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A key discovery at the TiO2/dye/electrolyte interface: slow local charge compensation and a reversible electric field
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
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2015 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 26, 16744-16751 p.Article 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.

Place, publisher, year, edition, pages
2015. Vol. 17, no 26, 16744-16751 p.
National Category
Physical Sciences Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-258901DOI: 10.1039/c5cp01274kISI: 000356874000012PubMedID: 26061451OAI: oai:DiVA.org:uu-258901DiVA: diva2:842596
Funder
Swedish Research Council
Available from: 2015-07-21 Created: 2015-07-21 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Exploring Electronic Processes at the Mesoporous TiO2/Dye/Electrolyte Interface
Open this publication in new window or tab >>Exploring Electronic Processes at the Mesoporous TiO2/Dye/Electrolyte Interface
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. 86 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1464
Keyword
dye-sensitized solar cells, dye regeneration, Stark effect, the local electric field, cationic effect
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-310191 (URN)978-91-554-9780-4 (ISBN)
Public defence
2017-02-10, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
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Supervisors
Available from: 2017-01-16 Created: 2016-12-12 Last updated: 2017-01-17

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Yang, WenxingPazoki, MeysamEriksson, Anna I. K.Hao, YanBoschloo, Gerrit

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