A series of cobalt-containing redox couples, based on [Co(1,10-phenanthroline)(3)](ClO4)(2) and its derivatives, were prepared for use as regenerators/shuttles in dye-sensitized solar cells featuring modified TiO2 photoelectrodes. Surface modification and trap-state passivation of the TiO2 nanoparticle film electrodes were accomplished via atomic layer deposition of an ultrathin alumina coating. Electron lifetimes were then extracted from open-circuit voltage decay measurements. Cells employing alumina barrier/passivation layers exhibited higher open-circuit voltages as shuttles with more positive redox potentials were used, with the Co(5-nitro-phen)(3)(3+ 2+) couple exhibiting the highest V-oc (0.844 V). Analysis of the open-circuit voltages and electron lifetimes indicate Marcus normal-region behavior for back electron transfer front the TiO2 photoanode to these compounds.