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Reduced crystallinity and enhanced charge transport by melt annealing of an organic semiconductor on single layer graphene
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
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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.

Place, publisher, year, edition, pages
2016. Vol. 4, no 19, 4143-4149 p.
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:umu:diva-120203DOI: 10.1039/C6TC00625FISI: 000376041700006OAI: oai:DiVA.org:umu-120203DiVA: diva2:927138
Available from: 2016-05-11 Created: 2016-05-11 Last updated: 2017-03-31Bibliographically approved
In thesis
1. Carbon nanotubes and graphene polymer composites for opto-electronic applications
Open this publication in new window or tab >>Carbon nanotubes and graphene polymer composites for opto-electronic applications
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Carbon nanotubes are carbon based structures with outstanding electronical and mechanical properties. They are used in a wide range of applications, usually embedded in polymer in the form of composites, in order to affect the electronic behavior of the matrix material. However, as the nanotubes properties are directly dependent on their intrinsic structure, it is necessary to select specific nanotubes depending on the application, which can be a complicated and inefficient process. This makes it attractive to be able to reduce the amount of material used in the composites.

In this thesis, focus is placed on the electrical properties of the composites. A simple patterning method is presented which allows the use of extremely low amounts of nanotubes in order to increase the electrical conductivity of diverse polymers such as polystyrene (PS) or poly(3-hexylthiophene) (P3HT). This method is called nanoimprint lithography and uses a flexible mold in order to pattern composite films, leading to the creation of conducting nanotube networks, resulting in vertically conducting samples (from the bottom of the film to the top of the imprinted patterns).

In parallel, X-ray diffraction measurements have been conducted on thin P3HT polymer films. These were prepared on either silicon substrate or on graphene, and the influence of the processing conditions as well as of the substrate on the crystallinity of the polymer have been investigated. The knowledge of the crystalline structure of P3HT is of great importance as it influences its electronic properties. Establishing a link between the processing conditions and the resulting crystallinity is therefore vital in order to be able to make opto-electronic devices such as transistor or photovoltaic cells.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2016. 57 p.
Keyword
carbon nanotubes, polythiophene, electrical conductivity, crystallography, graphene, nanoimprint lithography, synchrotron diffraction
National Category
Nano Technology
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-119779 (URN)978-91-7601-478-3 (ISBN)
Public defence
2016-06-01, KB3B1, KBC, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2016-05-11 Created: 2016-04-27 Last updated: 2016-05-26Bibliographically approved
2. Vertical charge transport in conjugated polymers
Open this publication in new window or tab >>Vertical charge transport in conjugated polymers
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
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:nbn:se:umu:diva-133180 (URN)978-91-7601-686-2 (ISBN)
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

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