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Energy Alignment and Surface Dipoles of Rylene Dyes adsorbed to TiO2 nanoparticles
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, Physics, Department of Physics and Astronomy.
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.
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2011 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 13, no 32, 14767-14774 p.Article in journal (Refereed) Published
Abstract [en]

The energy loss in dye-sensitized solar cells calculated from the energy difference between the lowest electronic transition of the dye and the obtained open-circuit voltage is often 1 eV or even more. To minimize this loss, it is important to accurately determine the energy alignment at the TiO2/dye/redox-mediator interface. In this study, we compared the results from electrochemistry and photoelectron spectroscopy for determining the energy alignment of three rylene dyes, two of which absorb relatively far in the red. The trends observed with the methods were different, as in the former, the energy alignment is measured relative to an external reference and includes contributions from solvent reorganization energies, while in the latter, it is measured relative to the energetics of the TiO2 and is lacking such contributions. The influence of the dyes' dipole moments on the energetics of the TiO2 was also measured and explained some of the differences in trends. Finally, we compared the injection efficiencies of the two red-absorbing dyes and found that the differences in injection efficiencies can be better explained using the energy alignment determined from photoelectron spectroscopy. This shows that the method for measuring the energetics of a DSC should be chosen according to what process one intends to study.

Place, publisher, year, edition, pages
2011. Vol. 13, no 32, 14767-14774 p.
National Category
Natural Sciences
Research subject
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-150046DOI: 10.1039/c1cp20911fISI: 000293516200058OAI: oai:DiVA.org:uu-150046DiVA: diva2:406260
Available from: 2011-03-25 Created: 2011-03-25 Last updated: 2014-09-30Bibliographically approved
In thesis
1. Characterisation of Organic Dyes for Solid State Dye-Sensitized Solar Cells
Open this publication in new window or tab >>Characterisation of Organic Dyes for Solid State Dye-Sensitized Solar Cells
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Energy from the sun can be converted to low cost electricity using dye-sensitized solar cells (DSCs). Dye molecules adsorbed to the surface of mesoporous TiO2 absorb light and inject electrons into the semiconductor. They are then regenerated by the reduced redox species from an electrolyte, typically consisting of the iodide/tri-iodide redox couple in an organic solvent. In a solid state version of the DSC, the liquid electrolyte is replaced by an organic hole conductor. Solid state DSCs using 2,2'7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD) have reached conversion efficiencies of up to 6 %, which is about half of the efficiency of the best iodide/tri-iodide cells.

 

Measurement techniques, such as spectroelectrochemistry and photo-induced absorption spectroscopy (PIA), were developed and applied to study the working mechanism of organic dyes in solid state DSCs under solar cell operating conditions. The energy alignment of the different solar cell components was studied by spectroelectrochemistry and the results were compared to photoelectron spectroscopy. PIA was used to study the injection and regeneration processes. For the first time, it was shown here that the results of PIA are influenced by an electric field due to the electrons injected into the TiO2. This electric field causes a shift in the absorption spectrum of dye molecules adsorbed to the TiO2 surface due to the Stark effect.

 

Taking the Stark effect into consideration during the data analysis, mechanistic differences between solid state and conventional DSCs were found. A perylene dye, ID176, was only able to efficiently inject electrons into the TiO2 in presence of lithium ions and in absence of a solvent. As a result, the sensitiser worked surprisingly well in solid state DSCs but not in liquid electrolyte ones. Regeneration of oxidised dye molecules by spiro-MeOTAD was found to be fast and efficient and spiro-MeOTAD could even reduce excited dye molecules.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2011. 89 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 814
Keyword
energy alignment, hole conductor, injection, interface, perylene, photo-induced absorption, regeneration, spectroelectrochemistry, spiro-MeOTAD, Stark effect, titanium dioxide
National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-150047 (URN)978-91-554-8042-4 (ISBN)
Public defence
2011-05-13, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2011-04-20 Created: 2011-03-25 Last updated: 2011-05-04Bibliographically approved

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Cappel, UteJohansson, Erik M. J.Hagfeldt, AndersBoschloo, GerritRensmo, Håkan
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