A new direction in dye-sensitized solar cells redox mediator development: In situ fine-tuning of the cobalt(II)/(III) redox potential through lewis base interactions

Dye-sensitized solar cells (DSCs) are an attractive renewable energy technology currently under intense investigation. In recent years, one area of major interest has been the exploration of alternatives to the classical iodide/triiodide redox shuttle, with particular attention focused on cobalt complexes with the general formula [Co(L)_n]^2+/3+. We introduce a new approach to designing redox mediators that involves the application of [Co(PY5Me₂)(MeCN)]^2+/3+ complexes, where PY5Me₂ is the pentadentate ligand, 2,6-bis(1,1-bis(2-pyridyl)ethyl) pyridine. It is shown, by X-ray crystallography, that the axial acetonitrile (MeCN) ligand can be replaced by more strongly coordinating Lewis bases (B) to give complexes with the general formula [Co(PY5Me₂)(B)] ^2+/3+, where B = 4-tert-butylpyridine (tBP) or N-methylbenzimidazole (NMBI). These commonly applied DSC electrolyte components are used for the first time to fine-tune the potential of the redox couple to the requirements of the dye through coordinative interactions with the Co^II/III centers. Application of electrolytes based on the [Co(PY5Me₂)(NMBI)] ^2+/3+ complex in combination with a commercially available organic sensitizer has enabled us to attain DSC efficiencies of 8.4% and 9.2% at a simulated light intensity of 100% sun (1000 W m^-2 AM1.5 G) and at 10% sun, respectively, higher than analogous devices applying the [Co(bpy) ₃]^2+/3+ redox couple, and an open circuit voltage (V _oc) of almost 1.0 V at 100% sun for devices constructed with the tBP complex.