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Towards flexible organic electronics: photoelectron spectroscopy of surfaces and interfaces
Linköping University, Department of Science and Technology. Linköping University, The Institute of Technology.
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Intensive studies of conjugated molecules and polymers are carried out all over the world with the intent of obtaining cheap and efficient organic electronic devices. The most mature application at the moment is the light-emitting diode, but also photovoltaic cells and different types of transistors shows promising results. Interest in these materials is based on possibilities of 'simple' and cheap processing techniques, comparing to inorganic compounds, in the manufacturing of devices. The understanding of the electronic and chemical structure of the surfaces and interfaces of these materials is a basic requirement for understanding the characteristics of the potential devices. Understanding the electronic structure of the pristine materials enables conclusions to be drawn concerning electrical and optical properties in these materials. The behaviour of the interface between metals and conjugated materials is one of the primary factors determining the suitability of using certain electrode/organic material combinations in device applications.

With this motivation, the electronic structure of both conjugated molecules and polymers surfaces and their interfaces to metals (and insulators) have been studied with mainly photoelectron spectroscopy (PES). In some cases complementary techniques have been needed and performed. This includes the four-point probe technique for determining surface resistance and atomic force morphology for determining surface morphology. As well as synchrotron-based techniques, such as near-edge X-ray absorption spectroscopy and resonant photoemission have been used. The main results compromised in this thesis are summarized below.

Poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT-PSS) is an aqueous colloidal dispersion consisting of doped conjugated polymer PEDOT with counter ions provided by the PSS chains. PEDOT-PSS films have previously proven to have a grain-like structure in which the grains have a ~30 Å thick insulating PSS outermost layer. The conductivity of thin PEDOT-PSS films has been improved through adding some high-boiling-point solvents to the PEDOT-PSS blend. The major reason for this increase is a rearrangement of the morphology, in terms of an increase in the PEDOT-to-PSS ratio in the surface region (i.e. the insulating PPS layer is decreased for each grain).

The initial stages of interface of PEDOT-PSS with aluminum for contacting purposes has also been examined. Due to the many components in the PEDOT-PSS film its reactions with alurninum was difficult to deduce. Therefore the aluminum interfaces with model molecules of each of the components of PEDOT-PSS were investigated to discern this. Phenyl-capped EDOT-trimer was used as a model oligomer for neutral PEDOT. It has been shown that aluminum preferentially interacts and forms covalent bonds with C-S carbons that causes a rearrangement of the charge density within the oligomer and breaks then-conjugation. In PEDOT-PSS blends the PEDOT part is left intact and alurninurn preferentially reacts with the SO3-H+ and/or SO3- species of the PSS part.

A specific blend of conjugated materials used in photovoltaic cells is a one to four mixture of APFO-3 (a low band gap copolymer based on alternating fluorene and donor-acceptor-donor units) to PCBM (soluble C60 derivative). The electrode systems studied are the widely used Al and Al/LiF contacts. We demonstrate a thickness dependent effect of the LiF layer in the Al/LiF/organic structure. LiF has a protective effect for all thickness preventing formation at the Al/organic interface of Al-organic complexes that destroy the Π-conjugation. In addition to this, there are two other beneficial effects (depending on LiF thickness). Decomposition of LiF occurs for thin enough layers in which the LiF species are in contact with both the organic film and the A1 atoms. This results in Li-doping of the organic films and creates a low workfunction contact. For thicker (multi)layers, the dipole formed at the LiP/organic interface is retained as no decomposition of the LiF occurs upon Al deposition.

We have shown the occurrence of interfacial dipoles at C60/LiF/Al interfaces and confirmed interfacial dipoles at Alq3/Al, C60/Al and Alq3/LiF/Al interfaces through vacuum level shifts. There is strong interaction with the substrates in all cases. There is evidence of covalent interaction between both Alq3 and C60 films with the AI substrates. The added LiF layer (between AI substrate and the organic film) prevents the covalent bonds from forming and the LiF does not dissociate in any case, unlike what is found in literature for the reverse order of deposition. For both Alq3 and C60 there is charge transfer from the Al substrate to the organic film through the LiF layer. However, if the thickness of the LiF layer exceeds 25 Å this charge transfer is blocked. The evolution of the electronic structure upon n-doping of the first Alq3 monolayer observed here is different from previous studies of n-doping mer-Alq3, indicating that there is preferential deposition and/or formation of the unusual facial isomer of Alq3 on the LiF/Al substrate. Our results are the first reported photoemission spectra of this isomer and its n-doped state.

The electronic structure of two new low band gap polymers (APFO-3 and APFO-7) based on donor-acceptor-donor groups copolymerized with fluorine units has been characterized. The valence band of APFO-3 seems to be highly dispersed and derived from orbitals delocalized over the whole polymer chain, where as the conduction band is nearly flat as it is derived from orbitals localized on the acceptor units. The existence of a dispersed valence band would predict good hole transporting properties, where as a flat conduction band would be expected to produce poor electron transporting properties. The electronic structure of APFO-7 has similarities to APFO-7 but it is also less clear. The larger size of the acceptor unit seem to distort both the valence band and conduction band shape as compared to APFO-3, however, so further work is needed to understand the more complex APFO-7 system.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet , 2004. , 56 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 895
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-22734Local ID: 2039ISBN: 91-85295-34-5 (print)OAI: oai:DiVA.org:liu-22734DiVA: diva2:243047
Public defence
2004-10-01, K3, Kåkenhus, Campus Norrköping, Norrköping, 10:15 (Swedish)
Opponent
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2013-01-29
List of papers
1. The effects of solvents on the morphology and sheet resistance in poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS) films
Open this publication in new window or tab >>The effects of solvents on the morphology and sheet resistance in poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS) films
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2003 (English)In: Synthetic metals, ISSN 0379-6779, E-ISSN 1879-3290, Vol. 139, no 1, 1-10 p.Article in journal (Refereed) Published
Abstract [en]

Films of poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS), prepared by coating the aqueous PEDOT–PSS dispersion and by coating a mixture of the PEDOT–PSS dispersion and different solvents, have been studied using four-point probe conductivity measurements, atomic force microscopy and photoelectron spectroscopy. The electrical conductivity of thin films of the second type (further on called PEDOT–PSS–solvents) was increased by a factor of about 600 as compared to films of the first type (further on called PEDOT–PSS–pristine). Morphological and physical changes occur in the polymer film due to the presence of the solvent mixture, the most striking being that the ratio of PEDOT-to-PSS in the surface region of the films is increased by a factor of ∼2–3. This increase of PEDOT at the surface indicates that the thickness of the insulating PSS ‘shell’ that surrounds the conducting PEDOT–PSS grains is reduced. The (partial) reduction of the excess PSS layer that surrounds the conducting PEDOT–PSS grains is proposed to lead to an increased and improved connectivity between such grains in the film and hence a higher conductivity. When PEDOT–PSS–solvents receives a post-coating heat treatment, the increased PEDOT-to-PSS ratio at the surface is maintained or even slightly improved, as is the increase in electrical conductivity, even though spectroscopy show that the solvent molecules are removed. This suggests that screening or doping by the solvents throughout the film are not likely to be the key mechanisms for the improved conductivity and supports our proposed mechanism of improved conductivity through improved connectivity between the conducting grains.

Keyword
AFM, Conducting polymers, PEDOT-PSS, Polymer blend, Sorbitol, XPS
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-46525 (URN)10.1016/S0379-6779(02)01259-6 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-13
2. X-ray photoelectron spectroscopy study of the metal/polymer contacts involving aluminum and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) derivatives
Open this publication in new window or tab >>X-ray photoelectron spectroscopy study of the metal/polymer contacts involving aluminum and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) derivatives
2003 (English)In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 18, no 5, 1219-1226 p.Article in journal (Refereed) Published
Abstract [en]

The contact formed between aluminum and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) (PEDOT-PSS) derivatives was studied using x-ray photoelectron spectroscopy. The aluminum/PEDOT-PSS contact contains an interfacial layer formed by chemical reactions between aluminum and mainly poly(styrenesulfonic acid) (PSSH). These chemical interactions were studied with the help of model systems (PSSH, benzenesulfonic acid, and sodium benzenesulfonate). The preferred reaction site of aluminum is the SO3 and SO3H+ groups of the PSS chains, giving rise to C-S-Al(-O) and C-O-Al species. The resulting contact formed consists of an insulating aluminum/PSS layer and a thin region of partially dedoped PEDOT-PSS. There is significant aluminum diffusion into films of the highly conducting form of PEDOT-PSS that have substantially less PSS at the surface. Hence, no (thick) aluminum/PSS layer is formed in this case, though the PEDOT chains close to the aluminum contact will still be partially dedoped as for the aluminum/PEDOT-PSS case.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-48652 (URN)10.1557/JMR.2003.0167 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-12
3. Phenyl-capped EDOT trimer: its chemical and electronic structure and its interface with aluminum
Open this publication in new window or tab >>Phenyl-capped EDOT trimer: its chemical and electronic structure and its interface with aluminum
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2003 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 107, no 39, 10793-10800 p.Article in journal (Refereed) Published
Abstract [en]

The chemical and electronic properties of a phenyl-capped 3,4-(ethylenedioxy)thiophene trimer (EDOT trimer) and its interface formation with aluminum have been studied. Thin EDOT trimer films were prepared on clean gold substrates through in-situ vapor deposition. Aluminum was deposited stepwise on top of the EDOT trimer, and the initial stages of interface formation were investigated by photoelectron spectroscopy. The organic/metal interface formed was not completely abrupt; some degree of diffusion of aluminum into the EDOT trimer film occurred. The aluminum atoms preferentially react with the α-position of the trimer (C−S carbon atoms) forming covalent bonds. The formation of these covalent bonds causes a break in the π-conjugation of the system due to the introduction of sp3 defects. The charge density also is somewhat redistributed within the oligomer as a whole, mainly affecting the neighboring atoms:  sulfur and β-position of the trimer (C=C−O carbon atoms). Once the C−S carbon sites are saturated, the aluminum instead reacts with the less favorable carbon atom of the ethylene bridge (C−O−C carbons). Worth noting is the decrease in work function from 5.2 eV for sputter cleaned gold to 4.1 eV upon deposition of the EDOT trimer. Our results have several implications for organic electronics. The sp3 defects introduced by the aluminum−EDOT contacting will influence the charge injection into the material across the EDOT trimer/aluminum interface negatively. The change in work function could potentially be used to modify gold contacts for electron injection into molecules with low electron affinity.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-47757 (URN)10.1021/jp034249p (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-13
4. Photoelectron spectroscopy of the contact between the cathode and the active layers in plastic solar cells: the role of LiF
Open this publication in new window or tab >>Photoelectron spectroscopy of the contact between the cathode and the active layers in plastic solar cells: the role of LiF
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2005 (English)In: Japanese Journal of Applied Physics, ISSN 0021-4922, E-ISSN 1347-4065, Vol. 44, no 6A, 3695-3701 p.Article in journal (Refereed) Published
Abstract [en]

The surfaces and electrode interfaces of a polymer blend used in prototype solar cells have been characterized with photoelectron spectroscopy. The polymer blend in question is a 1:4 mixture of APFO-3:PCBM. Based on surface analysis of the pristine film we can conclude that the surface of the blend is a 1:1 mixture of APFO-3 and PCBM. The electrode systems studied are the widely used Al and Al/LiF contacts. LiF prevents formation at the Al/organic interface of Al-organic complexes that destroy the π-conjugation. In addition to this, there are two other beneficial, thickness dependent, effects. Decomposition of LiF occurs for thin enough layers in which the LiF species are in contact with both the organic film and the Al atoms, which creates a low workfunction contact. For thicker (multi)layers, the dipole formed at the LiF/organic interface is retained as no decomposition of the LiF occurs upon Al deposition.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-30346 (URN)10.1143/JJAP.44.3695 (DOI)15883 (Local ID)15883 (Archive number)15883 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-12-13
5. Photoemission of Alq3 and C60 films on Al and LiF/Al substrates
Open this publication in new window or tab >>Photoemission of Alq3 and C60 films on Al and LiF/Al substrates
2005 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 98, no 1, 14901-14907 p.Article in journal (Refereed) Published
Abstract [en]

Photoemission studies of thin films of Alq3 and C60 deposited on Al and LiF/Al substrates have been performed in order to deduce the interactions of the organic films with the substrates. For all cases there is evidence of strong interaction resulting in the formation of interfacial dipoles. Attempts to explain the origin of these interfacial dipoles and the type of interface formed in each case have been done through analysis of the valence electronic structure and core levels of the materials. The origin of the interfacial dipoles is mainly covalent interaction when the organic films are deposited on Al substrates, and charge transfer between the organic molecules and the metal through the LiF sandwich layer when the organic films are deposited on LiF/Al substrates. For thick-enough LiF films, however, there is no interaction between the organic films and the substrates. In no case does the LiF dissociate, unlike what is found for the reverse order of deposition. Two charge-transfer-induced gap states are found for (sub)monolayer films of Alq3 deposited on LiF/Al. We propose that the formation of two gap states corresponds to negatively charged fac-Alq3.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-32253 (URN)10.1063/1.1929884 (DOI)18133 (Local ID)18133 (Archive number)18133 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-12-13
6. Electronic structure of novel low band gap conjugated polymers
Open this publication in new window or tab >>Electronic structure of novel low band gap conjugated polymers
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The electronic structures of two novel low band gap conjugated polymers have been studied. Both materials are conjugated alternating copolymers based on fluorene units and low band gap donor-acceptor-donor units. The emphasis in this study has been to study the valence and the conduction bands. In particular the degree of localization or delocalization along the polymers and the symmetry of these states have been studied, since these features can be related to their transport properties. The main experimental part of this work is photoelectron spectroscopy, near-edge X-ray photon absorption and resonant photoemission. These techniques have been used to probe the frontier electronic structure of these systems. The experimental results are interpreted with the help of density functional theory calculations. The valence bands are dispersed originating from orbitals delocalized along the polymer chain shile the conduction bands are more flat, as they are derived from orbitals localized on the acceptor units. This band structure would predict good hole transporting properties but poor electron transport.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-88047 (URN)
Available from: 2013-01-29 Created: 2013-01-29 Last updated: 2017-02-03

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