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Modelling Charge Transport for Organic Solar Cells within Marcus Theory
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics. Linköping University, Faculty of Science & Engineering.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With the technological advancement of modern society, electronic devices are getting progressively more integrated in our everyday lives. Their continuouslygrowing presence is generating numerous concerns about costs, efficiency and the environmental impact of the electronic waste. In this context, organic electronics is finding its way through the market, allowing for potentially low-cost, light, flexible, transparent and environmentally friendly electronics. Despite the numerous successes of organic electronics, the functioning of several categories of organic devices still represents a technological challenge, due to problems like low efficiencies and stabilities (degradation over time).

Organic devices are composed by one or more organic materials depending on the particular application. The conformation and electronic structure of the organic molecules as well as their supramolecular arrangement in the single phase or at the interface are known to strongly a affect the mobility and/or the efficiency of the device. While there is consensus on the fundamental physics of organic devices, we still lack a detailed comprehensive theory able to fully explain experimental data. In this thesis we focus on trying to expand our knowledge of charge transport in organic materials through theoretical modelling and simulation of organic electronic devices. While the methodology developed is generally valid for any organic device, we will particularly focus on the case represented by organic photovoltaics.

The morphology of the system is obtained by molecular dynamics simulations. Marcus theory is used to calculate the hopping rate of the charge carriers and subsequently study the possibility of free charge carriers production in an organic solar cell. The theory is then compared both with Kinetic Monte Carlo simulations and with experiments to identify the main pitfalls of the actual theory and ways to improve it. The Marcus rate between two molecules depends on the molecular orbital energies, the transfer integral between the two molecules and the reorganization energy. The orbital energies and the transfer integrals between two neighbouring molecules are obtained through quantum mechanical calculations in vacuum. Electrostatic effects of the environment are included through atomic charges and atomic polarizabilities, producing a correction both to the orbital energy and to the reorganization energy. We have studied several systems in the single phase (polyphenylene vinylene, C60, PC61BM) and at the interface between two organic materials (anthracene/C60, TQ1/PC71BM).

We show how a combination of different methodologies can be used to obtain a realistic ab-initio model of organic devices taking into account environmental effects. This allows us to obtain qualitative agreement with experimental data of mobility in the single phase and to determine whether or not two materials are suitable to be used together in an organic solar cell.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017. , p. 54
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1815
National Category
Theoretical Chemistry Condensed Matter Physics Physical Chemistry
Identifiers
URN: urn:nbn:se:liu:diva-133329DOI: 10.3384/diss.diva-133329ISBN: 9789176856192 (print)OAI: oai:DiVA.org:liu-133329DiVA, id: diva2:1058072
Public defence
2017-02-10, Planck (J206), Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2016-12-20 Created: 2016-12-20 Last updated: 2017-01-17Bibliographically approved
List of papers
1. Transition fields in organic materials: From percolation to inverted Marcus regime. A consistent Monte Carlo simulation in disordered PPV
Open this publication in new window or tab >>Transition fields in organic materials: From percolation to inverted Marcus regime. A consistent Monte Carlo simulation in disordered PPV
2015 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 142, no 9, p. 094503-Article in journal (Refereed) Published
Abstract [en]

In this article, we analyze the electric field dependence of the hole mobility in disordered poly (p-phenylene vinylene). The charge carrier mobility is obtained from Monte Carlo simulations. Depending on the field strength three regions can be identified: the percolation region, the correlation region, and the inverted region. Each region is characterized by a different conduction mechanism and thus a different functional dependence of the mobility on the electric field. Earlier studies have highlighted that Poole-Frenkel law, which appears in the correlation region, is based on the type of correlation caused by randomly distributed electric dipoles. This behavior is thus observed in a limited range of field strengths, and by studying a broader range of electric fields, a more fundamental understanding of the transport mechanism is obtained. We identify the electric fields determining the transitions between the different conduction mechanisms in the material and we explain their physical origin. In principle, this allows us to characterize the mobility field dependence for any organic material. Additionally, we study the charge carrier trapping mechanisms due to diagonal and off-diagonal disorder, respectively. (C) 2015 AIP Publishing LLC.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-117234 (URN)10.1063/1.4913733 (DOI)000350973900041 ()25747090 (PubMedID)
Note

Funding Agencies|Swedish Research Council (VR); MATTER Network; SERC (Swedish e-Science Research Center)

Available from: 2015-04-22 Created: 2015-04-21 Last updated: 2017-12-04
2. Effect of Polarization on the Mobility of C60: A Kinetic Monte-Carlo Study
Open this publication in new window or tab >>Effect of Polarization on the Mobility of C60: A Kinetic Monte-Carlo Study
Show others...
2016 (English)In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 12, no 2, p. 812-824Article in journal (Refereed) Published
Abstract [en]

We present a study of mobility field and temperature dependence for C60 with Kinetic Monte-Carlo simulations. We propose a new scheme to take into account polarization effects in organic materials through atomic induced dipoles on nearby molecules. This leads to an energy correction for the single site energies and to an external reorganization happening after each hopping. The inclusion of polarization allows us to obtain a good agreement with experiments for both mobility field and temperature dependence.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2016
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-122989 (URN)10.1021/acs.jctc.5b00975 (DOI)000370112900032 ()
Note

Vid tiden för disputation förelåg publikationen endast som manuskript

Funding agencies:  SeRC (Swedish e-Science Research Center)

Available from: 2015-12-01 Created: 2015-12-01 Last updated: 2017-12-01Bibliographically approved
3. Theoretical Study of the Charge-Transfer State Separation within Marcus Theory: The C-60-Anthracene Case Study
Open this publication in new window or tab >>Theoretical Study of the Charge-Transfer State Separation within Marcus Theory: The C-60-Anthracene Case Study
2016 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 37, p. 24722-24736Article in journal (Refereed) Published
Abstract [en]

We study, within Marcus theory, the possibility of the charge-transfer (CT) state splitting at organic interfaces and a subsequent transport of the free charge carriers to the electrodes. As a case study we analyze model anthracene-C-60 interfaces. Kinetic Monte Carlo (KMC) simulations on the cold CT state were performed at a range of applied electric fields, and with the fields applied at a range of angles to the interface to simulate the action of the electric field in a bulk heterojunction (BHJ) interface. The results show that the inclusion of polarization in our model increases CT state dissociation and charge collection. The effect of the electric field on CT state splitting and free charge carrier conduction is analyzed in detail with and without polarization. Also, depending on the relative orientation of the anthracene and C-60 molecules at the interface, CT state splitting shows different behavior with respect to both applied field strength and applied field angle. The importance of the hot CT in helping the charge carrier dissociation is also analyzed in our scheme.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2016
Keyword
organic solar cell; charge transfer state; splitting separation; interface; Marcus theory; kinetic Monte Carlo
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-132221 (URN)10.1021/acsami.6b06645 (DOI)000384033600054 ()27561228 (PubMedID)
Note

Funding Agencies|SERC (Swedish e-Science Research Center)

Available from: 2016-10-25 Created: 2016-10-21 Last updated: 2018-03-22

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