Dong-Myung Shin

Photochemistry of chalcone and the application of chalcone-derivatives in photo-alignment layer of liquid crystal display

Abstract We synthesized chalcone derivatives and introduced them as a side chain unit in the backbone of polyimide for photo-alignment layers. The rate of photoreaction was followed by the disappearance of the CCCO bond in the chalcone moiety using UV–visible spectroscopy. The absorption peak at around 340 nm decreased significantly in less than a few minutes at room temperature. We studied the effect of the length of alkyl chain in chalcone derivatives and found that the alignment properties are very much dependent on the chain length.

Simple method for measuring the high pretilt angle of nematic liquid crystals

Pretilt angle measurable by using the conventional crystal rotation method is limited to about 0°–20° and 70°–90°. We propose a method by which we can measure the pretilt angle by using the crystal rotation method even if the pretilt angle is between 20° and 70°. In this method, the determination of a pretilt angle is achieved by finding the rotation angles where transmittances are maximum and minimum in the periodic transmittance characteristics obtained with the variation of rotation angle.

Surface-induced alignment of pentacene by photo-alignment technology for organic thin film transistors

A series of highly soluble maleimide-based polymers with photoreactive pendant group were synthesized and used as a gate dielectric insulator for organic thin film transistors. Photoalignment properties were characterized by UV-visible spectroscopy and the alignment of liquid crystals. Photopolymerization of polymer-coated organic thin film transistors with linearly polarized UV light induces anisotropy in field-effect mobility. The gate dielectric insulator based on the photoalignable maleimide shows higher field-effect mobility and on/off current ratio than those from a SiO2 gate dielectric insulator with the values of 0.3 cm2 V−1 s−1 and 104, respectively. Our results demonstrate that the photo-induced anisotropy of alignment films to control the molecular order of semiconducting pentacene is a promising technology for improving the performance of organic thin film transistors.

Influence of oligothiophene layer on PVK-based blue light-emitting diode

Abstract The α-sexithiophene, α-6T, was used as the hole injecting layer in polymer-based electroluminescent (EL) device. The hole basis of injection mechanism at the ITO/dye-doped poly(N-vinylcarbazole) (PVK) layer was investigated on the Fowler–Nordheim tunneling theory. The tunneling mechanism of holes is not applicable for the EL device that is fabricated in ITO/α-6T/dye-doped PVK/Al. We propose that the carrier injection barrier migrates from electrode (ITO)/organic layer (dye-doped PVK) to organic layer(α-6T)/organic layer (dye-doped PVK).

The characteristics of polyimide photoalignment layer with chalcone derivatives produced by linear polarized UV light

The photoalignment of liquid crystal (LC) molecules located onto polyimide films with chalcone derivatives using linearly polarized UV (LPUV) light is investigated. The LPUV light irradiation generated dimerization products of the chalcones followed by isomerization of the chalcone derivatives. The alignment directions of LC molecules were either homeotropic or planar with respect to plane of polyimide film, depending upon the alkyl chain length attached on the chalcones.

Influence of oligothiophene layer on the light emitting efficiency

Abstract The effect of α -sexithiophene ( α –6T) layers on the light emitting diode (LED) were studied. The α –6T was used for a buffer layer in electroluminescent (EL) devices. Enhanced carrier (hole) injection and improved emission efficiency were observed. Carrier injection characteristics were investigated as a function of α –6T layer thickness. The efficiency of the electroluminescence was proportional to the thickness of α —6T layer. The highest efficiency was observed at 600 A of α –6T layer, which was about 1.5 times higher than that of device without α –6T layer. The device with α –6T layer showed an operating voltage lowered by 2 V. The α –6T layer can substitute hole blocking layer, and control charge injecting properties.

Highly Efficient Color Filter Using Fluorescent Materials, (Z)-3-(4′-(dimethylamino)biphenyl-4-yl)-2-phenylacrylonitrile

The transmittance and color characteristics of color filters are directly related to the energy efficiency and image quality of LCD panels. We tried to investigate the effect of fluorescent dyes, (Z)-3-(4′-(dimethylamino)biphenyl-4-yl)-2-phenylacrylonitrile, on the transmittance and the color gamut of LCD panel. The fluorescent dye was synthesized and dispersed in polymer matrix and implemented at the lamp side before the color filters. Energy transfer from blue to green using fluorescent dye 1 can be applied to increase energy efficiency of green color resist. The fluorescent films using dye 1 can improve the transmittance about 4–5% and the color gamut and color purity that can be inferred from the color coordinate change from (X = 0.309, Y = 0.467) to (X = 0.325, Y = 0.521).

Dual Emission from Organic Light-Emitting Diodes Using Deep Red Novel Phosphorescent Iridium(III) Complexes

A new phosphorescent Iridium complex, Ir(TMP-TT)2(acac) was synthesized for organic light-emitting diodes (OLEDs). The ligand, TMP-TT, was synthesized by Suzuki coupling reaction, and Ir(TMP-TT)2(acac) was synthesized by Nonoyama reaction. The UV-Visible absorption peak and shoulders of Ir(TMP-TT)2(acac) were measured to be at 366, 420, 490 nm and its photoluminescence (PL) emission spectrums showed blue light-emission peaked 440 nm at 366 nm of UV absorption red light-emission peaked 638 nm at 490nm of UV absorption in chloroform solvent. Optoelectronic properties of device with the structure of ITO / PEDOT (40 nm) / TFB (30 nm) / RH-02 : Ir(TMP-TT)2(acac) (30 nm : 10%) / Bphen (40 nm) / Liq / Al was measured. The relative intensity of the dual wavelength emissions at 540 nm and 638 nm varied with applying voltage. This study shows that Ir(TMP-TT)2(acac) can be a strong dopant candidate for the white light-emitting OLEDs.

Surface Morphology of Photo-Crosslinkable Polyimide after Irradiation of Polarized UV Light

Abstract Photo-crosslinkable polyimide(PI) which contains CF3 moiety was synthesized. Polarized UV light transformed ketone group of PI to hydroxyl group, which was confirmed by IR and UV-visible spectroscopy. We investigated the dichroic UV-absorption before and after photo-reaction with linearly polarized light. The grooves were formed on the surface of PI film after irradiation with polarized UV-light. It was interesting to observe that the alignment of grooves was perpendicular to the polarized director, while the orientation of LC was parallel to the groove alignment.

Temperature-Dependent I-V Characteristics and Thermal Annealing Effects of C22-Quinolium(TCNQ) Langmuir-Blodgett Films

Abstract Electrical properties and thermal annealing effects of C22-quinoliuni(TCNQ) Langmuir-Blodgett(LB) films were studied. Typical current-voltage(I-V) characteristics along the perpendicular direction show an anomalous behavior of breakdown near the electric-field strength of 106 V/cm. From the study of I-V characteristics in the high-electric field region, it is found that there is a contribution of joule heating in addition to the electrical effect. To see the thermal influence of the specimen, current was measured as a function of temperature(20 ∼ 180°C). It shows that the current increases about 4 orders of magnitude near 60 ∼ 70°C, and remains constant for a while up to ∼ 150 °C and then suddenly drops. Such increase of current near 60 ∼ 70 °Cseems to be related to a softness of alkyl chains. Besides the electrical measurements, UV/visible absorption(300 ∼ 800nm) of the thermally annealed sample was measured to see the internal-structure change. It is found there are four characteristic peaks. A...