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Inhomogeneous charge transfer within monolayer zinc phthalocyanine absorbed on TiO2(110)
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. (Surface physics)ORCID iD: 0000-0002-9663-7705
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
The University of Queensland.
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
Show others and affiliations
2012 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 15, p. 154703-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.

Place, publisher, year, edition, pages
2012. Vol. 136, no 15, p. 154703-
Keywords [en]
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: urn:nbn:se:kth:diva-47336DOI: 10.1063/1.3699072ISI: 000303147000034Scopus ID: 2-s2.0-84860168047OAI: oai:DiVA.org:kth-47336DiVA, id: diva2:454798
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
In thesis
1. Molecular Interaction of Thin Film Photosensitive Organic Dyes on TiO2 Surfaces
Open this publication in new window or tab >>Molecular Interaction of Thin Film Photosensitive Organic Dyes on TiO2 Surfaces
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. p. xiii, 62
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2011:14
Keywords
photoelectron spectroscopy, X-ray absorption spectroscopy, organic semiconductor, oxides, adsorption, dye sensitization, electronic structure, charge transfer
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-47354 (URN)978-91-7501-114-8 (ISBN)
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
2. Structure-dependent charge transfer at the interafce between organic thin films, and metals and metal oxides
Open this publication in new window or tab >>Structure-dependent charge transfer at the interafce between organic thin films, and metals and metal oxides
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. p. xi, 74
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2013:06
Keywords
photoelectric spectroscopy, X-ray absorption spectroscopy, organic semiconductors, phthalocyanine, charge transfer, electronic structure, dye sensitization
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-134841 (URN)
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

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