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Khan A.A.,University of Cambridge | Rughoobur G.,Electronic Devices and Materials Group | Kamarudin M.A.,University of Cambridge | Sepe A.,Adolphe Merkle Institute | And 4 more authors.
Organic Electronics: physics, materials, applications | Year: 2016

Discotic liquid crystals (DLCs) are considered promising materials for organo-electronic applications. Columnar alignment of DLCs leads to anisotropic charge transport with high charge carrier mobility. However, pure DLCs exhibit low intrinsic charge carrier density which limits bulk conductivity. This research studies the alignment and conductivity properties of small molecule triphenylene-based DLCs to develop hole transport layers for potential applications in organic semiconductor devices. Binary mixtures of homologous DLCs of the hexakis(n-alkyloxy)triphenylene series (HAT6 and HAT10) are formulated. Mesophase characteristics and columnar alignment of these mixtures are characterized using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). Alignment, orientation and order of columnar packing in the mixtures is studied using X-ray diffraction (XRD) and grazing incidence wide angle X-ray scattering (GIWAXS) measurements. It is identified that binary mixture formation strongly effects the columnar alignment in solution processed films. Furthermore, to increase charge carrier density in the DLC films a strong electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is added as a p-type dopant, followed by an extensive characterization of its doping effect. POM, DSC thermal scans, UV-visible spectroscopy, photo-luminescence spectroscopy (PL) and I-V measurements are utilized to characterize and establish the improvement of hole conduction in the doped films. It is observed that F4TCNQ-doped triphenylene DLC films exhibit two-fold increase in hole conductivity, making the materials highly relevant for charge transport applications. © 2016 Published by Elsevier B.V.

Miyazaki E.,Electronic Devices and Materials Group | Tagawa M.,Kobe University | Yokota K.,Kobe University | Yokota R.,Japan Aerospace Exploration Agency | And 2 more authors.
Acta Astronautica | Year: 2010

Silicon containing polyimide is proposed as an atomic-oxygen (AO)-tolerant material for Low Earth Orbit flight. For this study, commercially available polysiloxane-block-polyimide film is selected for investigation. An AO beam is irradiated on the polysiloxane-block-polyimide film at the Combined Space Effects Test Facility of JAXA in Tsukuba, Japan. To investigate the AO tolerance, mass change measurement, cross-sectional transmission electron microscopic (TEM) observation, and X-ray photoelectron spectroscopic (XPS) analysis are performed. Results show that the mass loss of polysiloxane-block-polyimide is one one-hundredth or less than that of Kapton® H: Cross-sectional TEM observation and XPS analysis reveals that the AO protective SiO2 layer is self-organized by AO irradiation. Furthermore, the self-organized SiO2 layer is intentionally damaged to investigate reorganization of a new layer on it. Further AO irradiation of the damaged surface revealed that the new layer is built with a 500-nm-deep eroded region. The result verifies the "self-healing" ability of polysiloxane-block-polyimide. These results suggest that polysiloxane-block-polyimide film has high potential to provide many advantages of a space-use material, especially for LEO spacecraft. © 2009 Elsevier Ltd. All rights reserved.

Shimamura H.,Electronic Devices and Materials Group | Nakamura T.,Hokkaido University
Polymer Degradation and Stability | Year: 2010

The degradation of the mechanical properties of polyimide films was evaluated by means of tensile tests after exposure to a low earth orbit (LEO) environment. Polyimide films irradiated with atomic oxygen (AO), ultraviolet (UV) light, and electron beam (EB) rays using ground simulation facilities were also evaluated similarly and compared. In these experiments tensile stress (7.0 MPa or less) was applied to the samples in order to assess its effects on mechanical properties. The mechanical properties of the flight samples decreased concomitantly with increased exposure duration. The fracture surfaces exhibited characteristic radiated patterns initiating from the exposed surfaces which showed a rough texture. In the AO-irradiated samples the mechanical properties degraded and the surface texture developed as the AO fluence increased; similar fracture surfaces appeared in the flight samples. In contrast, UV and EB irradiation had little impact on mechanical properties. Based on these results, the eroded surfaces by AO irradiation served as the starting points of the rupture, resulting in degradation of mechanical properties of polyimide films exposed to a LEO environment. The tensile stress states induced no difference in evaluations. © 2009 Elsevier Ltd. All rights reserved.

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