Performance Enhancement and Limitations of Cobalt Bipyridyl Redox Shuttles in Dye-Sensitized Solar Cells

A series of one-electron outersphere cobalt bipyridyl redox couples were used as redox shuttles in dye-sensitized solar cells (DSSCs). Atomic layer deposition was used to deposit an ultrathin coating of alumina on nanoparticle-based TiO2 DSSC photoanodes, which results in significantly improved quantum yields for all of the DSSCs containing outersphere redox systems. However, a significant discrepancy in performance remains between DSSCs containing the different cobalt redox shuttles. Variation of the driving force for regeneration by ∼500 mV, by employing [Ru(bpy)2(4,4′-dicarboxy-bpy)](PF6)2 as a dye, combined with concentration dependence studies indicates that the cobalt redox couples are not limited by dye regeneration; however, in certain cases the iodide electrolyte was, one of the very few systems where alternate redox couples perform significantly better than triiodide/iodide. Electron lifetimes were measured with the open circuit voltage decay technique. The differences in the lifetimes (recombination kinetics) of DSSCs employing cobalt redox couples correlate well with the differences in the incident photon-to-current efficiencies (IPCEs), providing strong evidence that the external quantum efficiencies of DSSCs with cobalt polypyridyl redox couples are limited by recombination. We further found that, contrary to previous reports, the cobalt(III/II) tris(4,4′-dimethyl-2,2′-bipyridine) couple can produce comparable external quantum yields to cobalt(III/II) tris(4,4′-di-tert-butyl-2,2′-bipyridine) when employed as redox shuttles in DSSCs. However, the photovoltaic performances of both are constrained by mass transport of the oxidized species through the nanoparticle photoelectrode.