Orientation Of Pentacene Research Articles

An Orientation‐Controlled Pentacene Film Aligned by Photoaligned Polyimide for Organic Thin‐Film Transistor Applications

AbstractWe have demonstrated that the photoalignment method could be used to control the structural anisotropy of pentacene films, an active semiconducting layer, in thin‐film transistors (TFTs) with conspicuously anisotropic electrical characteristics. The photoaligned pentacene films were characterized with respect to structure and morphology using X‐ray diffraction and atomic force microscopy. Compared to the pentacene films that are not controlled, a maximal 25‐times increase in field‐effect mobility (up to 0.75 cm V–1 s–1) has been achieved in the photoaligned pentacene‐based TFTs by aligning pentacene orientation parallel to the direction of current flow with the help of a photoaligned polyimide layer. Mobility anisotropic ratio in the range of 2.7–8.3 for the current flow parallel and perpendicular to the alignment of the photoaligned pentacene films have been observed for photoaligned pentacene‐based TFTs.

Structural evolution of pentacene on a Ag(110) surface

Structural evolution of pentacene thin films on Ag(110) crystal, during molecular deposition and during a subsequent change of substrate temperature, was investigated in situ by a molecular beam epitaxy low-energy electron diffraction technique. The pentacene molecules exhibit high mobility on the Ag(110) surface at room temperature and the adsorbate shows a structural evolution from disorder in the submonolayer to an ordered structure when it reaches one monolayer. Heating the substrate to $145\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ results in no further structural transition. The ordered monolayer structure on Ag(110) has two domains mirrored at the crystal axis of the silver substrate. Molecular mechanics calculations indicate that the pentacene molecules lie flat on the Ag(110) surface, which is in good agreement with the experimental results. The calculation also gives the favorable position; i.e., the orientation of pentacene on the Ag(110).

Improvement of Characteristics of Organic Thin-Film Transistor with Anodized Gate Insulator by an Electrolyte Solution and Low-Voltage Driving of Liquid Crystal by Organic Thin-Film Transistors

The characteristics of pentacene-based organic thin-film transistors (OTFTs) on plastic substrates with anodized gate insulators using different electrolyte solutions were investigated. The characteristics were significantly improved by using ammonium borate as the solution. The results show a good current-saturation characteristic, field-effect mobility of 0.51 cm2/Vs and a current on/off ratio of 105 at a low drain voltage of 3 V. To investigate the reason for the improvements, the structural and electrical characteristics of OTFTs and insulators were examined using various methods such as X-ray diffraction, atomic force microscope, capacitance-voltage measurement and secondary ion mass spectroscopy. From the results of these experiments, it was found that the orientation of pentacene was improved and the concentration of impurities in the insulator decreased in the case of ammonium borate. We also examined low-voltage driving of liquid crystal devices by the OTFT, targeting its application for flexible displays. Improvement of characteristics by the solution was also confirmed in the driving.

Orientation of pentacene films using surface alignment layers and its influence on thin-film transistor characteristics

We have investigated the effect of surface order on the orientation and mobility of pentacene. The surface order was created using monolayers and polymers that are normally used to align liquid crystals. Rubbed polyvinylalcohol layers were found to align approximately 27% of the pentacene grains within a 30° range. When introduced in a thin-film transistor, they were found to enhance the saturation current by a factor of 2.5. A mechanism for this enhancement is proposed.