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The key to achieving high-efficiency dye-sensitized solar cells (DSSCs) is the realization of a redox shuttle which exhibits quantitative dye regeneration with a minimal driving force. Since the electron diffusion length, L n , is controlled by recombination to the redox shuttle, an optimal redox shuttle must balance the kinetics of these two key electron-transfer reactions. In this work the dye regeneration efficiency, η reg , and the electron diffusion length were determined for DSSCs employing cobalt tris(bipyridine), [Co(bpy) 3 ] 3+/2+ , and cobalt bis(trithiacyclononane), [Co(ttcn) 2 ] 3+/2+ , redox shuttles from optical and incident photon to current efficiency (IPCE) measurements of the cells under front side and back side illumination directions. The regeneration of the D35cpdt dye was found to be quantitative with [Co(ttcn) 2 ] 3+/2+ ; however, dye regeneration with the current champion redox shuttle [Co(bpy) 3 ] 3+/2+ is suboptimal despite a larger driving force of the reaction. The electron diffusion length was found to be shorter for DSSCs with the [Co(ttcn) 2 ] 3+/2+ redox shuttle compared to [Co(bpy) 3 ] 3+/2+ , however, due to faster recombination. The self-exchange rate constants of the two redox shuttles were determined from cross-exchange measurements and were found to differ by over 4 orders of magnitude. Application of Marcus theory allowed the difference in self-exchange rate constants to quantitatively account for the differences in regeneration efficiency and electron diffusion length of the two redox shuttles. Atomic layer deposition (ALD) was used to add a single layer of alumina on the TiO 2 film prior to immersing it in the sensitizer solution. This treatment resulted in improved performance for DSSCs employing both redox shuttles; however, the improvement was shown to arise from different causes. The alumina layer reduces recombination to the redox shuttle and thereby increases L n for [Co(ttcn) 2 ] 3+/2+ . The alumina layer was also shown to improve the dye regeneration efficiency for the [Co(bpy) 3 ] 3+/2+ redox shuttle through reduction of recombination to the oxidized dye. These findings clearly demonstrate the fine balance between the regeneration and recombination reactions when outer-sphere redox shuttles are employed in DSSCs. Isolation of the efficiency-limiting reactions, however, allows for strategies to overcome these barriers to be identified and implemented.

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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.