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Liquid Redox Electrolytes for Dye-Sensitized Solar Cells
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on liquid redox electrolytes in dye-sensitized solar cells (DSCs). A liquid redox electrolyte, as one of the key constituents in DSCs, typically consists of a redox mediator, additives and a solvent. This thesis work concerns all these three aspects of liquid electrolytes, aiming through fundamental insights to enhance the photovoltaic performances of liquid DSCs.

Initial attention has been paid to the iodine concentration effects in ionic liquid (IL)-based electrolytes. It has been revealed that the higher iodine concentration required in IL-based electrolytes can be attributed to both triiodide mobility associated with the high viscosity of the IL, and chemical availability of triiodide. The concept of incompletely solvated ionic liquids (ISILs) has been introduced as a new type of electrolyte solvent for DSCs. It has been found that the photovoltaic performance of ISIL-based electrolytes can even rival that of organic solvent-based electrolytes. And most strikingly, ISIL-based electrolytes provide highly stable DSC devices under light-soaking conditions, as a result of the substantially lower vapor pressure of the ISIL system. A significant synergistic effect has been observed when both guanidinium thiocyanate and N-methylbenzimidazole are employed together in an IL-based electrolyte, exhibiting an optimal overall conversion efficiency.

Tetrathiafulvalene (TTF) has been investigated as an organic iodine-free redox couple in electrolytes for DSCs. An unexpected worse performance has been observed for the TTF system, albeit it possesses a particularly attractive positive redox potential. An organic, iodine-free thiolate/disulfide system has also been adopted as a redox couple in electrolytes for organic DSCs. An impressive efficiency of 6.0% has successfully been achieved by using this thiolate/disulfide redox couple in combination with a poly (3, 4-ethylenedioxythiophene) (PEDOT) counter electrode material under full sunlight illumination (AM 1.5G, 100 mW/cm2). Such high efficiency can even rival that of its counterpart DSC using a state-of-the-art iodine-based electrolyte in the systems studied.The cation effects of lithium, sodium and guanidinium ions in liquid electrolytes for DSCs have been scrutinized. The selection of the type of cations has been found to exert quite different impacts on the conduction band edge (CB) of the TiO2 and also on the electron recombination kinetics, therefore resulting in different photovoltaic behavior.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , 60 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2012:2
Keyword [en]
dye-sensitized solar cells, electrolytes, ionic liquids, redox couples, additives
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-64139ISBN: 978-91-7501-231-5 (print)OAI: oai:DiVA.org:kth-64139DiVA: diva2:483008
Public defence
2012-02-16, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (Finnish)
Opponent
Supervisors
Note
QC 20120124Available from: 2012-01-24 Created: 2012-01-24 Last updated: 2012-01-24Bibliographically approved
List of papers
1. Investigation of Iodine Concentration Effects in Electrolytes for Dye-Sensitized Solar Cells
Open this publication in new window or tab >>Investigation of Iodine Concentration Effects in Electrolytes for Dye-Sensitized Solar Cells
Show others...
2010 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 114, no 23, 10612-10620 p.Article in journal (Refereed) Published
Abstract [en]

The present work describes the effects of different iodine concentrations and iodine-to-iodide ratios in electrolytes for dye-sensitized solar cells based on low-viscous, binary ionic liquid and organic liquid solvents. Current-voltage characteristics, photoelectrochemical measurements, electrochemical impedance spectroscopy, and Raman spectroscopy were used for characterization. Optimal short-circuit current and overall conversion efficiency were achieved using intermediate and low iodine concentration in ionic liquid-based and acetonitrile-based electrolytes, respectively. Results from photoelectrochemical and Raman-spectroscopic measurements reveal that both triiodide mobility and chemical availability affect the optimal iodine concentration required in these two types of electrolytes. The higher iodine concentrations required for the ionic liquid-based electrolytes partly compensate for these effects, although negative effects from higher recombination losses and light absorption of iodine-containing species start to become significant.

Keyword
Chemical availability, Dye-Sensitized solar cell, Iodine concentration, Organic liquid, Overall conversion efficiency, Photoelectrochemical measurements, Photoelectrochemicals, Recombination loss, Spectroscopic measurements, Triiodide, Acetonitrile, Concentration (process), Conversion efficiency, Current voltage characteristics, Electrochemical corrosion, Electrochemical impedance spectroscopy, Electrolytes, Fused silica, Industrial chemicals, Iodine, Ionic liquids, Ions, Optimization, Photoelectrochemical cells, Raman spectroscopy, Solar cells, Switching circuits
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-27506 (URN)10.1021/jp1001918 (DOI)000278479700040 ()2-s2.0-77953510497 (Scopus ID)
Funder
StandUp
Note

QC 20110112

Available from: 2011-01-12 Created: 2010-12-13 Last updated: 2017-12-11Bibliographically approved
2. Incompletely Solvated Ionic Liquids as Electrolyte Solvents for Highly Stable Dye-Sensitized Solar Cells
Open this publication in new window or tab >>Incompletely Solvated Ionic Liquids as Electrolyte Solvents for Highly Stable Dye-Sensitized Solar Cells
(English)Article in journal (Other academic) Submitted
Abstract [en]

Ionic liquids have been intensively investigated as alternative stable electrolyte solvents for dye-sensitized solar cells (DSCs). A highest overall conversion efficiency of over 8% has been achieved using ionic-liquid-based electrolyte in combination with an iodide/triiodide redox couple. However, the relatively high viscosities of ionic liquids require higher iodine concentration in the electrolyte due to mass-transport limitations of the triiodide ions. The higher iodine concentration significantly reduces the photovoltaic performance, which normally are lower than those using organic solvent-based electrolytes. Here, the concept of incompletely solvated ionic liquids (ISILs) is introduced and represents a conceptually new type of electrolyte solvents for DSCs. It is found that the photovoltaic performance of ISIL-based electrolytes can rival that of organic solvent-based electrolytes. Furthermore, the vapor pressures of ISILs are found be considerably lower than that for pure organic solvent. Stability tests show that ISIL-based electrolytes provide highly stable DSCs under light-soaking conditions. Thus, ISIL-based electrolytes offer a new platform to develop more efficient and stable DSC devices of relevance to future large-scale applications.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-63821 (URN)
Funder
StandUp
Note

QS 20120328

Available from: 2012-01-24 Created: 2012-01-24 Last updated: 2016-08-16Bibliographically approved
3. Synergistic Effect of N-Methylbenzimidazole and Guanidinium Thiocyanate on the Performance of Dye-Sensitized Solar Cells Based on Ionic Liquid Electrolytes
Open this publication in new window or tab >>Synergistic Effect of N-Methylbenzimidazole and Guanidinium Thiocyanate on the Performance of Dye-Sensitized Solar Cells Based on Ionic Liquid Electrolytes
2010 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 114, no 50, 22330-22337 p.Article in journal (Refereed) Published
Abstract [en]

The effects of additives guanidinium thiocyanate (GSCN) and N-methylbenzimidazole (MBI) on the photovoltaic performance of dye-sensitized solar cells based on low-viscous, binary ionic liquid and organic liquid electrolytes were investigated. Addition of only GSCN to the electrolyte has a pronounced influence on the short-circuit current, owing largely to the positive shift of the conduction band edge potential, probably increasing the injection efficiency of the excited dye. When only MBI was added to the electrolyte, a significant improvement of the open-circuit voltage was found, which could be attributed to a negative shift of the TiO2 conduction band edge potential and a longer electron lifetime under open-circuit conditions. Synergistic effects were observed when GSCN and MBI were used together in the ionic liquid-based electrolyte. In this case, optimal open-circuit voltage and total conversion efficiency were obtained among the ionic liquid electrolytes studied mainly due to the more efficient retardation of the recombination loss reaction at the TiO2/electrolyte interface.

Keyword
band-edge movement, charge recombination, tio2, efficiency, conversion, molten, light, salts, power
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-28590 (URN)10.1021/jp1073686 (DOI)000285236800060 ()2-s2.0-79951629346 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationStandUp
Note

QC 20150720

Available from: 2011-01-17 Created: 2011-01-17 Last updated: 2017-12-11Bibliographically approved
4. Tetrathiafulvalene as a one-electron iodine-free organic redox mediator in electrolytes for dye-sensitized solar cells
Open this publication in new window or tab >>Tetrathiafulvalene as a one-electron iodine-free organic redox mediator in electrolytes for dye-sensitized solar cells
Show others...
2012 (English)In: RSC Advances, ISSN 2046-2069, Vol. 2, no 3, 1083-1087 p.Article in journal (Refereed) Published
Abstract [en]

Tetrathiafulvalene (TTF) was investigated as an organic iodine-free redox mediator in electrolytes for dye-sensitized, nanocrystalline solar cells (DSCs) and was compared to the commonly used iodide/triiodide system. The TTF system studied was determined to be a one-electron transfer system, although potentially exhibiting three well-defined oxidation states. Despite the slightly positive redox potential of TTF, electrolytes with TTF displayed around 200 mV lower open-circuit voltage than the iodide/triiodide system. This can mainly be ascribed to a much shorter electron lifetime in the TiO2 film. Mass transport limitations for redox species in TTF-based electrolytes were found to be serious. Electrochemical impedance measurements (EIS) show that the charge-transfer resistance at the counter electrode in the electrolyte with TTF is considerably larger than for the iodide/triiodide system. In addition, the light absorption of the TTF-based electrolyte is stronger than that for the iodide/triiodide system. Thus, DSCs with TTF-based electrolytes show worse photovoltaic performance than those with iodide/triiodide-based electrolytes. The differences in IV characteristics and charge-recombination behavior have also been elucidated.

Place, publisher, year, edition, pages
The Royal Society of Chemistry, 2012
Keyword
EFFICIENT, DERIVATIVES, COMPLEXES, COUPLE
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-63848 (URN)10.1039/C1RA00877C (DOI)000299177000049 ()2-s2.0-84859201345 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationStandUp
Note

QC 20120403

Updated from e-pub ahead of print

Available from: 2012-01-24 Created: 2012-01-24 Last updated: 2016-04-26Bibliographically approved
5. Efficient Organic-Dye-Sensitized Solar Cells Based on an Iodine-Free Electrolyte
Open this publication in new window or tab >>Efficient Organic-Dye-Sensitized Solar Cells Based on an Iodine-Free Electrolyte
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2010 (English)In: Angewandte Chemie International Edition, ISSN 1433-7851, E-ISSN 1521-3773, Vol. 49, no 40, 7328-7331 p.Article in journal (Refereed) Published
Keyword
organic dyes, photoelectrochemistry, redox chemistry, solar cells, thiadiazoles
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-46681 (URN)10.1002/anie.201003740 (DOI)000282916800034 ()2-s2.0-77957375686 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationStandUp
Note

QC 20111105

Available from: 2011-11-05 Created: 2011-11-04 Last updated: 2017-12-08Bibliographically approved
6. Organic Redox Couples and Organic Counter Electrode for Efficient Organic Dye-Sensitized Solar Cells
Open this publication in new window or tab >>Organic Redox Couples and Organic Counter Electrode for Efficient Organic Dye-Sensitized Solar Cells
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2011 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 133, no 24, 9413-9422 p.Article in journal (Refereed) Published
Abstract [en]

A series of organic thiolate/disulfide redox couples have been synthesized and have been studied systematically in dye-sensitized solar cells (DSCs) on the basis of an organic dye (TH305). Photophysical, photoelectrochemical, and photovoltaic measurements were performed in order to get insights into the effects of different redox couples on the performance of DSCs. The polymeric, organic poly(3,4-ethylenedioxythiophene) (PEDOT) material has also been introduced as counter electrode in this kind of noniodine-containing DSCs showing a promising conversion efficiency of 6.0% under AM 1.5G, 100 mW.cm(-2) light illumination. Detailed studies using electrochemical impedance spectroscopy and linear-sweep voltammetry reveal that the reduction of disulfide species is more efficient on the PEDOT counter electrode surface than on the commonly used platinized conducting glass electrode. Both pure and solvated ionic-liquid electrolytes based on a thiolate anion have been studied in the DSCs. The pure and solvated ionic-liquid-based electrolytes containing an organic redox couple render efficiencies of 3.4% and 1.2% under 10 mW.cm(-2) light illumination, respectively.

Keyword
IONIC LIQUID ELECTROLYTE, TEMPERATURE MOLTEN-SALTS, SOLVENT-FREE, POLYPYRIDINE COMPLEXES, HOLE CONDUCTORS, POLY(3, 4-ETHYLENEDIOXYTHIOPHENE), MEDIATORS, COMBINATION, PERFORMANCE, COBALT
National Category
Other Basic Medicine
Identifiers
urn:nbn:se:kth:diva-36232 (URN)10.1021/ja2030933 (DOI)000291915100055 ()2-s2.0-79959233553 (Scopus ID)
Funder
StandUp
Note

QC 20110711

Available from: 2011-07-11 Created: 2011-07-11 Last updated: 2017-12-11Bibliographically approved
7. Investigation of Cation Effects in the Electrolytes for Dye-Sensitized Solar Cells
Open this publication in new window or tab >>Investigation of Cation Effects in the Electrolytes for Dye-Sensitized Solar Cells
(English)Article in journal (Other academic) Submitted
Abstract [en]

A comparison of the effects of the cations of lithium, sodium and guanidinium in electrolytes for dye-sensitized solar cells have been investigated. Upon addition of cations to the reference electrolyte, short-circuit currents are generally found to be significantly enhanced, largely due to the positive shift of the conduction band edge (CB) in the TiO2. This probably results in an increase of the electron injection yield from the excited state of the sensitizing dye to the CB of TiO2. The open-circuit voltages for electrolytes incorporating lithium and sodium cations, however, are found to be slightly lower in comparison to the reference electrolyte. This may be attributed to the more positive shift of the conduction band edge in the TiO2. On the contrary, a slight improvement of the open-circuit voltage for electrolytes containing higher concentrations of guanidinium cations is observed relative to the reference electrolyte. This can most likely be ascribed to the collective effect of a much longer electron lifetime in the TiO2 and less positive shift of the CB. The electrolyte higher concentrations of guanidinium cations exhibits the optimal overall conversion efficiency among all the electrolytes investigated. the optimal performance can be attributed to the dual gain in both short-circuit current and open-circuit voltage with respect to the reference electrolyte.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-63827 (URN)
Note
QS 2012Available from: 2012-01-24 Created: 2012-01-24 Last updated: 2012-01-24Bibliographically approved

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