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Molecular Interaction of Thin Film Photosensitive Organic Dyes on TiO2 Surfaces
KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF. (Surface)ORCID iD: 0000-0002-9663-7705
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The photosensitive molecule adsorption on titanium dioxide (TiO2) forms the so-called “dye sensitized TiO2” system, a typical organic/oxide heterojunction, which is of great interest in catalysis and energy applications, e.g. dye-sensitized solar cell (DSSC). Traditionally, the transition metal complex dyes are the focus of the study. However, as the fast development of the organic semiconductors and invention of new pure organic dyes, it is necessary to expand the research horizon to cover these molecules and concrete the fundamental understanding of their basic properties, especially during sensitization.In this work, we focus on two different photosensitive molecules: phthalocyanines and triphenylamine-based dyes. Phthalocyanines are organic semiconductors with symmetric macro aromatic molecular structures. They possess good photoelectrical properties and good thermal and chemical stability, which make them widely used in the organic electronic industries. Triphenylamine-based dyes are new types of pure organic dyes which deliver high efficiency and reduce the cost of DSSC. They can be nominated as one of the strong candidates to substitute the ruthenium complex dyes in DSSC. The researches were carried out using classic surface science techniques on single crystal substrates and under ultrahigh vacuum condition. The photosensitive molecules were deposited by organic molecular beam deposition. The substrate reconstruction and ordering were checked by low energy electron diffraction. The molecular electronic, geometric structures and charge transfer properties were characterized by photoelectron spectroscopy, near edge X-ray absorption fine structure spectroscopy and resonant photoelectron spectroscopy (RPES). Scanning tunneling microscopy is used to directly image the molecular adsorption.For phthalocyanines, we select MgPc, ZnPc, FePc and TiOPc, which showed a general charge transfer from molecule to the substrate when adsorbed on rutile TiO2(110) surface with 1×1 and 1×2 reconstructions. This charge transfer can be prevented by modifying the TiO2 surface with pyridine derivatives (4-tert-butyl pyridine (4TBP), 2,2’-bipyridine and 4,4’-bipyridine), and furthermore the energy level alignment at the interface is modified by the surface dipole established by the pyridine molecules. Annealing also plays an important role to control the molecular structure and change the electronic structure together with the charge transfer properties, shown by TiOPc film. Special discussions were done for 4TBP for its ability to shift the substrate band bending by healing the oxygen vacancies, which makes it an important additive in the DSSC electrolyte. For the triphenylamine-based dye (TPAC), the systematic deposition enables the characterization of the coverage dependent changes of molecular electronic and geometric structures. The light polarization dependent charge transfer was revealed by RPES. Furthermore, the iodine doped TPAC on TiO2 were investigated to mimic the electrolyte/dye/TiO2 interface in the real DSSC.The whole work of this thesis aims to provide fundamental understanding of the interaction between photosensitive molecules on TiO2 surfaces at molecular level in the monolayer region, e.g. the formation of interfacial states and the coverage dependent atomic and electronic structures, etc. We explored the potential of the application of new dyes and modified of the existing system by identifying their advantage and disadvantage. The results may benefit the fields of dye syntheses, catalysis researches and designs of organic photovoltaic devices.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , xiii, 62 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2011:14
Keyword [en]
photoelectron spectroscopy, X-ray absorption spectroscopy, organic semiconductor, oxides, adsorption, dye sensitization, electronic structure, charge transfer
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:kth:diva-47354ISBN: 978-91-7501-114-8 OAI: oai:DiVA.org:kth-47354DiVA: diva2:455235
Public defence
2011-12-02, C2, KTH-Electrum, Isafjordsgatan 26, Kista,, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20111114Available from: 2011-11-14 Created: 2011-11-08 Last updated: 2011-11-14Bibliographically approved
List of papers
1. 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
2. Crystallization-Induced Charge-Transfer Change in TiOPc Thin Films Revealed by Resonant Photoemission Spectroscopy
Open this publication in new window or tab >>Crystallization-Induced Charge-Transfer Change in TiOPc Thin Films Revealed by Resonant Photoemission Spectroscopy
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2011 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 30, 14969-14977 p.Article in journal (Refereed) Published
Abstract [en]

Organic semiconductors usually demonstrate crystal structure dependent electronic properties, and through precise control of film structure, the performance of novel organic electronic devices can be greatly improved. Understanding the crystal structure dependent charge-transfer mechanism thus becomes critical. In this work, we have prepared amorphous titanyl phthalocyanine films by vacuum molecular beam evaporation and have further crystallized them through vacuum annealing. In the crystalline phase, an excited electron is rapidly transferred into neighboring molecules; while in the amorphous phase, it is mainly localized and recombines with the core hole as revealed by resonant photoemission spectroscopy (RPES). The fast electron transfer time is determined to be around 16 fs in the crystalline film, which is in good agreement with the charge-transfer hopping time estimated from the best device performance reported. The crystallized film shows more p-type characteristics than the amorphous with all the energy levels shifting toward the vacuum level. However, the greatly improved charge transfer is assigned to the molecular orbital coupling rather than this shift. From density functional theory and RPES, we specify the contribution of two differently coordinated nitrogen atoms (N2c and N3c) to the experimental results and illustrate that the N3c related orbital has experienced a dramatic change, which is keenly related to the improved charge transfer.

National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-37540 (URN)10.1021/jp1100363 (DOI)000293192100067 ()2-s2.0-79961039242 (Scopus ID)
Note
QC 20110816Available from: 2011-08-16 Created: 2011-08-15 Last updated: 2017-12-08Bibliographically approved
3. Structure-Dependent 4-Tert-Butyl Pyridine-Induced BandBending at TiO2 Surfaces
Open this publication in new window or tab >>Structure-Dependent 4-Tert-Butyl Pyridine-Induced BandBending at TiO2 Surfaces
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2011 (English)In: International Journal of Photoenergy (Online), ISSN 1110-662X, E-ISSN 1687-529X, Vol. 2011, 1-6 p.Article in journal (Refereed) Published
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-47350 (URN)10.1155/2011/401356 (DOI)000283223000001 ()2-s2.0-77957940667 (Scopus ID)
Note
QC 20111109Available from: 2011-11-08 Created: 2011-11-08 Last updated: 2017-12-08Bibliographically approved
4. 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
5. Adsorption geometry, molecular interaction, and charge transfer of triphenylamine-based dye on rutile TiO2(110)
Open this publication in new window or tab >>Adsorption geometry, molecular interaction, and charge transfer of triphenylamine-based dye on rutile TiO2(110)
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2010 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 133, no 22, 224704- p.Article in journal (Refereed) Published
Abstract [en]

The fast development of new organic sensitizers leads to the need for a better understanding of the complexity and significance of their adsorption processes on TiO2 surfaces. We have investigated a prototype of the triphenylamine-cyanoacrylic acid (donor-acceptor) on rutile TiO2 (110) surface with special attention on the monolayer region. This molecule belongs to the type of dye, some of which so far has delivered the record efficiency of 10%-10.3% for pure organic sensitizers [W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang, F. Wang, C. Pan, and P. Wang, Chem. Mater. 22, 1915 (2010)]. The molecular configuration of this dye on the TiO2 surface was found to vary with coverage and adopt gradually an upright geometry, as determined from near edge x-ray absorption fine structure spectroscopy. Due to the molecular interaction within the increasingly dense packed layer, the molecular electronic structure changes systematically: all energy levels shift to higher binding energies, as shown by photoelectron spectroscopy. Furthermore, the investigation of charge delocalization within the molecule was carried out by means of resonant photoelectron spectroscopy. A fast delocalization (similar to 1.8 fs) occurs at the donor part while a competing process between delocalization and localization takes place at the acceptor part. This depicts the "push-pull" concept in donor-acceptor molecular system in time scale. (C) 2010 American Institute of Physics. [doi:10.1063/1.3509389]

National Category
Atom and Molecular Physics and Optics Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-29370 (URN)10.1063/1.3509389 (DOI)000285477800019 ()2-s2.0-78650400126 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20110202Available from: 2011-02-02 Created: 2011-02-01 Last updated: 2017-12-11Bibliographically approved
6. Modification of Charge Transfer and Energy Level Alignment at Organic/TiO2 Interfaces
Open this publication in new window or tab >>Modification of Charge Transfer and Energy Level Alignment at Organic/TiO2 Interfaces
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2009 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, no 31, 13765-13771 p.Article in journal (Refereed) Published
Abstract [en]

Adsorption of titanyl phthalocyanine (TiOPc) on rutile TiO2(110) modified by a set of pyridine derivatives (2,2'-bipyridine, 4,4'-bipyridine, and 4-tert-butyl pyridine) has been investigated using synchrotron radiation based X-ray photoelectron spectroscopy (XPS). For the unmodified TiOPc/TiO2 system, a strong charge transfer is observed from the first layer TiOPc into the substrate, which leads to a molecular layer at the interface with a depleted highest occupied molecular orbital (HOMO). However, precovering the TiO2 surface with a saturated pyridine monolayer effectively reduce this process and leave the TiOPc in a less perturbed molecular state. Furthermore, the TiOPc HOMO and core levels are observed at different binding energies ranging by 0.3 eV on the three pyridine monolayers, which is ascribed to differences in surface potentials set up by the different pyridine/TiO2 systems.

Keyword
electronic-structure, titanyl phthalocyanine, tio2(110)-(1x1) surface, stm observation, solar-cells, thin-film, metal, pyridine, molecules, layer
Identifiers
urn:nbn:se:kth:diva-18640 (URN)10.1021/jp902814d (DOI)000268478700047 ()2-s2.0-68749107094 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
7. Changing adsorption mode of FePc on TiO2(110) by surface modification with bipyridine
Open this publication in new window or tab >>Changing adsorption mode of FePc on TiO2(110) by surface modification with bipyridine
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2008 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 129, no 7, 074707- p.Article in journal (Refereed) Published
Abstract [en]

Surface modification of reactive oxide substrates to obtain a less strongly interacting template for dye adsorption may be a way to enhance performance in dye-sensitized solar cells. In this work, we have investigated the electronic and structural properties of 4,4(')-bipyridine (bipy) as modifier adsorbed on the TiO2(110) surface. The modified surface is then coated with iron phthalocyanine (FePc) and the properties of this heterostructure are investigated with synchrotron based photoelectron spectroscopy, x-ray absorption spectroscopy, and scanning tunneling microscopy. We find that a saturated monolayer consisting of standing bipy molecules with one nitrogen atom pointing outward is formed on the oxide surface. FePc adsorb in molecular chains along the [001] direction on top of bipy and ordered in a tilted arrangement with adjacent molecules partially overlapping. Already from the first layer, the electronic properties of FePc resemble those of multilayer films. FePc alone is oxidized on the TiO2(110) surface, but preadsorbed bipy prevents this reaction. The energy level lineup at the interface is clarified.

Keyword
Direct energy conversion; Solar cells; Solar energy; Bipyridine; Dye adsorption; Dye-sensitized solar cells; Electronic and structural properties; Oxide substrates; Surface modifications; Adsorption; 4, 4' bipyridine; 4, 4'-bipyridyl; ferrous ion; indole derivative; iron phthalocyanine; pyridine derivative; titanium; titanium dioxide; unclassified drug; adsorption; article; artificial membrane; chemistry; electron; surface property; Electrons; Ferrous Compounds; Indoles; Membranes, Artificial; Pyridines; Surface Properties; Titanium
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-7781 (URN)10.1063/1.2969081 (DOI)000258643300041 ()19044792 (PubMedID)2-s2.0-50249106920 (Scopus ID)
Note
QC 20100812. Uppdaterad från Submitted till Published 20100812.Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2017-12-14Bibliographically approved
8. Adsorption of TiOPc on 1×1 and 1×2 reconstructed Rutile TiO2(110) Surfaces
Open this publication in new window or tab >>Adsorption of TiOPc on 1×1 and 1×2 reconstructed Rutile TiO2(110) Surfaces
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Ultrathin titanyl phthalocyanine (TiOPc) layers have been studied on rutile TiO2 (110) 1×1 and 1×2 surfaces by synchrotron radiation based photoelectron spectroscopy (PES) and scanning tunneling microscopy (STM). The adsorption mode of TiOPc on the surface is of great interest in that it is strongly related to the efficiency of the charge transfer process between dye and surface. Core level spectra show that two different adsorption states coexist on both surfaces; an interfacial state that has strong interaction with the substrate while the other retains its molecular characteristics. The bonding for the interfacial state is suggested to be mainly in the 1×1 areas. Annealing significantly changes the distribution of molecules with a preferential adsorption along steps and (1x2) strands.

National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-47344 (URN)
Note
QS 2011Available from: 2011-11-08 Created: 2011-11-08 Last updated: 2011-11-14Bibliographically approved

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