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Electron-lattice dynamics in π-conjugated systems
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics . Linköping University, The Institute of Technology. (Beräkningsfysik)
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The work presented in this thesis concerns the dynamics in π-conjugated hydrocarbon systems. Due to the molecular bonding structure of these systems there exists a coupling between the electronic system and the phonons of the lattice. If this interaction, which is referred to as the electron-phonon (e-ph) coupling, is sufficiently strong it may cause externally introduced charge carriers to self-localize in a polarization cloud of lattice distortions. These quasi-particles are, if singly charged, termed polarons, the localization length of which, aside from the e-ph coupling strength, also depend upon the structural and energetic disorder of the system. In disordered systems localization is strong and transport is facilitated by nonadiabatic hopping of charge carriers from one localized state to the next, whereas in well-ordered systems, where extended states are formed, adiabatic transport models apply.Despite great academic efforts a unified model for charge transport in π-conjugated systems is still lacking and further investigations are necessary to uncover the basic physics at hand in these systems. The call for such efforts has been the main guidelines for the work presented in this thesis and are related to the topics of papers I-IV. In order to capture the coupled electron-lattice dynamics, we use a methodological approach where we obtain the time-dependence of the electronic degrees of freedom from the solutions to the time-dependent Schrödinger equation and determine the ionic motion in the evolving charge density distribution by simultaneously solving the lattice equation of motion within the potential field of the ions. The Hamiltonian used to describe the system is derived from an extension of the famous Su-Schrieffer-Heeger (SSH) model extended to three-dimensional systems.In papers I-III we explore the impact of phenylene ring torsion on delocalization and transport properties in poly(para-phenylene vinylene) (PPV). The physics that we are particularly interested in relates to the reduced electron transfer integral strength across the interconnection between the phenylene rings and the vinylene groups upon ring torsion. Keeping this in mind, we demonstrate in paper I the impact of static ring torsion on intrachain mobility and provide a detailed analysis of the influence of the potential barriers (due to consecutive ring torsion) on the nature of charge carrier propagation. In paper II we extend our initial approach to include also the dynamics of ring torsion. We show that without any externally applied electric field, this type of dynamics is the dominant property controlling intrachain propagation, but that when an external electric field is applied, charge carriers may traverse the potential barriers through a process that involves nonadiabatic effects and a temporary delocalization of the polaron state. Finally, in paper III we study the impact of the lattice dynamics on the electron localization properties in PPV and show that the phenylene ring torsion modes couples strongly to the electronic wave function which gives rise to electron localization at room temperature.In papers IV and V we focus on the dynamics of molecular crystals using a stack of pentacene molecules in the single crystal configuration as a model system, but study, in paper IV, the transport as a function of the intermolecular interaction strength, J. We observe a smooth transition from a nonadiabatic to an adiabatic polaron drift process over the regime 20<J<120 meV. For intermolecular interaction strengths above J≈120 meV the polaron is no longer stable and transport becomes band-like. In paper V, finally, we study the internal conversion processes in these systems, which is the dominant relaxation channel from higher lying states. This process involves the transfer of energy from the electronic system to the lattice. Our results show that this process is strongly nonadiabatic and that the relaxation time associated with large energy excitations is limited by transitions made between states of different bands.

Abstract [sv]

I dagens samhälle är elektroniken ett allt viktigare och större inslag i vår vardag. Vi ser på TV, talar i mobiltelefoner, och arbetar på datorer. I hjärtat av denna teknologi finner vi diskreta komponenter och integrerade kretsar utformade främst för att styra strömmen av elektroner genom halvledande material. Traditionellt sett har kisel eller olika former av legeringar använts som det aktiva materialet i dessa komponenter och kretsar, men under de senaste 20 åren har såväl transistorer som solceller och lysdioder realiserats där det aktiva materialet är organiskt, d.v.s., kolbaserat.Vi befinner oss för tillfället mitt uppe i det kommersiella genombrottet för organisk elektronik. Redan idag säljs många MP3-spelare och mobiltelefoner med små skärmar där varje pixelelementen utgörs av organiska ljusemitterande dioder (OLEDs), men teknologin håller redan på att introduceras i mer storskaliga produkter som datorskärmar och TV-apparater som därigenom skulle kunna göras energieffektivare, tunnare, flexiblare och på sikt också billigare. Andra tekniska tillämpningsområden för organisk elektronik som förutspås en lysande framtid är RFID-märkning, organiska solceller, och elektronik tryckt på papper, men även smarta textiler och bioelektronik har stor utvecklingspotential.Den kanske största utmaningen kvarstår dock, att skapa elektroniska kretsar och komponenter uppbyggda kring enskilda molekyler, s.k. molekylär elektronik. Mycket snart närmar vi oss den fysikaliska gränsen för hur små komponenter som vi kan realisera med traditionella icke-organiska material som kisel och en stor drivkraft bakom forskningen på halvledande organiska material har varit just visionen om molekylär elektronik som inte är mer än några miljondelars milimeter stora. För detta ändamål krävs en mycket nogrann kontroll av tillverkningsprocesserna liksom en detaljförståelse för hur molekylerna leder ström och hur denna förmåga kan manipuleras för att realisera såväl traditionella som nya komponenter.I denna avhandling presenteras en översikt av den fysik som möjliggör ledningsförmåga hos särskilda klasser av organiska material, s.k. π-konjugerade system, samt de forskningsresultat som utgör mitt bidrag till denna disciplin. En av utmaningarna på området är den komplexitet som de organiska materialen erbjuder: laddningsprocesserna påverkas nämligen av en rad olika faktorer såsom laddningstäthet, temperatur, pålagd spänning, samt molekylernas former och inbördes struktur. I detta arbete har jag utifrån en vidareutveckling av existerande modeller genom numeriska datasimuleringar undersökt effekten av de senare tre faktorerna på elektronstrukturen, laddnigstransporten och energidissipation i denna klass av material.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2008. , 49 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1215
Keyword [en]
charge transport, electron-lattice dynamics, polaron, adiabatic transport, electron localization, internal conversion
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:liu:diva-12590ISBN: 978-91-7393-788-7 (print)OAI: oai:DiVA.org:liu-12590DiVA: diva2:1781
Public defence
2008-10-31, Planck, Fysikhuset, Campus Valla, Linköping University, Linköping, 10:15 (English)
Opponent
Supervisors
Projects
Center of Organic Electronics (COE)
Available from: 2007-03-06 Created: 2008-09-16 Last updated: 2009-03-11Bibliographically approved
List of papers
1. Impact of ring torsion on the intrachain mobility in conjugated polymers
Open this publication in new window or tab >>Impact of ring torsion on the intrachain mobility in conjugated polymers
2007 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 10, 104304- p.Article in journal (Refereed) Published
Abstract [en]

Wehave developed a fully three-dimensional model based on the solutionof the time-dependent Schrödinger equation for studies of polaron mobilityin twisted polymer chains. Variations in ring torsion angles alonga conjugated polymer chain are shown to have a strongeffect on the intrachain charge carrier mobility. An increase inring torsion between two neighboring monomers can cause electron localizationand then result in a transition of the type oftransport from adiabatic polaron drift to nonadiabatic polaron hopping. Inparticular, we show the sensitivity for such a transition inthe case of random variations in the ring torsion anglesalong a poly(phenylene vinylene) chain. The effective energy barrier associatedwith the change in torsion angle also depends on theapplied electric-field strength, and by increasing the field strength atransition back to adiabatic transport can be obtained.

Keyword
electron-lattice dynamics, carrier mobility, localised states, polarons, hopping conduction
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-16943 (URN)10.1103/PhysRevB.75.104304 (DOI)
Projects
Center of Organic Electronics (COE)
Note

Original Publication: Magnus Hultell and Sven Stafström, Impact of ring torsion on the intrachain mobility in conjugated polymers, 2008, Physical Review B. Condensed Matter and Materials Physics, (75), 10, 104304. http://dx.doi.org/10.1103/PhysRevB.75.104304 Copyright: American Physical Society http://www.aps.org/

Available from: 2009-02-25 Created: 2009-02-25 Last updated: 2016-01-07Bibliographically approved
2. Impact of ring torsion dynamics on intrachain charge transport in conjugated polymers
Open this publication in new window or tab >>Impact of ring torsion dynamics on intrachain charge transport in conjugated polymers
2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, Vol. 79, no 1, 014302- p.Article in journal (Refereed) Published
Abstract [en]

Based on an approach including both the time-dependent Schrodinger equation and an effective Newtons equation for the ionic motion, we study the impact of ring torsion dynamics on the intrachain charge transport process in conjugated polymers. As model systems we have used single chains of poly(para-phenylene-vinylene). Without any external electric field, the dynamics of the phenyl ring torsion is the dominant property controlling intrachain charge propagation. The charge is coupled to both ring torsions and bond lengths distortions, which results in a significantly more localized polaron state than in a planar chain. In the presence of an electric field, the charge can breach the barriers caused by ring torsions, a process that involves nonadiabatic effects and a temporary delocalization of the polaron state.

Keyword
conducting polymers, localised states, polarons, Schrodinger equation
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-16835 (URN)10.1103/PhysRevB.79.014302 (DOI)
Note
Original Publication: Magnus Hultell and Sven Stafström, Impact of ring torsion dynamics on intrachain charge transport in conjugated polymers, 2009, PHYSICAL REVIEW B, (79), 1, 014302. http://dx.doi.org/10.1103/PhysRevB.79.014302 Copyright: American Physical Society http://www.aps.org/ Available from: 2009-02-25 Created: 2009-02-20 Last updated: 2009-04-30Bibliographically approved
3. The effect of lattice dynamics on electron localization in poly-(para-phenylene vinylene)
Open this publication in new window or tab >>The effect of lattice dynamics on electron localization in poly-(para-phenylene vinylene)
2009 (English)In: Synthetic metals, ISSN 0379-6779, Vol. 159, no 21-22, 2219-2221 p.Article in journal (Refereed) Published
Abstract [en]

The lattice dynamics in poly-(para-phenylenevinylene) and its effect on the electron localization properties have been investigated. The simulations were performed using a hybrid quantum mechanical/molecular dynamics approach including the Pariser–Parr–Pople Hamiltonian. It is found that the dynamic disorder that exists at room temperature induces electron localization. The most important lattice modes in the context of localization are the torsional modes of the phenylene rings, that are shown to couple strongly to the electronic system. Since these modes occur at low frequencies, they will also have a strong impact on intra-chain charge transport.

Keyword
Electron localization; Molecular dynamics; Torsional modes
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-12589 (URN)10.1016/j.synthmet.2009.08.035 (DOI)
Available from: 2008-09-16 Created: 2008-09-16 Last updated: 2010-04-22
4. Polaron dynamics in highly ordered molecular crystals
Open this publication in new window or tab >>Polaron dynamics in highly ordered molecular crystals
2006 (English)In: Chemical Physics Letters, ISSN 0009-2614, Vol. 428, no 4-6, 446-450 p.Article in journal (Refereed) Published
Abstract [en]

From a numerical solution of the time-dependent Schrödinger equation and the lattice equation of motion we obtain a microscopic picture of polaron transport in highly ordered molecular crystals in the presence of an external electric field. We have chosen the pentacene single crystal as a model system, but study the transport as a function of the intermolecular interaction strength, J. We observe a smooth transition from a nonadiabatic to an adiabatic polaronic drift process over the regime 20 < J < 120 meV. For intermolecular interaction strengths above 120 meV the polaron is no longer stable and the transport becomes band like.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-14208 (URN)10.1016/j.cplett.2006.07.042 (DOI)
Available from: 2007-03-06 Created: 2007-03-06
5. Nonradiative relaxation processes in molecular crystals
Open this publication in new window or tab >>Nonradiative relaxation processes in molecular crystals
2008 (English)In: Journal of Luminescence, ISSN 0022-2313, Vol. 128, no 12, 2019-2026 p.Article in journal (Refereed) Published
Abstract [en]

Internal conversion is the dominant relaxation channel from higher lying excited states in molecular crystals and involves the transfer of energy from the electronic system to the lattice. In this work, we present results from simulations of the nonradiative relaxation process with an emphasis on both intra- and interband transitions. We find the internal conversion process to be strongly nonadiabatic and the associated relaxation time in the case of large energy excitations to be limited by the transitions made between states of different bands.

Keyword
Internal conversion, Frenkel excitons, Interstate transitions, Pentacene single crystal
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-12587 (URN)10.1016/j.jlumin.2008.07.001 (DOI)
Projects
Center of Organic Electronics
Available from: 2008-09-16 Created: 2008-09-16 Last updated: 2009-04-30

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