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Regeneration and Recombination kinetics in Cobalt Polypyridine based Dye-Sensitized Solar Cells, explained using Marcus theory
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
Institute of chemical science and engineering, Ecole Polytechnique Fédérale de Lausanne.
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2013 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 15, no 19, 7087-7097 p.Article in journal (Refereed) Published
Abstract [en]

Regeneration and recombination kinetics was investigated for dye-sensitized solar cells (DSCs) using a series of different cobalt polypyridine redox couples, ranging in redox potential in between 0.34 and 1.20 V vs. NHE. Marcus theory was applied to explain the rate of electron transfer. The regeneration kinetics for a number of different dyes (L0, D35, Y123, Z907) by most of the cobalt redox shuttles investigated occurred in the Marcus normal region. The calculated reorganization energies for the regeneration reaction ranged between 0.59 and 0.69 eV for the different organic and organometallic dyes investigated. Under the experimental conditions employed, the regeneration efficiency decreased when cobalt complexes with a driving force for regeneration of 0.4 eV and less were employed. The regeneration efficiency was found to depend on the structure of the dye and the concentration of the redox couples. [Co(bpy-pz)2]2+, which has a driving force for regeneration of 0.25 eV for the triphenylamine based organic dye, D35, was found to regenerate 84 % of the dye molecules, when a high concentration of the cobalt complex was used. Recombination kinetics between electrons in TiO2 and cobalt (III) species in the electrolyte was also studied using steady state dark current measurements. This reaction occurred in the Marcus inverted region for most of the cobalt complexes, and recombination losses are thus not expected to be problematic for D35-sensitized DSCs employing cobalt complexes with high redox potentials. Recombination mediated by surface states was, however, found to significantly influence the result for the cobalt complexes with most positive redox potentials. The calculated system reorganization energies using Marcus theory from the regeneration kinetics and steady state current measurements were very similar, indicating that they are mostly determined by the cobalt mediator.

Place, publisher, year, edition, pages
2013. Vol. 15, no 19, 7087-7097 p.
National Category
Physical Chemistry
URN: urn:nbn:se:uu:diva-192668DOI: 10.1039/c3cp50997dISI: 000317980600013OAI: oai:DiVA.org:uu-192668DiVA: diva2:600441
Available from: 2013-01-24 Created: 2013-01-24 Last updated: 2013-06-04Bibliographically approved
In thesis
1. Alternative Redox Couples for Dye-Sensitized Solar Cells
Open this publication in new window or tab >>Alternative Redox Couples for Dye-Sensitized Solar Cells
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Dye-sensitized solar cells (DSCs) convert sunlight to electricity at a low cost. In the DSC, a dye anchored to a mesoporous TiO2 semiconductor is responsible for capturing the sunlight. The resulting excited dye injects an electron into the conduction band of the TiO2 and is in turn regenerated by a redox mediator, normally iodide/triiodide, in a surrounding electrolyte. The success of the iodide/triiodide redox couple is mainly attributed to its slow interception of electrons at the TiO2 surface, which suppresses recombination losses in the DSC.

One of the main limitations with the iodide/triiodide redox couple is, however, the large driving force needed for regeneration, which minimizes the open circuit voltage and thus the energy conversion efficiency. In this thesis, alternative redox couples to the iodide/triiodide redox couple have been investigated. These redox couples include the one-electron transition metal complexes, ferrocene and cobalt polypyridine complexes. The use of one-electron redox couples in the DSC has previously been shown to lead to poor photovoltaic performances, because of increased recombination.

Cobalt redox couples were here found to give surprisingly high efficiencies in combination with the triphenylamine-based organic dye, D35. The success of the D35 dye, in combination with cobalt redox couples, was mainly attributed to the introduction of steric alkoxy chains on the dye, which supress recombination losses. By introducing steric substituents on the dye, rather than on the redox couple, mass transport limitations could in addition be avoided, which previously has been suggested to limit the performance of cobalt complexes in the DSC. The result of this study formed the basis for the world record efficiency of DSCs of 12.3 % using cobalt redox couples.

Interfacial electron-transfer processes in cobalt-based DSCs were investigated to gain information of advantages and limitations using cobalt redox couples in the DSC. The redox potentials of cobalt redox couples are easily tuned by changing the coordination sphere of the complexes, and regeneration and recombination kinetics were systematically investigated by increasing the redox potential of the cobalt complexes. Our hope is that this thesis can be a guideline for future design of new redox systems in DSCs. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. 80 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1017
redox mediator, triphenylamine, cobalt, ferrocene, titanium dioxide, regeneration, recombination
National Category
Physical Chemistry
Research subject
Physical Chemistry
urn:nbn:se:uu:diva-192694 (URN)978-91-554-8595-5 (ISBN)
Public defence
2013-03-22, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Available from: 2013-03-01 Created: 2013-01-24 Last updated: 2013-04-02Bibliographically approved

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Feldt, SandraBoschloo, GerritHagfeldt, Anders
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