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 CoII/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 (Voc) of almost 1.0 V at 100% sun for devices constructed with
the tBP complex.