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Interface Engineering in Organic Electronics
Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry . Linköping University, The Institute of Technology.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Organic electronics is a field covering all applications and devices where one or several of the active components are made of organic material, such as organic light emitting diodes, organic solar cells, organic thin film transistors, organic magnets for spintronics etc. In all of the applications mentioned above, transport of charges across both inorganic/organic and organic/organic interfaces play a key role for device performance. In order to achieve high efficiencies and longer life-times, proper matching of the electronic energy levels of the different materials is needed.

The aim of the research presented in this thesis has been to explore different routes to optimize interface energetics and gain deeper knowledge of the mechanisms that govern charge transport over the interface. Photoelectron spectroscopy (PES) is a method well suited to study both interactions between different materials taking place at surfaces as well as interface energetics.

One way to achieve proper matching of interfaces energy levels is by adding a dipole layer. In the three first papers presented in the thesis, the method of adding a monolayer of small organic molecules to change the work function of the surface is investigated. We start with a model system consisting of a nickel surface and PPDA molecules where we have strong interaction and mixing of orbitals between the molecule and the metal surface. The second system consists of a gold surface and TDAE molecules with weaker interaction with integer electron transfer and finally in the third paper an organic surface VPP-PEDOT-Tos is modified, with TDAE, to create a transparent low work function organic electrode. In the fourth paper, we focus on gaining deeper understanding of the Integer Charge Transfer (ICT) model and the mechanisms governing the alignment of energy levels at organic/(in)organic interfaces and in the fifth paper we continue to challenge this model by using it to predict the behavior of a bilayer device, in terms of energy level alignment.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2011. , 38 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1411
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-72236ISBN: 978-91-7393-018-5 (print)OAI: oai:DiVA.org:liu-72236DiVA: diva2:458629
Public defence
2011-12-14, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2011-11-23 Created: 2011-11-23 Last updated: 2013-09-12Bibliographically approved
List of papers
1. Characterization of the interface dipole at the paraphenylenediamine-nickel interface: A joint theoretical and experimental study
Open this publication in new window or tab >>Characterization of the interface dipole at the paraphenylenediamine-nickel interface: A joint theoretical and experimental study
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2005 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 122, no 8, 84712- p.Article in journal (Refereed) Published
Abstract [en]

In organic-based (opto)electronic devices, charge injection into conjugated materials is governed to a large extent by the metal-organic interface dipole. Controlling the injection of charges requires a better understanding of the fundamental origin of the interface dipole. In this context, photoelectron spectroscopies and density functional theory calculations are used to investigate the interaction between para-phenylenediamine (PPDA), an electron donor, and a polycrystalline nickel surface. The interface dipole formed upon chemisorption of one PPDA monolayer strongly modifies the work function of the nickel surface from 5.10 to 3.55 eV. The work function decrease of 1.55 eV is explained by the electron-donor character of PPDA and the modification of the electronic density at the metal surface. PPDA monolayers are composed of tilted molecules interacting via the nitrogen lone-pair and PPDA molecules chemisorbed parallel to the surface via their π-electron density. Annealing the monolayer leads to dehydrogenation of PPDA activated by the nickel surface, as found for other amines.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-24542 (URN)10.1063/1.1851507 (DOI)6701 (Local ID)6701 (Archive number)6701 (OAI)
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2017-02-03
2. Integer charge transfer at the tetrakis(dimethylamino)ethylene/Au interface
Open this publication in new window or tab >>Integer charge transfer at the tetrakis(dimethylamino)ethylene/Au interface
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2008 (English)In: Applied Physics Letters, ISSN 0003-6951, Vol. 92, no 16, 163302-1-163302-3 p.Article in journal (Refereed) Published
Abstract [en]

In organic-based electronics, interfacial properties have a profound impact on device performance. The lineup of energy levels is usually dependent on interface dipoles, which may arise from charge transfer reactions. In many applications, metal-organic junctions are prepared under ambient conditions, where direct overlap of the organic system from the metal bands is prevented due to presence of oxides and/or hydrocarbons. We present direct experimental and theoretical evidence showing that the interface energetic for such systems is governed by exchange of an integer amount of electrons.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-20776 (URN)10.1063/1.2912818 (DOI)
Available from: 2009-09-18 Created: 2009-09-18 Last updated: 2013-09-12
3. Transparent low-work-function indium tin oxide electrode obtained by molecular scale interface engineering
Open this publication in new window or tab >>Transparent low-work-function indium tin oxide electrode obtained by molecular scale interface engineering
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2004 (English)In: Applied Physics Letters, ISSN 0003-6951, Vol. 85, no 9, 1616-1618 p.Article in journal (Refereed) Published
Abstract [en]

Transparent low-work-function indium tin oxide (ITO) electrode was obtained by using molecular scale interface engineering. The modified ITO surface may be used as electron injecting electrode in polymer light-emitting devices. ITO surfaces, exposed to TDAE molecules, were found to be stable upon exposure to air, and to mild annealing. Photoelectron spectroscopy measurements show that the low-work-function of the modified electrode remains upon exposure to air in gentle annealing.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-45659 (URN)10.1063/1.1785873 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2013-09-12
4. Self assembled monolayer engineered interfaces for determination of charge transfer and charge separated states
Open this publication in new window or tab >>Self assembled monolayer engineered interfaces for determination of charge transfer and charge separated states
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Most interfaces in organic electronics consists of weakly interacting organic/(in)organic material interfaces where the interaction is limited to charge transfer via tunnelling. In order to optimize device structure and performance, it is of great importance to understand the rules that govern the energy level alignment at those interfaces. The integer charge transfer (ICT) model is a model used to explain and predict the interaction and energy level alignment behaviour from the so-called integer charge transfer energy, EICT values. In this paper we investigate two phenomena that could influence the absolute value of EICT at hybrid organic and organic-organic interfaces and provide experimentally-derived quantitative data on the strength of the effects.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-72233 (URN)
Available from: 2011-11-23 Created: 2011-11-23 Last updated: 2011-11-23Bibliographically approved
5. Energy level alignment at metal-organic and organic-organic interfaces with Alq3 and NTCDA
Open this publication in new window or tab >>Energy level alignment at metal-organic and organic-organic interfaces with Alq3 and NTCDA
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The energy level alignment behavior of the widely used materials tris-(8-hydroxyquinoline)aluminum (Alq3) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) is investigated. The Integer Charge Transfer (ICT) model is successfully used to predict their overall behavior at weakly-interacting hybridorganic and organic-organic interfaces, including NTCDA/Alq3 bilayers. The EICT- of NTCDA is measured to be 4.35 eV and the EICT+ of Alq3 is found to be 4.3 eV. The Alq3 films furthermore feature an interface dipole in absence of charge transfer due to the intrinsic dipole of the molecule and ordering effects.

Keyword
energy level alignment, interface properties, organic electronics, Fermi level pinning, bilayer structure, giant surface potential
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-72234 (URN)
Available from: 2011-11-23 Created: 2011-11-23 Last updated: 2011-11-23Bibliographically approved

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