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Interfaces in Dye-Sensitized Oxide / Hole-Conductor Heterojunctions for Solar Cell Applications
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics I.
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nanoporous dye-sensitized solar cells (DSSC) are promising devices for solar to electric energy conversion. In this thesis photoelectron spectroscopy (PES), x-ray absorption spectroscopy (XAS) and photovoltaic measurements are used for studies of the key interfaces in the DSSC.

Photovoltaic properties of new combinations of TiO2/dye/hole-conductor heterojunctions were demonstrated and their interfacial structures were studied. Three different types of hole-conductor materials were investigated: Triarylamine derivatives, a conducting polymer and CuI. The difference in photocurrent and photovoltage properties of the heterojunction due to small changes in the hole-conductor material was followed. Also a series of dye molecules were used to measure the influence of the dye on the photovoltaic properties. Differences in both the energy-level matching and the geometric structure of the interfaces in the different heterojunctions were studied by PES. This combination of photovoltaic and PES measurements shows the possibility to link the interfacial electronic and molecular structure to the functional properties of the device.

Three effective dyes used in the DSSC, Ru(dcbpy)2(NCS)2, Ru(tcterpy)(NCS)3 and an organic dye were studied in detail using PES and XAS and resonant core hole decay spectroscopy. The results gave information of the frontier electronic structure of the dyes and how the dyes are bonded to the TiO2 surface.

Finally, the hole-conductor mechanism in a conducting polymer was investigated theoretically using semi-empirical and ab-initio methods.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2006. , p. 53
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 188
Keywords [en]
Physics, Photoelectron spectroscopy, Solar cells, Heterojunction
Keywords [sv]
Fysik
Identifiers
URN: urn:nbn:se:uu:diva-6892ISBN: 91-554-6575-7 (print)OAI: oai:DiVA.org:uu-6892DiVA, id: diva2:168412
Public defence
2006-05-29, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsv.1, Uppsala, 13:15
Opponent
Supervisors
Available from: 2006-05-08 Created: 2006-05-08 Last updated: 2012-10-09Bibliographically approved
List of papers
1. Electronic and Molecular Surface Structure of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2 Adsorbed from Solution onto Nanostructured TiO2: A Photoelectron Spectroscopy Study
Open this publication in new window or tab >>Electronic and Molecular Surface Structure of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2 Adsorbed from Solution onto Nanostructured TiO2: A Photoelectron Spectroscopy Study
2005 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 109, no 47, p. 22256-22263Article in journal (Refereed) Published
Abstract [en]

The element specificity of photoelectron spectroscopy (PES) has been used to compare the electronic and molecular structure of the dyes Ru(tcterpy)(NCS)3 (BD) and Ru(dcbpy)2(NCS)2adsorbed from solution onto nanostructured TiO2. Ru(dcbpy)2(NCS)2 was investigated in its acid (N3) and in its 2-fold deprotonated form (N719) having tetrabutylammonium (TBA+) as counterions. A comparison of the O1s spectra for the dyes indicates that the interactions through the carboxylate groups with the TiO2 surface are very similar for the dyes. However, we observe that some of the dye molecules also interact through the NCS groups when adsorbed at the TiO2 surface. Comparing the N719 and the N3 molecule, the fraction of NCS groups interacting through the sulfur atoms is smaller for N719 than for N3. We also note that the counterion TBA+ is coadsorbed with the N719 and BD molecules although the amount was smaller than expected from the molecular formulas. Comparing the valence levels for the dyes adsorbed on TiO2, the position of the highest occupied electronic energy level is similar for N3 and N719, while that for BD is lower by 0.25 eV relative to that of the other complexes.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-94528 (URN)10.1021/jp0525282 (DOI)
Available from: 2006-05-08 Created: 2006-05-08 Last updated: 2017-12-14Bibliographically approved
2. Frontier electronic structures of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2: A photoelectron spectroscopy study
Open this publication in new window or tab >>Frontier electronic structures of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2: A photoelectron spectroscopy study
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2007 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 126, no 24, p. 244303-Article in journal (Refereed) Published
Abstract [en]

The frontier electronic structures of Ru(tcterpy)(NCS)(3) [black dye (BD)] and Ru(dcbpy)(2)(NCS)(2) (N719) have been investigated by photoelectron spectroscopy (PES), X-ray absorption spectroscopy (XAS) and resonant photoelectron spectroscopy (RPES). N1s XAS has been used to probe the nitrogen contribution in the unoccupied density of states, and PES, together with RPES over the N1s edge, has been used to delineate the character of the occupied density of states. The experimental findings of the frontier electron structure are compared to calculations of the partial density of states for the nitrogens in the different ligands (NCS and terpyridine/bipyridine) and for Ru4d. The result indicates large similarities between the two complexes. Specifically, the valence level spectra show two well separated structures at low binding energy. The experimental results indicate that the outermost structure in the valence region largely has a Ru4d character but with a substantial character also from the NCS ligand. Interestingly, the second lowest structure also has a significant Ru4d character mixed into the structure otherwise dominated by NCS. Comparing the two complexes the BD valence structures lowest in binding energy contains a large contribution from the NCS ligands but almost no contribution from the terpyridine ligands, while for N719 also some contribution from the bipyridine ligands is mixed into the energy levels.

Keywords
dyes, electronic density of states, photoelectron spectra, X-ray absorption spectra, valence bands, binding energy
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-94529 (URN)10.1063/1.2738066 (DOI)000247625800021 ()
Available from: 2006-05-08 Created: 2006-05-08 Last updated: 2017-12-14Bibliographically approved
3. Electronic and Molecular Surface Structure of a Polyene-diphenylaniline Dye Adsorbed from Solution onto Nanoporous TiO2
Open this publication in new window or tab >>Electronic and Molecular Surface Structure of a Polyene-diphenylaniline Dye Adsorbed from Solution onto Nanoporous TiO2
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2007 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 111, no 24, p. 8580-8586Article in journal (Refereed) Published
Abstract [en]

The surface electronic and molecular structure of a new organic chromophore useful for dye-sensitized nanostructured solar cells has been investigated by means of electron spectroscopy. Initially the use of a simple molecular system containing the polyene-diphenylaniline chromophore in a solar cell device was verified. The electronic and molecular surface structure of the functional dye-sensitized interface was then investigated in detail by a combination of core level spectroscopy, valence level spectroscopy, X-ray absorption spectroscopy, and resonant photoemission spectroscopy. The results indicate a dominating orientation of the molecule at the surface, having the diphenylaniline moiety pointing out from the surface. Valence level spectroscopy, X-ray absorption spectroscopy, and resonant photoemission spectroscopy were used to experimentally delineate the frontier electronic structure of the molecule, and the experimental spectra were analyzed against theoretical spectra, based on density functional theory. Together the investigation gives insight into energy matching of the molecular electronic states with respect to the TiO2 substrate as well as the localization of the frontier electronic states and the direction of the charge-transfer absorption process with regards to the TiO2 surface.

National Category
Chemical Sciences
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-94530 (URN)10.1021/jp068771y (DOI)000247215200028 ()
Available from: 2006-05-08 Created: 2006-05-08 Last updated: 2017-12-14Bibliographically approved
4. Photovoltaic and Interfacial Properties of Heterojunctions Containing Dye-sensitized Dense TiO2 and Triarylamine derivatives
Open this publication in new window or tab >>Photovoltaic and Interfacial Properties of Heterojunctions Containing Dye-sensitized Dense TiO2 and Triarylamine derivatives
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2007 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 19, no 8, p. 2071-2078Article in journal (Refereed) Published
Abstract [en]

A series of solid-state heterojunctions comprising a dense TiO2 film electrode as an electron conductor, a ruthenium polypyridine complex (Ru(dcbpy)2(NCS)2) as a light-absorbing dye, and different triarylamine derivatives as hole conductors were prepared, and their photovoltaic properties as well as the molecular and electronic interfacial structures were investigated. The photovoltaic properties were compared to systems containing the hole conductors dissolved in an organic solvent as well as to a system containing a liquid electrolyte containing the iodide/tri-iodide redox couple. Two of the solid-state heterojunctions showed conversion efficiencies close to those of the system containing the iodide/tri-iodide redox couple, while one system was clearly less efficient. To explain the differences in photovoltaic properties the electronic and molecular interfacial structures of the solid-state heterojunctions were investigated by photoelectron spectroscopy (PES). By valence level PES the electronic energy levels highest in energy for the dye and the hole conductors were mapped, and the differences in energy matching partly explain the trends in photovoltaic properties. Differences in the molecular surface structure of the heterojunctions were also observed from the N Is core level measurements. Specifically it was found that the smaller hole conductor, showing low photocurrent yield, is inserted into the dye layer.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-94531 (URN)10.1021/cm062498v (DOI)000245549500028 ()
Available from: 2006-05-08 Created: 2006-05-08 Last updated: 2017-12-14Bibliographically approved
5. Interfacial Properties of Photovoltaic TiO2/dye/PEDOT-PSS Heterojunctions
Open this publication in new window or tab >>Interfacial Properties of Photovoltaic TiO2/dye/PEDOT-PSS Heterojunctions
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2005 (English)In: Synthetic metals, ISSN 0379-6779, E-ISSN 1879-3290, Vol. 149, no 2-3, p. 157-167Article in journal (Refereed) Published
Abstract [en]

Systems comprising a dense TiO2 film electrode, a ruthenium polypyridine dye and a PEDOT–PSS(poly(3,4-ethylenedioxythiophene)–poly(4-styrenesulphonate)) film were prepared. The heterojunctions were shown to have photovoltaicproperties, with the dye absorbing the light, the TiO2 acting as an electron conducting material and PEDOT–PSS acting as a hole transport material. A series of dyes was used to investigate their influence on the photocurrent and the photovoltage characteristics of the heterojunction. These results were compared to a photoelectrochemical system in which the PEDOT–PSS was replaced by a liquid electrolyte containing triiodide/iodide redox-couple.

Photoelectron spectroscopy (PES) was used to monitor the interfacialproperties of the heterojunction and the investigation points out effects of importance when assembling the materials together to a functional unit. Specifically, it was concluded that the interaction with the dye clearly affects the structure ofPEDOT–PSS, both with respect to the surface composition of PSS relative to PEDOT and with respect to the chemical state of the sulphur in the polymers. Moreover, a comparison of the Ru3d and the valence band spectra of the two different interfaces (dye/TiO2 and dye/PEDOT–PSS) indicates that the energy level structure of the dyes compared to the substrate is different for the two surfaces. Thus, in the combined energy level picture under dark conditions, the energy levels in TiO2 relative to the energy levels inPEDOT–PSS depend on the dye.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-94532 (URN)10.1016/j.synthmet.2004.12.004 (DOI)
Available from: 2006-05-08 Created: 2006-05-08 Last updated: 2017-12-14Bibliographically approved
6. Interfacial properties of the nanostructured dye-sensitized solid heterojunction TiO2/RuL2(NCS)2/CuI
Open this publication in new window or tab >>Interfacial properties of the nanostructured dye-sensitized solid heterojunction TiO2/RuL2(NCS)2/CuI
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2004 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 120, no 23, p. 11224-11232Article in journal (Refereed) Published
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-94533 (URN)10.1063/1.1739399 (DOI)
Available from: 2006-05-08 Created: 2006-05-08 Last updated: 2017-12-14Bibliographically approved
7. Electronic structure and mechanism for conductivity in thiophene oligomers and regioregular polymer
Open this publication in new window or tab >>Electronic structure and mechanism for conductivity in thiophene oligomers and regioregular polymer
2004 (English)In: Synthetic metals, ISSN 0379-6779, E-ISSN 1879-3290, Vol. 144, no 2, p. 183-191Article in journal (Refereed) Published
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-94534 (URN)10.1016/j.synthmet.2004.03.005 (DOI)
Available from: 2006-05-08 Created: 2006-05-08 Last updated: 2017-12-14Bibliographically approved

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