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, 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.

Dye-sensitized electron transfer from TiO2 to oxidized triphenylamines that follows first-order kinetics

“… 6 – 13 This is likely due to the dominance of I – /I 3 – as the prototypical redox mediator employed in DSSCs that is not easily amenable to systematic studies. 14 , 15 However, alternative redox mediators, such as Co(III/II) polypyridyl complexes, 16 – 23 triphenylamine or phenothiazine, 24 , 25 have provided new opportunities to gain fundamental information on how sensitive charge recombination is to the electrolyte composition, 6 , 26 molecular structure, 12 , 27 or the thermodynamic driving force. 7 – 9 …”

Insights into the excited state dynamics of Fe(ii) polypyridyl complexes from variable-temperature ultrafast spectroscopy

“…Information about reorganization energies associated with the excited states of [Ru(bpy) 3 ] 2+ are known,38 but we believe that a better comparison is afforded from self-exchange measurements of Co( iii ) complexes where reduction of a low-spin d 6 complex typically yields a high-spin species as the product. Hamann et al have in fact recently published a study of the self-exchange parameters of various Co( iii ) complexes in the context of their work on Co-based redox shuttles for use in dye-sensitized solar cells 39. These researchers report a reorganization energy associated with self-exchange of [Co(bpy) 3 ] 2+ of 3.21 eV.…”

Light-induced electrolyte improvement in cobalt tris(bipyridine)-mediated dye-sensitized solar cells

“…However, less attention has been dedicated to the one-electron, outer-sphere transition-metal complexes recently qualied as attractive redox shuttle alternatives. Such redox complexes in particular offer a high photovoltage of DSSCs, [4][5][6][7] and tunable redox potential and coordination sphere that can be used to control the driving forces and kinetics of the two rate-limiting electron transfer processes (including dye regeneration and electron recombination in the device 8,9 ) via structural changes in the complex ligands. 7,[10][11][12] Paradoxically, one-electron redox couples of this type offer fast regeneration of the oxidized dye molecules, but at the same time suffer from fast recombination loss reactions.…”

“…5,[13][14][15] Co(II/III)-polypyridyl redox systems are the most studied and to date the most successfully employed in DSSCs, mainly due to the advantageous large inner-sphere reorganization energy necessarily for spin transition which reduces the interfacial charge-transfer loss. 9,[16][17][18] Nevertheless, the recombination issue at the electrode/electrolyte interface still prevails for such redox mediators. One reason is attributed to the sluggish transport of the bulky complex ions, especially so when high-boiling and typically viscous solvents are used.…”