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A Theoretical Study of Charge Transport in Molecular Crystals
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics. Linköping University, The Institute of Technology.
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The main objective of this thesis is to provide a deeper understanding of the charge transport phenomena occuring in molecular crystals. The focus is on the stability and the dynamics of the polaron as the charge carrier.

To achieve this goal, a series of numerical calculations are performed using the semi-emprical "Holstien-Peierls" model. The model considers both intra- (Holstein) and inter- (Peierls) molecular interactions, in particular the electron-phonon interactions.

First, the stability of the polaron in an ordered two dimensional molecular lattice with an excess charge is studied using Resilient backPropagation, RPROP, algorithm. The stability is defined by the "polaron formation energy". This formation energy is obtained for a wide range of parameter sets including both intra- and inter-molecular electron-phonon coupling strengths and their vibrational frequencies, transfer intergral and electric field. We found that the polaron formation energies lying in the range of 50-100 meV are more interesting for our studies.

The second step to cover is the dynamical behaviour of the polaron. Using the stable polaron solutions acheived in the first step, an electric field is applied as an external force, pushing the charge to move. We observed that the polaron remains stable and moves with a constant velocity for only a limited range of parameter sets. Finally, the impact of disorder and temperature on the charge dynamics is considered. Adding disorder to the system will result in a more restricted parameter set space for which the polaron is dynamically stable and mobile.

Temperature is included in the Newtonian equations of motion via a random force. We observed that the polaron remains localized and moves with a diffusive behaviour up to a certain temperature. If the temperature increases to values above this critical temperature, the localized polaron becomes delocalized.

All this research work is coded in MATLAB software , allowing us to run the calculations, test and validate our results.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. , 41 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1560
Keyword [en]
Molecular Crystals, Charge transport, Polaron, Holstein model, Peierls coupling
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-88111Local ID: LIU-TEK-LIC-2012:45ISBN: 978-91-7519-731-9 (print)OAI: oai:DiVA.org:liu-88111DiVA: diva2:601769
Presentation
2013-02-04, Schrödinger, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 13:15 (English)
Opponent
Supervisors
Available from: 2013-01-30 Created: 2013-01-30 Last updated: 2013-01-30Bibliographically approved
List of papers
1. Polaron stability in molecular crystals
Open this publication in new window or tab >>Polaron stability in molecular crystals
2012 (English)In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 376, no 22, 1807-1811 p.Article in journal (Refereed) Published
Abstract [en]

A semi-empirical Peierls-Holstein model is applied to studies of the stability of polarons in two-dimensional molecular crystal systems. Calculations for a broad range of intra- and inter-molecular parameters within this model were performed in order to obtain detailed knowledge concerning the stability of the polaron solution with respect to a rigid lattice band solution. For realistic values of the parameters the polaron solution is stable with a polaron energy in the range 50-100 meV. A metastable polaron solution is also identified. The polarons that result from our model are highly localized and it is questionable if adiabatic polaron transport can occur in the system.

Place, publisher, year, edition, pages
Elsevier, 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-78584 (URN)10.1016/j.physleta.2012.04.007 (DOI)000304338100010 ()
Note
Funding Agencies|Swedish Research Council (VR)||Available from: 2012-06-15 Created: 2012-06-15 Last updated: 2017-12-07
2. Polaron dynamics in a two-dimensional Holstein-Peierls system
Open this publication in new window or tab >>Polaron dynamics in a two-dimensional Holstein-Peierls system
2013 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, no 18, 184104- p.Article in journal (Refereed) Published
Abstract [en]

A semiclassical model for studying charge transport in a two-dimensional molecular lattice is presented and applied to both a well ordered system and a system with disorder. The model includes both intra- and inter-molecular electron-lattice interactions and the focus of the studies is to describe the dynamics of a charge carrier in the system. In particular, we study the dynamics of the system in which the polaron solution is dynamically stable. It is found that the parameter space for which the polaron is moving through the system is quite restricted and that the polaron is immobile for large electron-phonon coupling and weak intermolecular electron interactions and dynamically unstable and disassociates into a delocalized electronic state decoupled from the lattice for small electron-phonon coupling and strong intermolecular electron interactions. Disorder further limits the parameter space in which the polaron is mobile.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-88110 (URN)10.1063/1.4803691 (DOI)000319290800009 ()23676026 (PubMedID)
Note

Funding Agencies|Swedish Research Council (VR)||

Available from: 2013-01-30 Created: 2013-01-30 Last updated: 2017-12-06Bibliographically approved

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