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Vertical charge transport in conjugated polymers
Umeå University, Faculty of Science and Technology, Department of Physics.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Conjugated polymers are novel organic electronic materials highly important for organic photovoltaic applications. Charge transport is one of the key properties which defines the performance of conjugated polymers in electronic devices.

This work aims to explore the charge transport anisotropy in thin films of P3HT, one of the most common conjugated polymers. Using X-ray diffraction techniques and charge transport measurements, the relation between vertical charge transport through thin P3HT films and structure of the films was established.

It was shown that particular orientations of crystalline domains of P3HT, namely face-on and chain-on, are beneficial for vertical charge transport. These orientations provide the efficient pathways for the charges to be transported vertically, either via π-π stacking interaction between the adjacent conjugated chains, or via the conjugated chain backbones. It was also demonstrated that particular orientations of crystallites are favourable for the formation of interconnected percolated pathways providing enhanced vertical charge transport across the film.

Deposition of P3HT on most commonly used silicon substrates typically results in the formation of mostly edge-on orientation of crystallites which is unfavourable for vertical charge transport. Nanoimprint lithography was demonstrated as a powerful processing method for reorienting the edge-on crystalline domains of P3HT into chain-on (vertical) orientation. It is also shown that thin P3HT films with preferentially face-on orientations of crystallites can be deposited on graphene surface by spin coating.

Using patterning of thin P3HT films by nanoimprint lithography, unprecedentedly high average vertical mobilities in the range of 3.1-10.6 cm2 V-1 s-1 were achieved in undoped P3HT.

These results demonstrate that charge transport in thin films of a relatively simple and well-known conjugated polymer P3HT can be significantly improved using optimization of crystallinity,orientation of crystallites, polymer chain orientation and alignment in the films.

Place, publisher, year, edition, pages
Umeå: Umeå University , 2017. , 88 p.
Keyword [en]
organic electronics, conjugated polymers, nanotechnology, nanoimprint lithography, graphene, P3HT, charge transport, X-ray diffraction, structure, crystallite orientation
National Category
Nano Technology Textile, Rubber and Polymeric Materials
Research subject
Materials Science
Identifiers
URN: urn:nbn:se:umu:diva-133180ISBN: 978-91-7601-686-2 (print)OAI: oai:DiVA.org:umu-133180DiVA: diva2:1086327
Public defence
2017-05-17, N420, Naturvetarhuset, Universitetsvägen, 901 87, Umeå, 09:00 (English)
Opponent
Supervisors
Available from: 2017-04-26 Created: 2017-03-31 Last updated: 2017-04-24Bibliographically approved
List of papers
1. Enhanced Vertical Charge Transport in a Semiconducting P3HT Thin Film on Single Layer Graphene
Open this publication in new window or tab >>Enhanced Vertical Charge Transport in a Semiconducting P3HT Thin Film on Single Layer Graphene
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2015 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 25, no 5, 664-670 p.Article in journal (Refereed) Published
Abstract [en]

The crystallization and electrical characterization of the semiconducting polymer poly(3-hexylthiophene) (P3HT) on a single layer graphene sheet is reported. Grazing incidence X-ray diffraction revealed that P3HT crystallizes with a mixture of face-on and edge-on lamellar orientations on graphene compared to mainly edge-on on a silicon substrate. Moreover, whereas ultrathin (10 nm) P3HT films form well oriented face-on and edge-on lamellae, thicker (50 nm) films form a mosaic of lamellae oriented at different angles from the graphene substrate. This mosaic of crystallites with - stacking oriented homogeneously at various angles inside the film favors the creation of a continuous pathway of interconnected crystallites, and results in a strong enhancement in vertical charge transport and charge carrier mobility in the thicker P3HT film. These results provide a better understanding of polythiophene crystallization on graphene, and should help the design of more efficient graphene based organic devices by control of the crystallinity of the semiconducting film.

Keyword
graphene, organic semiconductor, P3HT, crystallization, charge transport
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-100760 (URN)10.1002/adfm.201403418 (DOI)000349225400001 ()
Available from: 2015-04-26 Created: 2015-03-09 Last updated: 2017-03-31Bibliographically approved
2. Reduced crystallinity and enhanced charge transport by melt annealing of an organic semiconductor on single layer graphene
Open this publication in new window or tab >>Reduced crystallinity and enhanced charge transport by melt annealing of an organic semiconductor on single layer graphene
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2016 (English)In: Journal of Materials Chemistry C, ISSN 2050-7526, Vol. 4, no 19, 4143-4149 p.Article in journal (Refereed) Published
Abstract [en]

We report on the effect of the annealing temperature on the crystallization and the electrical properties of the semiconducting polymer poly(3-hexylthiophene) (P3HT) on single layer graphene. Electrical characterization showed that heating the P3HT film above the melting point (Tm) resulted in a higher vertical charge carrier mobility. Grazing incidence X-ray diffraction (GIXD) revealed that the film was actually less crystalline overall, but that it consisted of a much higher number of face-on crystallites. We moreover show that annealing above Tm removes the existing seeds still present in the film at lower temperatures and enhances face-on formation. These results provide a better understanding of the influence of the annealing temperature on polythiophene crystallization on graphene, and it shows that the annealing at higher temperature induces a more favorable crystalline orientation which enhances charge transport, despite the reduction in the overall crystallinity. These results should help in the design of more efficient graphene based organic electronic devices by controlling the crystalline morphology of the semiconducting film.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:umu:diva-120203 (URN)10.1039/C6TC00625F (DOI)000376041700006 ()
Available from: 2016-05-11 Created: 2016-05-11 Last updated: 2017-03-31Bibliographically approved
3. Ultrahigh Mobility in an Organic Semiconductor by Vertical Chain Alignment
Open this publication in new window or tab >>Ultrahigh Mobility in an Organic Semiconductor by Vertical Chain Alignment
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2016 (English)In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 28, no 12, 2359-2366 p.Article in journal (Refereed) Published
Abstract [en]

A method to produce highly efficient and long-range vertical charge transport is demonstrated in an undoped polythiophene thin film, with average mobilities above 3.1 cm(2) V-1 s(-1). These record high mobilities are achieved by controlled orientation of the polymer crystallites enabling the most efficient and fastest charge transport along the chain backbones and across multiple chains. The significant increase in mobility shown here may present a new route to producing faster and more efficient optoelectronic devices based on organic materials. [GRAPHICS] .

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-119277 (URN)10.1002/adma.201503422 (DOI)000372459600009 ()26813586 (PubMedID)
Available from: 2016-06-02 Created: 2016-04-15 Last updated: 2017-03-31Bibliographically approved

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