Kinetics of Regeneration and Recombination Reactions in Dye-Sensitized Solar Cells Employing Cobalt Redox Shuttles

Abstract

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, Ln, 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, ηreg, 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 TiO2 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 Ln 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.