Mr. Yusuke Suenaga Profile

Yusuke Suenaga

at Ritsumeikan Univ

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Proceedings Article | 21 February 2012 Paper

Vibration-induced mobility enhancement for a polymer transistor

Yuuki Kondo, Tomonori Hiraki, Yuusuke Suenaga, Tomonori Hanasaki, Ichiro Fujieda

Proceedings Volume 8258, 82581A (2012) https://doi.org/10.1117/12.906567

KEYWORDS: Transistors, Ultrasonography, Polymers, Molecules, Semiconductor materials, Thin films, Silicon, Semiconductors, Electrodes, Field effect transistors

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Charge transport in an organic thin-film transistor (OTFT) is controlled by many factors such as molecular packing in the semiconductor material and the contact property at the source/drain electrode. One approach utilizes an alignment layer to influence the molecular packing. Charge transport becomes anisotropic. However, additional processes are required to form such a structured layer. Solution processes offer more pathways in influencing the molecular packing. These include the use of solvent mixtures for adjusting the evaporation-induced flows, the temperature gradient in molten materials, drop-casting on tilted substrates, and other flow-induced processes. Common to these approaches is the fact that some forms of forces introduce directionality in semiconductor materials. Here, we propose to agitate organic molecules in a solution by applying ultrasound vibrations during the solvent evaporation. The vibration would translate and rotate the molecules and this might introduce ordering in the organic layer when the solvent evaporation is completed. In experiment, we fabricated bottom-contact polymer transistors by dispensing a poly(3- hexylthiophene)/1,2,4-trichlorobenzene solution on a substrate and subsequently drying it in a container immersed in an ultrasound bath. The average field effect mobility of the transistors prepared from a 0.1wt% solution with 30-min ultrasound vibration was 2.5 times higher than that of the control devices prepared without the vibration. We attribute this result to enhanced ordering of the P3HT molecules in the vibrated solution. Atomic-force microscope observation revealed longer polymer chains for the samples prepared with the vibration. We attribute this mobility enhancement to changes in molecular packing during the solvent evaporation.

Proceedings Article | 21 February 2012 Paper

A photo-aligned self-assembled monolayer for polymer transistors

Tatsuhiko Kawaguchi, Takehiro Okura, Yuusuke Suenaga, Tomonori Hanasaki, Ichiro Fujieda

Proceedings Volume 8258, 825819 (2012) https://doi.org/10.1117/12.906559

KEYWORDS: Ultraviolet radiation, Transistors, Anisotropy, Absorbance, Molecules, Self-assembled monolayers, Polarization, Polarizers, Electrodes, Organic semiconductors

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There is a continuing interest in improving electrical characteristics of an organic thin-film transistor (OTFT). One can accomplish this by controlling molecular orientations of semiconductor materials in the vicinities of an insulating layer as well as an electrode material. First, it is widely known that a self-assembled monolayer (SAM) is effective for this purpose. Second, a thin structured layer underneath an organic semiconductor material is effective for aligning the organic molecules in a specific direction. Irradiating azobenzene compounds with ultraviolet light converts trans isomers into cis-forms. When exposed to linearly-polarized ultraviolet light, the difference in the absorbance between the two isomers leads to a state where the azobenzene molecules are aligned perpendicular to the polarization direction of the ultraviolet light. Such a photo-alignment layer results in anisotropic charge transport in an OTFT and the current flow along the channel direction is enhanced. In principle, we expect that combination of these two technologies (SAM and photo-alignment) would further improve the current flow in OTFTs. In experiment, we synthesized a compound 4-(3- (trichlorosilyl)propoxy)azobenzene (Azo-SAM) and used this material to align an organic semiconductor poly(3- hexylthiophene) (P3HT). We formed the Azo-SAM on a glass substrate, spin-coated a P3HT/1, 2, 4-trichlorobenzene solution, annealed in nitrogen atmosphere and exposed it to linearly-polarized ultraviolet light. Absorbance spectroscopy in the visible range revealed anisotropy in the two samples exposed to the two polarization directions orthogonal to each other. Fabrication of organic transistors with this photo-alignment SAM is under way.

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