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Structure-dependent charge transfer at the interafce between organic thin films, and metals and metal oxides
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The purpose of the research work, presented in this thesis is to offer a detailed atomic level study of interfaces created by adsorption of organic molecules on metals and metal oxides to point out significant impact of substrate, dye structure as well as different mediators on the charge transfer at these interfaces, which is proven to influence the device performance to a great extent.

Adsorption of organic photosensitive molecules on metals and metal-oxides is the main focus of this thesis. Phthalocyanines which are organic semiconductors offer a broad range of properties, such as thermal and chemical stability, high charge mobility and strong absorption coefficient in the visible and near-IR regions, which make them very attractive to be applied in various systems and devices. Fuel cells, organic field-effect transistors (OFETs), organic light emitting diodes (OLEDs) and solar cells are examples of phthalocyanine’s applications. The main focus of this work is to characterize the interfaces of Dye Sensitized Solar Cells (DSSCs).

DSSC was invented by Michael Grätzel and Brian O’Regan in 1988. At the heart of this cell there is an oxide which is coated by a photosensitive dye. Under illumination, an electron is excited from HOMO to LUMO of the molecule, which can be further transferred to the conduction band of the oxide by a proper energy level alignment. The original state of the dye is regenerated by electron donation via the electrolyte, which usually is an organic solvent containing a redox couple e.g., iodide/triiodide. The iodide is regenerated by reduction of triiodide at the counter electrode. To improve the functionality of the cell, different additives can be added to the electrolyte.

To mimic the interfaces of this cell, molecular layers of MPc (M: Fe, Zn, Mg) are adsorbed on both metallic surfaces, Au(111) and Pt(111), and rutile TiO2(110). Layers of iodine were inserted between metallic substrates and dyes to investigate the electronic properties and charge transfer at these multi-interface systems. 4-tert-butyl pyridine is a significant additive to the electrolyte and has proven to enhance the cell’s performance. This molecule was also adsorbed on Pt(111) and TiO2(110). Phthalocyanines were deposited by organic molecular beam deposition and 4TBP was evaporated at room temperature. Surface structures and reconstructions were confirmed by LEED measurements. Surface sensitive synchrotron radiation based spectroscopy methods, XPS and NEXAFS were applied to characterize these surfaces and interfaces. STM images directly give a topographical and electronic map over the surface. All measurements were carried out in UHV condition.

When MPc was adsorbed on Au(111) and TiO2(110), charge transfer from molecule to substrate is suggested, while the opposite holds for MPc adsorbed on Pt(111). Moreover, stronger interaction between MPc and Pt(111) and TiO2(110) compared to Au(111) also demonstrates the effect of substrate on the charge transfer at the interface. The stronger interaction observed for these two substrates disturbed the smooth growth of a monolayer; it also resulted in bending of the molecular plane. Interaction of MPc with metallic surfaces was modified by inserting iodine at the interface. Another substrate-related effect was observed when MgPc was adsorbed on TiO2(110);  and -cross linked surfaces, where the surface reconstruction directly affect the molecular configuration as well as electronic structure at the interface. Besides, it is shown that the d-orbital filling of the central metal atom in MPc plays an important role for the properties of the molecular layer as well as charge transfer at the interface.

Upon adsorption of 4TBP on Pt(111), C-H bond is dissociatively broken and molecules is adsorbed with N atoms down. Modification of surface by iodine, prevent this dissociation. In the low coverage of iodine, there is a competition between 4TBP and iodine to directly bind to Pt(111). Investigation on the adsorption of 4TBP on TiO2(110) illustrated that these molecules in low coverage regime, prefer the oxygen vacancy sites and their adsorption on these sites, results in a downward band bending at the substrate’s surface. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , xi, 74 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2013:06
Keyword [en]
photoelectric spectroscopy, X-ray absorption spectroscopy, organic semiconductors, phthalocyanine, charge transfer, electronic structure, dye sensitization
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-134841OAI: oai:DiVA.org:kth-134841DiVA: diva2:668335
Public defence
2013-12-12, D, Forum, KTH-ICT, Isafjordsgatan 39, Kista, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, 30459-1
Note

QC 20131203

Available from: 2013-12-03 Created: 2013-11-29 Last updated: 2013-12-03Bibliographically approved
List of papers
1. Molecular layers of ZnPc and FePc on Au(111) surface: Charge transfer and chemical interaction
Open this publication in new window or tab >>Molecular layers of ZnPc and FePc on Au(111) surface: Charge transfer and chemical interaction
2012 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 137, no 8, 084705- p.Article in journal (Refereed) Published
Abstract [en]

We have studied zinc phthalocyanine (ZnPc) and iron phthalocyanine (FePc) thick films and monolayers on Au(111) using photoelectron spectroscopy and x-ray absorption spectroscopy. Both molecules are adsorbed flat on the surface at monolayer. ZnPc keeps this orientation in all investigated coverages, whereas FePc molecules stand up in the thick film. The stronger inter-molecular interaction of FePc molecules leads to change of orientation, as well as higher conductivity in FePc layer in comparison with ZnPc, which is reflected in thickness-dependent differences in core-level shifts. Work function changes indicate that both molecules donate charge to Au; through the pi-system. However, the Fe3d derived lowest unoccupied molecular orbital receives charge from the substrate when forming an interface state at the Fermi level. Thus, the central atom plays an important role in mediating the charge, but the charge transfer as a whole is a balance between the two different charge transfer channels; pi-system and the central atom.

National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-103381 (URN)10.1063/1.4746119 (DOI)000308416800048 ()2-s2.0-84865745575 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20121015

Available from: 2012-10-15 Created: 2012-10-11 Last updated: 2017-12-07Bibliographically approved
2. Reduced Au-MPc hole injection barrier by an intermediate iodine layer
Open this publication in new window or tab >>Reduced Au-MPc hole injection barrier by an intermediate iodine layer
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(English)Manuscript (preprint) (Other academic)
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-135977 (URN)
Note

QS 2013

Available from: 2013-12-03 Created: 2013-12-03 Last updated: 2013-12-03Bibliographically approved
3. Effect of the iodineon the site-dependent charge transfer at the Pt(111)-ZnPc interface
Open this publication in new window or tab >>Effect of the iodineon the site-dependent charge transfer at the Pt(111)-ZnPc interface
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(English)Manuscript (preprint) (Other academic)
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-135979 (URN)
Note

QS 2013

Available from: 2013-12-03 Created: 2013-12-03 Last updated: 2013-12-03Bibliographically approved
4. Inhomogeneous charge transfer within monolayer zinc phthalocyanine absorbed on TiO2(110)
Open this publication in new window or tab >>Inhomogeneous charge transfer within monolayer zinc phthalocyanine absorbed on TiO2(110)
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2012 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 15, 154703- p.Article in journal (Refereed) Published
Abstract [en]

The d-orbital contribution from the transition metal centers of phthalocyanine brings difficulties to understand the role of the organic ligands and their molecular frontier orbitals when it adsorbs on oxide surfaces. Here we use zinc phthalocyanine (ZnPc)/TiO2(110) as a model system where the zinc d-orbitals are located deep below the organic orbitals leaving room for a detailed study of the interaction between the organic ligand and the substrate. A charge depletion from the highest occupied molecular orbital is observed, and a consequent shift of N1s and C1s to higher binding energy in photoelectron spectroscopy (PES). A detailed comparison of peak shifts in PES and near-edge X-ray absorption fine structure spectroscopy illustrates a slightly uneven charge distribution within the molecular plane and an inhomogeneous charge transfer screening between the center and periphery of the organic ligand: faster in the periphery and slower at the center, which is different from other metal phthalocyanine, e. g., FePc/TiO2. Our results indicate that the metal center can substantially influence the electronic properties of the organic ligand at the interface by introducing an additional charge transfer channel to the inner molecular part.

Keyword
Binding energy, Electronic properties, Ligands, Monolayers, Nitrogen compounds, Photoelectron spectroscopy, Quantum chemistry, Titanium dioxide, Transition metals, X ray absorption fine structure spectroscopy, Zinc compounds
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-47336 (URN)10.1063/1.3699072 (DOI)000303147000034 ()2-s2.0-84860168047 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20120521. Updated from submitted to published.

Available from: 2011-11-08 Created: 2011-11-08 Last updated: 2017-12-08Bibliographically approved
5. Charge transfer and band bending on TiO2(110)-MgPc
Open this publication in new window or tab >>Charge transfer and band bending on TiO2(110)-MgPc
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(English)Manuscript (preprint) (Other academic)
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-135981 (URN)
Note

QS 2013

Available from: 2013-12-03 Created: 2013-12-03 Last updated: 2013-12-03Bibliographically approved
6. Dissociative bonding of 4-tert-butyl pyridine to Pt(111) and surface passivation by iodine
Open this publication in new window or tab >>Dissociative bonding of 4-tert-butyl pyridine to Pt(111) and surface passivation by iodine
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(English)Manuscript (preprint) (Other academic)
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-135982 (URN)
Note

QS 2013

Available from: 2013-12-03 Created: 2013-12-03 Last updated: 2013-12-03Bibliographically approved
7. 4-tert-Butyl Pyridine Bond Site and Band Bending on TiO2(110)
Open this publication in new window or tab >>4-tert-Butyl Pyridine Bond Site and Band Bending on TiO2(110)
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2010 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 114, no 5, 2315-2320 p.Article in journal (Refereed) Published
Abstract [en]

In the present work, we study the bonding of 4-tert-butyl pyridine (4TBP) to the TiO2(110) surface using photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. The results show that at low coverage, 4TBP adsorbs preferentially on oxygen vacancies. The calculated adsorption energy at the vacancies is 120 kJ/mol larger than that oil the five-fold-coordinated Ti4+ sites located in the rows on the TiO2 surface. The vacancy is "healed" by 4TBP, and the related gap state is strongly reduced through charge transfer into empty pi* orbitals on the pyridine ring. This leads to a change in surface band bending by 0.2 eV toward lower binding energies. The band bending does not change with further 4TBP deposition when saturating the surface to monolayer coverage, where the TiO2 surface is effectively protected against further adsorption by the dense 4TBP layer.

Keyword
sensitized solar-cells, rutile 110 surface, nanocrystalline tio2, electronic-structure, oxygen vacancies, charge-transfer, adsorption, stm, molecules, defects
National Category
Physical Chemistry Materials Engineering
Identifiers
urn:nbn:se:kth:diva-19179 (URN)10.1021/jp911038r (DOI)000274269700047 ()2-s2.0-77249086226 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20110124Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved

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