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.
A new low-spin (LS) cobalt(II) outer-sphere redox shuttle (OSRS) [Co(PY5Me 2 )(CN)] + , where PY5Me 2 represents the pentadentate ligand 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine, has been synthesized and characterized for its potential application in dye-sensitized solar cells (DSSCs). Introduction of the strong field CN − ligand into the open axial coordination site forced the cobalt(II) complex, [Co-(PY5Me 2 )(CN)] + , to become LS based upon the complex's magnetic susceptibility (1.91 ± 0.02 μ B ), determined by the Evans method. Interestingly, dimerization and subsequent cobalt hexacyanide cluster formation of the [Co(PY5Me 2 )(CN)] + monomer was observed upon long-term solvent exposure or addition of a supporting electrolyte for electrochemical characterization. Although long-term stability of the [Co(PY5Me 2 )-(CN)] + complex made it difficult to fabricate liquid electrolytes for DSSC applications, short-term stability in neat solvent afforded the opportunity to isolate the self-exchange kinetics of [Co(PY5Me 2 )(CN)] 2+/+ via stopped-flow spectroscopy. Use of Marcus theory provided a smaller than expected self-exchange rate constant of 20 ± 5.5 M −1 s −1 for [Co(PY5Me 2 )(CN)] 2+/+ , which we attribute to a Jahn−Teller effect observed from the collected monomer crystallographic data. When compared sideby-side to cobalt tris(2,2′-bipyridine), [Co(bpy) 3 ] 3+ , DSSCs employing [Co(PY5Me 2 )(CN)] 2+ are expected to achieve superior charge collection, which result from a smaller rate constant, k et , for recombination based upon simple dark J−E measurements of the two redox shuttles. Given the negative redox potential (0.254 V vs NHE) of [Co(PY5Me 2 )(CN)] 2+/+ and the slow recombination kinetics, [Co(PY5Me 2 )(CN)] 2+/+ becomes an attractive OSRS to regenerate near IR absorbing sensitizers in solid-state DSSC devices.
A cobalt(IV/III) redox shuttle, cobalt tris(2-(p-tolyl)pyridine), [Co(ptpy)], was synthesized and investigated for use in dye-sensitized solar cells, DSSCs. An incredibly fast self-exchange rate constant of (9.2 ± 3.9) × 10 M s was determined for [Co(ptpy)], making it an ideal candidate for dye regeneration. To avoid fast recombination and solubility limitations, we utilized a tandem electrolyte containing [Co(ptpy)] and cobalt tris(2,2'-bipyridine), [Co(bpy)]. An improved short circuit current density is achieved for DSSCs employing the tandem electrolyte, compared to electrolytes containing only [Co(bpy)], consistent with superior dye regeneration expected based on predictions using Marcus theory, which is also discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.