Fast Low-Spin Cobalt Complex Redox Shuttles for Dye-Sensitized Solar Cells

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Abstract
A low-spin cobalt(II) complex, cobalt bis(trithiacyclononane), [Co(ttcn)2]3+/2+, was investigated for use as a redox shuttle in dye-sensitized solar cells, DSSCs. This unique cobalt complex redox shuttle is stable, transparent, and easy to synthesize from commercial ligands and has attractive energetic and kinetic features for use in DSSCs. Initial results indicate that the overall performance is limited by recombination. Variation of the sensitizer and deposition of an ultrathin coating of alumina on nanoparticle-based TiO2 DSSC photoanodes reduced recombination, which resulted in significantly improved quantum yields. The photovoltaic behavior was compared to the current record efficiency cobalt tris-bipyridine, [Co(bpy)3]3+/2+, redox shuttle and produced similar results. Further use of high extinction organic sensitizers with only ∼200 mV of driving force for regeneration was examined, which produced efficiencies of over 2%; importantly, regeneration is not rate-limiting in this system, thus demonstrating the promise of using such fast redox shuttles.