Substituted Polypyridine Complexes of Cobalt(II/III) as Efficient Electron-Transfer Mediators in Dye-Sensitized Solar Cells

A number of cobalt complexes of substituted polypyridine ligands were synthesized and investigated as possible alternatives to the volatile and corrosive iodide/triiodide redox couple commonly used as an electron-transfer mediator in dye-sensitized solar cells (DSSCs). The extinction coefficients in the visible spectrum are on the order of 102 M-1 cm-1 for the majority of these complexes, diminishing competition with the light-harvesting dye. Cyclic voltammetric studies revealed a dramatic surface dependence of the heterogeneous electron-transfer rate, which is surprisingly different for gold, carbon, and platinum electrodes. DSSCs were assembled using a mediator that consisted of a mixture of Co(II) and Co(III) complexes in a 10:1 ratio. DSSCs containing these mediators were used to characterize incident photon-to-current conversion efficiency and photoelectrochemical responses. The best performing of these mediators were identified and subjected to further study. As suggested by electrochemical results, gold and carbon are superior cathode materials to platinum, and no evidence of corrosion on any cathode material was observed. Addition of lithium salts to the mediator solution resulted in a dramatic improvement in cell performance. The observed Li+ effect is explained in terms of the recombination of injected electrons in the photoanode with the oxidized mediator. The best mediator, based on tris(4,4¢-di-tert-butyl-2,2¢-dipyridyl)- cobalt(II/III) perchlorate, resulted in DSSCs exhibiting efficiencies within 80% of that of a comparable iodide/ triiodide-mediated DSSC. Due to the commercial availability of the ligand and the simplicity with which the complex can be made, this new mediator represents a nonvolatile, noncorrosive, and practical alternative as an efficient electron-transfer mediator in DSSCs.