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Characterisation of Organic Dyes for Solid State Dye-Sensitized Solar Cells
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Physical Chemistry.
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 [en]
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: urn:nbn:se:uu:diva-150047ISBN: 978-91-554-8042-4OAI: oai:DiVA.org:uu-150047DiVA: diva2:406937
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
List of papers
1. Dye regeneration by spiro-MeOTAD in solid state dye-sensitized solar cells studied by photoinduced absorption spectroscopy and spectroelectrochemistry.
Open this publication in new window or tab >>Dye regeneration by spiro-MeOTAD in solid state dye-sensitized solar cells studied by photoinduced absorption spectroscopy and spectroelectrochemistry.
2009 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, no 15, 6275-6281 p.Article in journal (Refereed) Published
Abstract [en]

Photoinduced absorption (PIA) spectroscopy is presented as a tool for the systematic study of dye regeneration and pore filling in solid state dye-sensitized solar cells (DSC). Oxidn. potentials and extinction coeffs. for oxidized species of the perylene dye, ID28, on TiO2 and of the hole conductor, 2,2'7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD), were detd. by spectroelectrochem. The onset of oxidn. of a solid film of spiro-MeOTAD was found to be 0.15 V vs. Fc/Fc+ and extinction coeffs. of spiro-MeOTAD+ were found to be 33 000 M-1 cm-1 at 507 nm and 8500 M-1 cm-1 at 690 nm. Electrons in TiO2 films were shown to alter the ground-state absorption spectra of ID28 attached to TiO2. PIA measurements indicated a good contact between ID28 and spiro-MeOTAD for different spiro-MeOTAD concns. for both 2- and 6-micro m thick TiO2 films. We discuss the possibility of estg. the quality of pore filling from the positions of absorption peaks. Results suggested that with a spiro-MeOTAD concn. of 300 mg mL-1 in chlorobenzene, a uniform distribution of spiro-MeOTAD in the pores of the 6-micro m thick TiO2 film could be achieved.

National Category
Chemical Sciences
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-100979 (URN)10.1021/jp811196h (DOI)000265030200058 ()
Available from: 2009-04-15 Created: 2009-04-15 Last updated: 2011-05-06Bibliographically approved
2. The influence of local electric fields on photoinduced absorption in dye-sensitized solar cells.
Open this publication in new window or tab >>The influence of local electric fields on photoinduced absorption in dye-sensitized solar cells.
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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.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-135810 (URN)10.1021/ja102334h (DOI)000279561200067 ()20552960 (PubMedID)
Available from: 2010-12-09 Created: 2010-12-08 Last updated: 2011-05-04Bibliographically approved
3. A broadly absorbing perylene dye for solid-state dye-sensitized solar cells.
Open this publication in new window or tab >>A broadly absorbing perylene dye for solid-state dye-sensitized solar cells.
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2009 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, no 33, 14595-14597 p.Article in journal (Refereed) Published
Abstract [en]

We present a new perylene sensitizer, ID 176, for dye-sensitized solar cells (DSCs). The dye has the capability for very high photocurrents due to strong absorption from 400 to over 700 rim. Photocurrents Of LIP to 9 mA cm(-2) were achieved in solid-state DSCs employing the hole conductor 2,2'7,7'-tetrakis-(NN-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD), with a conversion efficiency of 3.2%. In contrast, the sensitizer did not perform well in conjunction with liquid iodide/tri-iodide electrolytes, suggesting a difference in the injection and regeneration mechanisms in these two types of dye-sensitized solar cells.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-128305 (URN)10.1021/jp906409q (DOI)000268907500004 ()
Available from: 2010-07-21 Created: 2010-07-20 Last updated: 2012-02-10Bibliographically approved
4. Characterization of the Interface Properties and Processes in Solid State Dye-Sensitized Solar Cells Employing a Perylene Sensitizer
Open this publication in new window or tab >>Characterization of the Interface Properties and Processes in Solid State Dye-Sensitized Solar Cells Employing a Perylene Sensitizer
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2011 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 10, 4345-4358 p.Article in journal (Refereed) Published
Abstract [en]

We recently reported on a perylene sensitizer, ID176, which performs much better in solid state dye-sensitized solar cells than in those using liquid electrolytes with iodide/tri-iodide as the redox couple (J. Phys. Chem. C2009, 113, 14595-14597). Here, we present a characterization of the sensitizer and of the TiO2/dye interface by UV-visible absorption and fluorescence spectroscopy, spectroelectrochemistry, photoelectron spectroscopy, electroabsorption spectroscopy, photoinduced absorption spectroscopy, and femtosecond transient absorption measurements. We report that the absorption spectrum of the sensitizer is red-shifted by addition of lithium ions to the surface due to a downward shift of the excited state level of the sensitizer, which is of the same order of magnitude as the downward shift of the titanium dioxide conduction band edge. Results from photoelectron spectroscopy and electrochemistry suggest that the excited state is largely located below the conduction band edge of TiO2 but that there are states in the band gap of TiO2 which might be available for photoinduced electron injection. The sensitizer was able to efficiently inject into TiO2, when a lithium salt was present on the surface, while injection was much less effective in the absence of lithium ions or in the presence of solvent. In the presence of the hole conductor 2,2-,7,7-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9-spirobifluorene (spiro-MeOTAD) and LiTFSI, charge separation was monitored by the emergence of a Stark shift of the dye in transient absorption spectra, and both injection and regeneration appear to be completed within 1 ps. Regeneration by spiro-MeOTAD is therefore several orders of magnitude faster than regeneration by iodide, and ID176 can even be photoreduced by spiro-MeOTAD.

National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-149013 (URN)10.1021/jp111466h (DOI)000288113400064 ()
Available from: 2011-03-14 Created: 2011-03-14 Last updated: 2012-12-06Bibliographically approved
5. Energy Alignment and Surface Dipoles of Rylene Dyes adsorbed to TiO2 nanoparticles
Open this publication in new window or tab >>Energy Alignment and Surface Dipoles of Rylene Dyes adsorbed to TiO2 nanoparticles
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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.

National Category
Natural Sciences
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-150046 (URN)10.1039/c1cp20911f (DOI)000293516200058 ()
Available from: 2011-03-25 Created: 2011-03-25 Last updated: 2014-09-30Bibliographically approved

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