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The influence of local electric fields on photoinduced absorption in 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.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Physical Chemistry.
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2010 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 132, no 26, 9096-9101 p.Article in journal (Refereed) Published
Abstract [en]

The dye-sensitized solar cell (DSC) challenges conventional photovoltaics with its potential for low-cost production and its flexibility in terms of color and design Transient absorption spectroscopy is widely used to unravel the working mechanism of DSCs A surprising, unexplained feature observed in these studies is an apparent bleach of the ground-state absorption of the dye, under conditions where the dye is in the ground state. Here, we demonstrate that this feature can be attributed to a change of the local electric field affecting the absorption spectrum of the dye, an effect related to the Stark effect first reported in 1913 We present a method for measuring the effect of an externally applied electric field on the absorption of dye monolayers adsorbed on flat TiO2 substrates. The measured signal has the shape of the first derivative of the absorption spectra of the dyes and reverses sign along with the reversion of the direction of the change in dipole moment upon excitation relative to the TiO2 surface A very similar signal is observed in photoinduced absorption spectra of dye-sensitized TiO2 electrodes under solar cell conditions, demonstrating that the electric field across the dye molecules changes upon illumination This result has important implications for the analysis of transient absorption spectra of DSCs and other molecular optoelectronic devices and challenges the interpretation of many previously published results.

Place, publisher, year, edition, pages
2010. Vol. 132, no 26, 9096-9101 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-135810DOI: 10.1021/ja102334hISI: 000279561200067PubMedID: 20552960OAI: oai:DiVA.org:uu-135810DiVA: diva2:375862
Available from: 2010-12-09 Created: 2010-12-08 Last updated: 2017-12-11Bibliographically 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)
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Supervisors
Available from: 2011-04-20 Created: 2011-03-25 Last updated: 2011-05-04Bibliographically approved

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Cappel, Ute B.Feldt, Sandra M.Hagfeldt, AndersBoschloo, Gerrit

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