Jaehoon Park

Electro-optical characteristics of reflective liquid crystal mode using π-cell for low power operation

We demonstrated a reflective type of liquid crystal (LC) display with low power consumption using 180° twist and bend states in the π-cell. The driving voltage of the proposed operational mode depends on the cell and the LC parameters, such as the pretilt angle, the sample thickness, and the ratio of the sample thickness to the helical pitch of the LC. Under optimized conditions, this mode operates with less than 1.3 V and it exhibits excellent optical characteristics at all operational wavelengths.

Capacitance‐voltage characteristics of MIS capacitors using polymeric insulators

Abstract In this study, we investigate the capacitance‐voltage (C‐V) characteristics of metal‐insulator‐semiconductor (MIS) capacitors consisting of pentacene, as an organic semiconductor, and polymeric insulators such as poly(4‐vinylphenol) (PVP) or polystyrene (PS) prepared by spin‐coating process, to analyze the interfacial characteristics between pentacene and polymeric insulators. Compared with the device with PS, the MIS capacitor with PVP exhibited a pronounced shift in the flat‐band voltage according to the bias sweep direction. This hysteric feature in the C‐V characteristics is thought to be attributed to the trapped charges at the interface between pentacene and PVP owing to the hydrophilicity of PVP. From the experimental results, we can conclude that surface polarity of polymeric insulator has a critical effect on the interfacial properties, thereby affecting the bias stability of organic thin‐film transistors.

Characteristics of Organic Thin-Film Transistors with Self-Assembled Monolayer Formed in Gas-Phase on PVP Gate Insulator

The electrical characteristics of organic thin film transistors (OTFTs) with self-assembled monolayer (SAM) on the poly(4-vinylphenol) gate insulator have been investigated. The SAM was formed in gas-phase using the atomic layer deposition method onto gate insulator. It is observed that SAM modifies the surface polarity of gate insulator and influence the growth of the subsequent pentacene layer. Improvements of the crystallinity and directionality of pentacene molecules are confirmed by X-ray diffraction measurement. Also, the roughness of the pentacene thin film and the device performance are improved.

Synthesis and Properties of 6,13-Bis(4-propylphenyl)pentacene

Abstract In this study, 6,13‐Bis(4‐propylphenyl)pentacene was synthesized and characterized. The structures of the products that were obtained during the reaction steps were identified via FT‐IR and NMR spectroscopy and elemental analysis. The compound was found to be soluble in organic solvents like chloroform, dichloromethane, THF, and toluene. The charge transport mobility was 10‐4 cm2V‐1s‐1, and the on/off current ratio was 102, while the compound was 2.5 times more stable under oxidation in a solution compared with pentacene.

Pentacene Thin-Film Transistors with Hydrophobic Nanostructures on a Polymeric Gate Insulator

The effects of hydrophobic nanostructures on the growth of pentacene molecules and the electrical characteristics of pentacene thin-film transistors (TFTs) are investigated. X-ray diffraction results show that hydrophobic nanostructures are conducive to the upright growth of pentacene molecules to the substrate. Pentacene TFTs, in which hydrophobic nanostructures with a cowl shape were introduced at the interface between the pentacene layer and the gate insulator, thus exhibit a pronounced enhancement in the field-effect mobility, up to approximately 2.94 cm2/Vs. These results suggest that hydrophobic nanostructures are effective to the ordered growth of pentacene molecules for high-performance organic TFTs.

Scaling of Defect Domain by Reverse Twist in Chiral Hybrid In-Plane Switching LC Mode

We proposed a new Liquid Crystal (LC) mode named as chiral hybrid in-plane switching (CH-IPS) LC mode for LCD application. However, when LC is injected into a CH-IPS sample, the domains by reverse twist are induced usually. In order to remove such a defect, we investigate what the major factors which have influence on the creation of defects by the reverse twist domain are. By experiment, we define the major factors which have large influence on the formation of domains. By controlling these major factors properly, we can remove defect domains perfectly.

Electro-Optic Characteristics in a Cholesteric Phase of Bimesogenic Liquid Crystals

In this paper, we present electro-optic characteristics in cholesteric phase of the bimesogenic material with the helical twist power [HTP] agent BDH1218. The cholesteric phase of such mixture is more stable than blue phase and frequency-dependent in its alternative-current electric field response. It shows multi-stable-like domain structure characterized by the frequency and voltage applied. There are two driving methods for operation as a display. One is changing voltage under each of high and low frequency. The other is changing frequencies under a high voltage. We consider that the electro-optic characteristics of this cholecteric phase could be applied to electro-optic storage devices since all obtained gray levels are realized as memory states.