Spin-Doctoring Cobalt Redox Shuttles for Dye-Sensitized Solar Cells.

A new low-spin (LS) cobalt(II) outer-sphere redox shuttle (OSRS) [Co(PY5Me2)(CN)]+, where PY5Me2 represents the pentadentate ligand 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine, has been synthesized and characterized for its potential application in dye-sensitized solar cells (DSSCs). Introduction of the strong field CN- ligand into the open axial coordination site forced the cobalt(II) complex, [Co(PY5Me2)(CN)]+, to become LS based upon the complex's magnetic susceptibility (1.91 ± 0.02 µB), determined by the Evans method. Interestingly, dimerization and subsequent cobalt hexacyanide cluster formation of the [Co(PY5Me2)(CN)]+ monomer was observed upon long-term solvent exposure or addition of a supporting electrolyte for electrochemical characterization. Although long-term stability of the [Co(PY5Me2)(CN)]+ complex made it difficult to fabricate liquid electrolytes for DSSC applications, short-term stability in neat solvent afforded the opportunity to isolate the self-exchange kinetics of [Co(PY5Me2)(CN)]2+/+ via stopped-flow spectroscopy. Use of Marcus theory provided a smaller than expected self-exchange rate constant of 20 ± 5.5 M-1 s-1 for [Co(PY5Me2)(CN)]2+/+, which we attribute to a Jahn-Teller effect observed from the collected monomer crystallographic data. When compared side-by-side to cobalt tris(2,2'-bipyridine), [Co(bpy)3]3+, DSSCs employing [Co(PY5Me2)(CN)]2+ are expected to achieve superior charge collection, which result from a smaller rate constant, ket, for recombination based upon simple dark J- E measurements of the two redox shuttles. Given the negative redox potential (0.254 V vs NHE) of [Co(PY5Me2)(CN)]2+/+ and the slow recombination kinetics, [Co(PY5Me2)(CN)]2+/+ becomes an attractive OSRS to regenerate near IR absorbing sensitizers in solid-state DSSC devices.

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.