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Molecular degradation of D35 and K77 sensitizers when exposed to temperatures exceeding 100 °C investigated by photoelectron spectroscopy
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, 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.
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2016 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 12, 8598-8607 p.Article in journal (Refereed) Published
Abstract [en]

Degradation of the materials in dye-sensitized solar cells at elevated temperatures is critical for use in real applications. Both during fabrication of the solar cell and under real working conditions the solar cells will be exposed to heat. In this work, mesoporous TiO2 electrodes sensitized with the dyes D35 and K77 were subject to heat-treatment and the effects of this were thereafter investigated by photoelectron spectroscopy. For D35 it was found that heat-treatment changes the binding configuration inducing an increased interaction between the sulfur of the linker unit and the TiO2 surface. The interaction resulting from the change in binding configuration also affects the position of the HOMO level, where a shift of + 0.2 eV is observed when heated to 200 degrees C. For K77, parts of the thiocyanate units are detached and the nitrogen atom leaves the electrode whereas sulfur remains on the surface in various forms of sulfurous oxides. The total dye coverage of K77 gets reduced by heat-treatment. The HOMO level gets progressively less pronounced due to a loss of HOMO level electrons as a consequence of the lower dye coverage when heat-treated, which leads to a lower excitation rate and lower efficiency. The results are discussed in the context of performance for dye-sensitized solar cells.

Place, publisher, year, edition, pages
2016. Vol. 18, no 12, 8598-8607 p.
National Category
Atom and Molecular Physics and Optics Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-283259DOI: 10.1039/C5CP07921GISI: 000372249100031PubMedID: 26949128OAI: oai:DiVA.org:uu-283259DiVA: diva2:918857
Funder
Swedish Energy Agency, P221191-5Swedish Research Council FormasSwedish Research Council, 2014-6018, 2012-4721
Available from: 2016-04-12 Created: 2016-04-12 Last updated: 2017-11-30Bibliographically 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)
Opponent
Supervisors
Available from: 2016-09-14 Created: 2016-08-18 Last updated: 2016-09-22

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Oscarsson, JohanFredin, KristoferAhmadi, SarehEriksson, Anna I. K.Johansson, Erik M. J.Rensmo, Håkan

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