cells (DSC) were prepared from nanoporous TiO_2 electrodes with two different cobalt complex redox couples,propylene-l,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(ClO_4)_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 TiO_2.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~-/I_3~-.The increase of the lifetimes with Li~+ was interpreted with the decrease of the local concentration of Co~(III) near the surface of TiO_2.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).