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Design of organic dyes and cobalt polypyridine redox mediators for high-efficiency dye-sensitized solar cells
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Physical Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Physical Chemistry.
KTH, Organisk kemi.
KTH, Organisk kemi.
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2010 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 132, no 46, 16714-16724 p.Article in journal (Refereed) Published
Abstract [en]

Dye-sensitized solar cells (DSCs) with cobalt-based mediators with efficiencies surpassing the record for DSCs with iodide-free electrolytes were developed by selecting a suitable combination of a cobalt polypyridine complex and an organic sensitizer. The effect of the steric properties of two triphenylamine-based organic sensitizers and a series of cobalt polypyridine redox mediators on the overall device performance in DSCs as well as on transport and recombination processes in these devices was compared. The recombination and mass-transport limitations that, previously, have been found to limit the performance of these mediators were avoided by matching the properties of the dye and the cobalt redox mediator. Organic dyes with higher extinction coefficients than the standard ruthenium sensitizers were employed in DSCs in combination with outer-sphere redox mediators, enabling thinner TiO2 films to be used. Recombination was reduced further by introducing insulating butoxyl chains on the dye rather than on the cobalt redox mediator, enabling redox couples with higher diffusion coefficients and more suitable redox potential to be used, simultaneously improving the photocurrent and photovoltage of the device. Optimization of DSCs sensitized with a triphenylamine-based organic dye in combination with tris(2,2'-bipyridyl)cobalt(II/III) yielded solar cells with overall conversion efficiencies of 6.7% and open-circuit potentials of more than 0.9 V under 1000 W m(-2) AM1.5 G illumination. Excellent performance was also found under low light intensity indoor conditions.

Place, publisher, year, edition, pages
2010. Vol. 132, no 46, 16714-16724 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-139393DOI: 10.1021/ja1088869ISI: 000284792000064PubMedID: 21047080OAI: oai:DiVA.org:uu-139393DiVA: diva2:381305
Available from: 2010-12-27 Created: 2010-12-23 Last updated: 2017-12-11Bibliographically 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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1017
Keyword
redox mediator, triphenylamine, cobalt, ferrocene, titanium dioxide, regeneration, recombination
National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
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)
Opponent
Supervisors
Available from: 2013-03-01 Created: 2013-01-24 Last updated: 2013-04-02Bibliographically approved

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Feldt, Sandra M.Boschloo, GerritHagfeldt, Anders

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