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Towards Mixed Molecular Layers for Dye-Sensitized Solar Cells: A Photoelectron Spectroscopy Study
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
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 [en]
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: urn:nbn:se:uu:diva-301164ISBN: 978-91-554-9664-7OAI: oai:DiVA.org:uu-301164DiVA: diva2:953821
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
List of papers
1. Geometrical and energetical structural changes in organic dyes for dye-sensitized solar cells probed with photoelectron spectroscopy and DFT
Open this publication in new window or tab >>Geometrical and energetical structural changes in organic dyes for dye-sensitized solar cells probed with photoelectron spectroscopy and DFT
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2016 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 1, 252-260 p.Article in journal (Other academic) Published
Abstract [en]

The effects of alkoxy chain length in triarylamine based donor acceptor organic dyes are investigated with respect to the electronic and molecular surface structures on the performance of solar cells and the electron lifetime. The dyes were investigated when adsorbed on TiO2 in a configuration that can be used for dye sensitized solar cells (DSCs). Specifically, the two dyes D35 and D45 were compared using photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. The differences in solar cell characteristics when longer alkoxy chains are introduced in the dye donor unit are attributed to geometrical changes in dye packing while only minor differences were observed in the electronic structure. A higher dye load was observed for D45 on TiO2. However, D35 based solar cells result in higher photocurrent although the dye load is lower. This is explained by different geometrical structures of the dyes on the surface.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-230853 (URN)10.1039/c5cp04589d (DOI)000368755500027 ()
Funder
Swedish Research CouncilCarl Tryggers foundation Swedish Energy AgencyStandUp
Available from: 2014-08-31 Created: 2014-08-31 Last updated: 2016-08-25Bibliographically approved
2. Molecular degradation of D35 and K77 sensitizers when exposed to temperatures exceeding 100 °C investigated by photoelectron spectroscopy
Open this publication in new window or tab >>Molecular degradation of D35 and K77 sensitizers when exposed to temperatures exceeding 100 °C investigated by photoelectron spectroscopy
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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.

National Category
Atom and Molecular Physics and Optics Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-283259 (URN)10.1039/C5CP07921G (DOI)000372249100031 ()26949128 (PubMedID)
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: 2016-08-25Bibliographically approved
3. Coadsorption of Dye Molecules at TiO2 Surfaces: A Photoelectron Spectroscopy Study
Open this publication in new window or tab >>Coadsorption of Dye Molecules at TiO2 Surfaces: A Photoelectron Spectroscopy Study
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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) Published
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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-299720 (URN)10.1021/acs.jpcc.6b02521 (DOI)000378196200019 ()
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
4. Interface Structure Effects upon Co-Adsorption of Black Dye and D35 on TiO2
Open this publication in new window or tab >>Interface Structure Effects upon Co-Adsorption of Black Dye and D35 on TiO2
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-300851 (URN)
Available from: 2016-08-18 Created: 2016-08-15 Last updated: 2016-08-25
5. Mesoporous TiO2 microbead electrodes for solid state dye-sensitized solar cells
Open this publication in new window or tab >>Mesoporous TiO2 microbead electrodes for solid state dye-sensitized solar cells
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2014 (English)In: RSC Advances, ISSN 2046-2069, Vol. 4, no 91, 50295-50300 p.Article in journal (Refereed) Published
Abstract [en]

Mesoporous TiO2 microbead films have been investigated as working electrodes for solid state dye sensitized solar cells and 3.5% efficiency was achieved for 4 micrometer thick films under 1 sun illumination. Compared to conventional mesoporous solar cells, microbead films have higher porosity, increased open circuit voltage, lower fill factor and current density, faster transport time and lower electron lifetime. Cross sectional scanning electron microscopy results show that the pore filling of a solid hole conductor (spiro-OMeTAD) inside the entire mesoporous bead film is very good even for 4 micrometer thick films. The high porosity of the microbead film allows good penetration of spiro in thick films, while its high surface area ensures good dye coverage. X-ray photoelectron spectroscopy data reveals a lower density of intra-bandgap trap states for microbead films compared to conventional mesoporous TiO2 films, which could be in part responsible for faster transport of electrons and higher voltage in microbead films. Optimization of microbead films for solid state dye sensitized solar cells can be an interesting possibility for highly efficient and relatively thick film solid state solar cells.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-238099 (URN)10.1039/c4ra10049b (DOI)000343715000100 ()
Available from: 2014-12-15 Created: 2014-12-09 Last updated: 2016-08-25Bibliographically approved
6. Enhancement of p-Type Dye-Sensitized Solar Cell Performance by Supramolecular Assembly of Electron Donor and Acceptor
Open this publication in new window or tab >>Enhancement of p-Type Dye-Sensitized Solar Cell Performance by Supramolecular Assembly of Electron Donor and Acceptor
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2014 (English)In: Scientific Reports, ISSN 2045-2322, Vol. 4, 4282- p.Article in journal (Refereed) Published
Abstract [en]

Supramolecular interactions based on porphyrin and fullerene derivatives were successfully adopted to improve the photovoltaic performance of p-type dye-sensitized solar cells (DSCs). Photoelectron spectroscopy (PES) measurements suggest a change in binding configuration of ZnTCPP after co-sensitization with C60PPy, which could be ascribed to supramolecular interaction between ZnTCPP and C60PPy. The performance of the ZnTCPP/C60PPy-based p-type DSC has been increased by a factor of 4 in comparison with the DSC with the ZnTCPP alone. At 560 nm, the IPCE value of DSCs based on ZnTCPP/C60PPy was a factor of 10 greater than that generated by ZnTCPP-based DSCs. The influence of different electrolytes on charge extraction and electron lifetime was investigated and showed that the enhanced V-oc from the Co2+/(3+)(dtbp)(3)-based device is due to the positive E-F shift of NiO.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-222728 (URN)10.1038/srep04282 (DOI)000332374500002 ()
Available from: 2014-04-17 Created: 2014-04-14 Last updated: 2016-08-25

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