Bifurcation of Regeneration and Recombination in Dye-Sensitized Solar Cells via Electronic Manipulation of Tandem Cobalt Redox Shuttles

A cobalt­(IV/III) redox shuttle, cobalt tris­(2-(p-tolyl)­pyridine), [Co­(ptpy)₃]^+/0, was synthesized and investigated for use in dye-sensitized solar cells, DSSCs. An incredibly fast self-exchange rate constant of (9.2 ± 3.9) × 10⁸ M^–1 s^–1 was determined for [Co­(ptpy)₃]^+/0, making it an ideal candidate for dye regeneration. To avoid fast recombination and solubility limitations, we utilized a tandem electrolyte containing [Co­(ptpy)₃]^+/0 and cobalt tris­(2,2′-bipyridine), [Co­(bpy)₃]^3+/2+. An improved short circuit current density is achieved for DSSCs employing the tandem electrolyte, compared to electrolytes containing only [Co­(bpy)₃]^3+/2+, consistent with superior dye regeneration expected based on predictions using Marcus theory, which is also discussed.