The redox mediator in the electrolyte of the dye-sensitized solar cell (DSC) takes care of (1) reduction of the oxidized dye after photoinduced electron injection into TiO2, and (2) transport of charges in the electrolyte between the electrodes. The conventional redox mediator in the DSC is the iodide / triiodide (I-/I3-) couple. It has some major drawbacks: much voltage is lost in dye regeneration, it has a rather strong color, and it is very corrosive.
Cobalt polypyridine complexes are very interesting alternative redox mediators in DSC. Charge recombination is, however, much faster for these one-electron mediators than for the iodide system. By molecular modification of the dye, recombination of the electrons in TiO2 with the oxidized form of the redox couple can be suppressed. We achieved a breakthrough for cobalt mediator based DSC in 2010 by matching the steric properties of the cobalt mediator and the sensitizer dye (Feldt et al., J. Am. Chem. Soc. 132 (2010) 16714).
Cobalt complexes with bipyridines, terpyridines, phenanthrolines, etc., have a wide range of redox potentials, offering the prospect of a significant increase in the voltage output of DSCs (to more than one volt). Light absorption in the visible range is very low. Several issues still need to be resolved: because of the relatively large size of the complexes there can be mass transport limitations; other problems are solubility issues and long-term stability issues. The prospect for cobalt-based electrolytes is, however, very good: the current world record efficiency for a dye-sensitized solar cell (12.3%) is held by a DSC with a cobalt-based electrolyte.
(Cobalt-electrolytes and other R&D materials from our research are commercially available at Dyenamo AB.)