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Exploring Electronic Processes at the Mesoporous TiO2/Dye/Electrolyte Interface
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry. (Gerrit Boschloo)ORCID iD: 0000-0002-3440-9416
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Dye sensitized solar cells (DSSCs) are an attractive way to convert light into electricity. Its development requires a detailed understanding and kinetic optimization of various electronic processes, especially those occurring at the mesoporous TiO2/dye/electrolyte interface. This dissertation work is focused on the exploration of the various electronic processes at the sensitized-electrode/electrolyte interface by using various electrochemical and photochemical methods.

Firstly, an alternative redox couple—TEMPO/TEMPO·+ with a relatively high positive redox potential—is explored, aiming to reduce the energy loss during the dye regeneration process. Despite the fast dye regeneration, the charge recombination between the electrons in the conduction band of mesoporous TiO2 and the oxidized redox species is found to be the limiting factor of the device. Further, a more efficient tandem-electrolyte system is developed, leading to DSSCs with the power conversion efficiency of 10.5 % and 11.7 % at 1 sun and 0.5-sun illumination, respectively. An electron-transfer cascade process during dye regeneration by the redox mediators is discovered to be beneficial. Further stability studies on the device suggest the crucial role of TiO2/dye/electrolyte interfaces in the long-term stability of cobalt bipyridyl electrolyte-based DSSCs.

On the fundamental level, the local electric field and Stark effects at the TiO2/dye/electrolyte interface are investigated in various aspects—including the charge compensation mechanism, the factors affecting the electric field strength, as well as its impact on charge transfer kinetics. These results give further insights about the TiO2/dye/electrolyte interface, and contribute to the further development and understanding of DSSCs.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , p. 86
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1464
Keyword [en]
dye-sensitized solar cells, dye regeneration, Stark effect, the local electric field, cationic effect
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-310191ISBN: 978-91-554-9780-4 (print)OAI: oai:DiVA.org:uu-310191DiVA, id: diva2:1055554
Public defence
2017-02-10, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2017-01-16 Created: 2016-12-12 Last updated: 2017-01-17
List of papers
1. Efficient dye regeneration at low driving force achieved in triphenylamine dye LEG4 and TEMPO redox mediator based dye-sensitized solar cells
Open this publication in new window or tab >>Efficient dye regeneration at low driving force achieved in triphenylamine dye LEG4 and TEMPO redox mediator based dye-sensitized solar cells
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2015 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 24, p. 15868-15875Article in journal (Refereed) Published
Abstract [en]

Minimizing the driving force required for the regeneration of oxidized dyes using redox mediators in an electrolyte is essential to further improve the open-circuit voltage and efficiency of dye-sensitized solar cells (DSSCs). Appropriate combinations of redox mediators and dye molecules should be explored to achieve this goal. Herein, we present a triphenylamine dye, LEG4, in combination with a TEMPO-based electrolyte in acetonitrile (E-0 = 0.89 V vs. NHE), reaching an efficiency of up to 5.4% under one sun illumination and 40% performance improvement compared to the previously and widely used indoline dye D149. The origin of this improvement was found to be the increased dye regeneration efficiency of LEG4 using the TEMPO redox mediator, which regenerated more than 80% of the oxidized dye with a driving force of only similar to 0.2 eV. Detailed mechanistic studies further revealed that in addition to electron recombination to oxidized dyes, recombination of electrons from the conducting substrate and the mesoporous TiO2 film to the TEMPO+ redox species in the electrolyte accounts for the reduced short circuit current, compared to the state-of-the-art cobalt tris(bipyridine) electrolyte system. The diffusion length of the TEMPO-electrolyte based DSSCs was determined to be similar to 0.5 mu m, which is smaller than the similar to 2.8 mu m found for cobalt-electrolyte based DSSCs. These results show the advantages of using LEG4 as a sensitizer, compared to previously record indoline dyes, in combination with a TEMPO-based electrolyte. The low driving force for efficient dye regeneration presented by these results shows the potential to further improve the power conversion efficiency (PCE) of DSSCs by utilizing redox couples and dyes with a minimal need of driving force for high regeneration yields.

National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-258557 (URN)10.1039/c5cp01880c (DOI)000356056000044 ()26016854 (PubMedID)
Available from: 2015-07-15 Created: 2015-07-15 Last updated: 2017-12-04Bibliographically approved
2. A small electron donor in cobalt complex electrolyte significantly improves efficiency in dye-sensitized solar cells
Open this publication in new window or tab >>A small electron donor in cobalt complex electrolyte significantly improves efficiency in dye-sensitized solar cells
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2016 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 7, article id 13934Article in journal (Refereed) Published
Abstract [en]

Photoelectrochemical approach to solar energy conversion demands a kinetic optimization of various light-induced electron transfer processes. Of great importance are the redox mediator systems accomplishing the electron transfer processes at the semiconductor/electrolyte interface, therefore affecting profoundly the performance of various photoelectrochemical cells. Here, we develop a strategy-by addition of a small organic electron donor, tris(4-methoxyphenyl)amine, into state-of-art cobalt tris(bipyridine) redox electrolyte-to significantly improve the efficiency of dye-sensitized solar cells. The developed solar cells exhibit efficiency of 11.7 and 10.5%, at 0.46 and one-sun illumination, respectively, corresponding to a 26% efficiency improvement compared with the standard electrolyte. Preliminary stability tests showed the solar cell retained 90% of its initial efficiency after 250 h continuous one-sun light soaking. Detailed mechanistic studies reveal the crucial role of the electron transfer cascade processes within the new redox system.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-310194 (URN)10.1038/ncomms13934 (DOI)000390223200001 ()
Funder
Swedish Research CouncilSwedish Energy AgencyKnut and Alice Wallenberg FoundationStiftelsen Olle Engkvist Byggmästare
Note

Yan Hao and Wenxing Yang contributed equally to this work.

Available from: 2016-12-12 Created: 2016-12-12 Last updated: 2017-11-29Bibliographically approved
3. Thermal Stability Study of Dye-Sensitized Solar Cells with Cobalt Bipyridyl-based Electrolytes
Open this publication in new window or tab >>Thermal Stability Study of Dye-Sensitized Solar Cells with Cobalt Bipyridyl-based Electrolytes
2016 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 213, p. 879-886Article in journal, Meeting abstract (Refereed) Published
Abstract [en]

Dye-sensitized solar cells (DSSCs) with cobalt bipyridyl-based electrolytes can display higher solar cell performance than their iodide/triiodide counterpart. There is, however, little knowledge on their long term stability, which is a crucial aspect for potential commercial application. Herein, we studied the thermal stability of DSSCs using Co(bpy)(3)(2+/3+) redox electrolyte at 70 degrees C in the dark for 50 days, combining 3 different additives, 4-tert-butylpyridine (TBP), 1-methylimidazole (MBI) and 2,2'-bipyridyl (BPY), in a nonvolatile solvent 3-methoxypropionitrile (MPN). Significant voltage decreases were found for all the studied solar cells, with a mechanism involving both a positive shift of the conduction band edge potential of TiO2 and a decreased electron lifetime, characterized by time resolved transient modulation techniques. Furthermore electrochemical impedance spectroscopy and differential pulse voltammetry studies indicate that the stability of Co(bpy)(3)(3+) is limited, causing an increased diffusion resistance in the electrolyte, but, surprisingly, no substantial change of the short-circuit current density (Jsc) in the devices. Overall, the DSSCs fabricated with the addition of both MBI and BPY in the electrolyte show the highest stability, maintaining 96% of its initial efficiency after 50 days, resulting from the overall compensation effects between the open circuit voltage decrease and the Jsc increase. These results provide insights about the degradation mechanism and emphasize the importance of the stability of TiO2/dye/electrolyte interface for the device stability under thermal stress.

Keyword
Thermal stability, Dye sensitized solar cells, Cobalt electrolyte, Lewis base
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-304145 (URN)10.1016/j.electacta.2016.07.112 (DOI)000382251900105 ()
Conference
13th International Fischer Symposium, JUN 07-11, 2015, Lubeck, GERMANY
Available from: 2016-10-03 Created: 2016-10-03 Last updated: 2017-11-30Bibliographically approved
4. A key discovery at the TiO2/dye/electrolyte interface: slow local charge compensation and a reversible electric field
Open this publication in new window or tab >>A key discovery at the TiO2/dye/electrolyte interface: slow local charge compensation and a reversible electric field
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2015 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 26, p. 16744-16751Article in journal (Refereed) Published
Abstract [en]

Dye-sensitized mesoporous TiO2 films have been widely applied in energy and environmental science related research fields. The interaction between accumulated electrons inside TiO2 and cations in the surrounding electrolyte at the TiO2/dye/electrolyte interface is, however, still poorly understood. This interaction is undoubtedly important for both device performance and fundamental understanding. In the present study, Stark effects of an organic dye, LEG4, adsorbed on TiO2 were well characterized and used as a probe to monitor the local electric field at the TiO2/dye/electrolyte interface. By using time-resolved photo- and potential-induced absorption techniques, we found evidence for a slow (t > 0.1 s) local charge compensation mechanism, which follows electron accumulation inside the mesoporous TiO2. This slow local compensation was attributed to the penetration of cations from the electrolyte into the adsorbed dye layer, leading to a more localized charge compensation of the electrons inside TiO2. Importantly, when the electrons inside TiO2 were extracted, a remarkable reversal of the surface electric field was observed for the first time, which is attributed to the penetrated and/or adsorbed cations now being charge compensated by anions in the bulk electrolyte. A cation electrosorption model is developed to account for the overall process. These findings give new insights into the mesoporous TiO2/dye/electrolyte interface and the electron-cation interaction mechanism. Electrosorbed cations are proposed to act as electrostatic trap states for electrons in the mesoporous TiO2 electrode.

National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-258901 (URN)10.1039/c5cp01274k (DOI)000356874000012 ()26061451 (PubMedID)
Funder
Swedish Research Council
Available from: 2015-07-21 Created: 2015-07-21 Last updated: 2017-12-04Bibliographically approved
5. Studies on the Interfacial Electric Field and Stark Effect at the TiO2/Dye/Electrolyte Interface
Open this publication in new window or tab >>Studies on the Interfacial Electric Field and Stark Effect at the TiO2/Dye/Electrolyte Interface
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2016 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 39, p. 22215-22224Article in journal (Refereed) Published
Abstract [en]

Interfaces of dye-sensitized TiO2 nanoparticles with electrolytes or hole conductors have been widely applied in photoelectrochemical cells. However, the fundamental understanding of their properties and function is still poor. Herein, we demonstrate that the spectral changes that occur in the-visible spectrum of dye-sensitized TiO2 films upon (a) Li+ titration, (b) potentiostatic electron accumulation in mesoporous TiO2, and (c) photoinduced electron injection into TiO2 can be explained by the Stark effect, which can then be used to characterize the change in the local electric field at the TiO2/dye/electrolyte interface. A quantitative analysis of the Stark effect indicates that the compact (Helmholtz) layer capacitance at the TiO2/dye/electrolyte interface strongly affects the strength of the local electric field. Systematic studies show that the Helmholtz layer capacitance depends strongly on the Li+ concentration and surface dye coverage but is independent of the concentrations of other electrolytic species and the light intensity. These results illustrate the potential of Stark spectroscopy for the in situ study of the TiO2/dye/electrolyte interfaces and provide substantial new insights into these widely applied interfaces related to photoelectrochemistry and other research fields.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-307270 (URN)10.1021/acs.jpcc.6b07096 (DOI)000384959100012 ()
Funder
Swedish Energy AgencySwedish Research CouncilStandUp
Available from: 2016-11-14 Created: 2016-11-11 Last updated: 2017-11-29Bibliographically approved
6. Impact of Local Electric Fields on Charge-Transfer Processes at the TiO2/Dye/Electrolyte Interface
Open this publication in new window or tab >>Impact of Local Electric Fields on Charge-Transfer Processes at the TiO2/Dye/Electrolyte Interface
2017 (English)In: ACS Energy Letter, ISSN 2380-8195, Vol. 2, no 1, p. 161-167Article in journal (Refereed) Accepted
Abstract [en]

Photoinduced electron -transfer processes at the TiO2/ dye/electrolyte interface are vital for various emerging technologies. Here, the impact of the local electric field at this interface on the charge -transfer processes was investigated in two aspects: (a) charge recombination between the electrons accumulated within TiO2 and the photoxidized dye and (b) regeneration of the dyes by the cobalt bipyridyl redox mediators. The amplitude of the local electric field was changed by use of different cations in the electrolytic environment, in the order E-Ca(2+) > E-Mg(2+)> E-Na(+) > E-u(+) characterized by the transient absorption spectroscopy. For the charge recombination process, the kinetic time constant showed a remarkable linear correlation with the relative electric field strength, while for the regeneration process, no evident dependence was observed. These results collectively suggest the spatial confinement of the effects of the local electric field on the interfacial electron -transfer processes.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-310189 (URN)10.1021/acsenergylett.6b00568 (DOI)000392260400025 ()
Funder
Swedish Energy AgencySwedish Research CouncilStandUp
Available from: 2016-12-12 Created: 2016-12-12 Last updated: 2017-02-27Bibliographically approved

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