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Coadsorption of Dye Molecules at TiO2 Surfaces: A Photoelectron Spectroscopy Study
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
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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2016 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 23, 12484-12494 p.Article in journal (Refereed) PublishedText
Abstract [en]

The effects of coadsorbing the amphiphilic ruthenium-based dye Z907 (cis-bis(isothiocyanato)(2,20-bipyridy1-4,40-dicarboxylato)(4,40-dinony1-20-bipyridy1)-ruthenium(II)) with the coadsorbent DPA (n-decylphosphonic acid) and with the organic dye D35 ((E)-3-(5-(4-(bis(2',4'-dibutoxybiphenyl-4-yl)amino)phenyl)thiophen-2-yl)-2-cyanoacrylic acid) on mesoporous TiO2 were investigated using photoelectron spectroscopy (PES). Z907 is expected to adsorb to the TiO2 surface via the carboxylic acid groups. However, Z907 also shows signs of interacting with the TiO2 via the sulfur of the thiocyanate groups, and this interaction is affected by both the addition of DPA and D35. DPA, when added, exchanges with Z907 at the TiO2 surface, and each Z907 is replaced by six DPA molecules, but it does not affect the energy level alignment between Z907 and TiO2 substantially. Adding D35 to Z907 induces changes in the adsorption configuration of Z907 by the means of suppressing the interaction of the thiocyanate ligands and the TiO2 surface. The HOMO level of Z907 is shifted by the addition of D35. Coadsorbing Z907 with D35 thus gives changes at a molecular level, meaning that this is an example of collaborative sensitization.

Place, publisher, year, edition, pages
2016. Vol. 120, no 23, 12484-12494 p.
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-299720DOI: 10.1021/acs.jpcc.6b02521ISI: 000378196200019OAI: oai:DiVA.org:uu-299720DiVA: diva2:950010
Funder
Swedish Energy Agency, P221191-5Swedish Research Council FormasSwedish Research Council, 2014-6018 2012-4721
Available from: 2016-07-26 Created: 2016-07-26 Last updated: 2016-08-25Bibliographically approved
In thesis
1. Towards Mixed Molecular Layers for Dye-Sensitized Solar Cells: A Photoelectron Spectroscopy Study
Open this publication in new window or tab >>Towards Mixed Molecular Layers for Dye-Sensitized Solar Cells: A Photoelectron Spectroscopy Study
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The increasing demand for renewable energy has led to substantial research on different solar cell technologies. The dye-sensitized solar cell (DSC) is a technology utilizing dye molecules for light absorption. Dye molecules are adsorbed to a mesoporous semiconductor surface and after light absorption in the dye, charge separation occurs at this interface. Traditionally, DSCs have used layers of single dye species, but in recent efforts to enhance power conversion efficiency, more complex molecular layers have been designed to increase the light absorption. For example, the most efficient DSCs use a combination of two dye molecules, and such dye co-adsorption is studied in this thesis.

A key to highly efficient DSCs is to understand the dye/semiconductor interface from a molecular perspective. One way of gaining this understanding is by using an element specific, surface sensitive technique, such as photoelectron spectroscopy (PES).

In this thesis, PES is used to understand new complex dye/semiconductor interfaces. Dyes adsorbed to semiconductor surfaces are analyzed using PES in terms of geometric and electronic surface structure.  The investigations ultimately target the effects of co-adsorbing dyes with other dyes or co-adsorbents.

PES shows that Ru dyes can adsorb in mixed configurations to TiO2. Co-adsorption with an organic dye affects the configuration of the Ru dyes. As a consequence, shifts in energy level alignment and increased dye coverage are observed. The dyes are affected at a molecular level in ways beneficial for solar cell performance. This is called collaborative sensitization and is also observed in todays most efficient DSC.

Dye molecules are generally sensitive to high temperatures and the substantial decrease in power conversion efficiency after heat-treatment can be understood using PES. Furthermore, comparing two mesoscopic TiO2 morphologies used in DSCs show differences in trap state density in the band gap, explaining the photovoltage difference in DSCs comprising these morphologies. Using mixed molecular layers on NiO results in significant improvements of p-type DSC power conversion efficiency. PES shows that changed adsorption configuration contribute to this effect.

This thesis shows that PES studies can be used to obtain insight into functional properties of complex DSC interfaces at a molecular level.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 81 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1409
Keyword
dye-sensitized solar cell, DSC, mesoscopic solar cell, photoelectron spectroscopy, PES, XPS, interface, TiO2, NiO, co-adsorption, co-adsorbent, collaborative sensitization, mixed molecular layers
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-301164 (URN)978-91-554-9664-7 (ISBN)
Public defence
2016-10-06, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
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Available from: 2016-09-14 Created: 2016-08-18 Last updated: 2016-09-22

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Oscarsson, JohanHahlin, MariaJohansson, Erik M. J.Lindblad, RebeckaSiegbahn, HansRensmo, Håkan
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