Joon-Ung Lee

Washing effects on the anchoring energy and surface order parameter on rubbed polymer surfaces containing the trifluoromethyl moiety

The washing effects on liquid crystal (LC) alignment capability in a NLC, 4-n-pentyl-4′-cyanobiphenyl (5CB), on a rubbed polyimide (PI) surface containing the trifluoromethyl moiety were studied. The extrapolation length d e of 5CB on the rubbed PI surface decreases with the rubbing strength RS. Also, the large extrapolation length d e of 5CB for washing with IPA was measured at RS=114 mm. The polar anchoring energy of 5CB on the rubbed PI surface was decreased by the washing process; it increased with the rubbing strength RS on the PI surface. The surface order parameter S S of 5CB for all the washing processes is smaller than for the non-washing process. Consequently, the polar anchoring energy and surface order parameter S s in 5CB are largely attributed to washing effects.

Conduction mechanism and stability of organic light-emitting diodes depending on the Al-based cathodes

We have studied a conduction mechanism and stability of organic light-emitting diodes with a variation of cathode materials; Al, LiAl, and LiF/Al. The organic light-emitting diodes emit a light by an injection and transport of charge carriers such as holes and electrons. Thus, the charge transport is affected by the injection barrier at the interface and hopping process in the bulk. To have an efficient organic light-emitting diodes, understanding of the electrical conduction mechanism is important. By varying the cathode materials, the electron injection at the interface could be controlled because of the work function change at the cathode. We have performed the temperature-dependent electrical properties and stability of the organic light-emitting diodes in the temperature range from 10 K to 300 K. And the devices of LiAl and LiF/AI cathode, in the lifetime and luminous efficiency, are superior to those of pure-Al.

The Fracture Distribution in ITO Coating with Compressive Bending Stress on Polymer Substrates

In this paper, we investigated the fracture distribution in indium-tin-oxide (ITO) coating with compressive bending stress on polymer. Under compressive strain, the ITO island delaminates, buckles and cracks. As the mechanical compressive stress increases, the buckling width of ITO seems to be increased. These created cracks are related to well-defined distribution of mechanical stress in ITO island-arrays. We related. mechanical bending stress to crack distribution and derived theoretical equation of position-dependent bending stress. And, we verified the bending stress`s magnitude to crack distribution observed from optical photographs.

Liquid Crystal Alignment Effect on Photopolymer Surface Using an Acrylate Unit Photopolymerized by a Photoinitiator

Photoalignment materials of PGMAcr, poly[3-(acryloyloxy)-2-hydroxypropyl methacrylate] using an acrylate unit photopolymerized by a photo-initiator and PGMA4Ch, poly[3-(4-chalconyloxy)-2-hydroxypropyl methacrylate] using photodimerization by the chalcone group were synthesized. Also, the liquid crystal (LC) alignment capabilities on the photopolymer layers were studied. A good LC alignment with UV exposure on the PGMAcr surface can be obtained. However, LC alignment defects were observed on the PGMA4Ch surface. The LC alignment capability of the PGMAcr surface by the photoinitiator was better than that of the PGMA4Ch surface by the chalcone group which is a photosensitive moiety.

Electrical Conduction Mechanism in ITO/Alq 3 /Al Organic Light-emitting Diodes

We have used ITO/Alq/Al structure to study electrical conduction mechanism in organic light-emitting diodes. Current-voltage-luminance characteristics were measured at room temperature by varying the thickness of Alq layer from 60 to 400mm. We were able to confirm that there are three different mechanisms depending on the applied voltage region; ohmic, space-charge-limited current, and trap-charge-limit-current mechanism. And the maximum luminous efficiency was obtained when the thickness of Alq layer is 200nm.

Negative resistance and current oscillation phenomena in oriented polypropylene films

The conduction characteristics in oriented polypropylene (OPP) film were studied over field intensities between 10 and 300 MV/m and a range of temperature from 5 to 55 C. The range of the conduction characteristics observed at 15 C was divided into five regions with increasing field intensity. Particularly, in the region from 70 to 82 MV/m voltage-controlled negative resistance was displayed. In the negative resistance region, current oscillations were also observed and the mobility of carriers was 1.44 {times} 10{sup {minus}13} m{sup 2}/V s. The negative resistance characteristics could be explained by Gibbons` theory.

The status of electrical insulation technology in Korea

The recent progress of apparatus technology in the Korean power industries is due to the governmental drive to upgrade design and manufacturing capabilities to achieve competitiveness in world markets. In the following, the electrical insulation technology development processes for electric apparatus and cables in industries, the basic research efforts in the university laboratories, and the recent research and development activities in the research institute, KERI, are described.

TSC properties of epoxy composites cured with acid-anhydride

The TSC spectroscopy was applied to investigate an influence of structural change and interfacial state on the electrical properties of epoxy composites cured with acid-anhydride. A addition of over abundant hardener and inorganic filler separated a /spl beta/ peak near 10[/spl deg/C] into two peaks of /spl beta//sub 1/(10/spl deg/C) and /spl beta//sub 2/(20/spl deg/C) and increased magnitude of TSC spectra between /spl alpha/ and /spl beta/ peak, which would be closely related with the process of thermal oxidation and separation of hardeners phenyl group according to an introduction of interfacial deformation, an expansion of free volume and a reduction of structural packing density. And the treatment of silane coupling agents on filler improved interfacial deformation to decrease the amplitude of TSC spectra.

Effects of PEDOT:PSS Buffer Layer in a Device Structure of ITO/PEDOT:PSS/TPD/Alq 3 /Cathode

We have investigated the effects of hole-injection buffer layer in organic light-emitting diodes using poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate)(PEDOT:PSS) in a device structure of . Polymer PEDOT:PSS buffer layer was made by spin casting method. Current-voltage, luminance-voltage characteristics and efficiency of device were measured at room temperature with a variation of cathode materials; Al, LiF/Al, LiAl, and Ca/Al. The device with LiF/Al cathode shows an improvement of external quantum efficiency approximately by a factor of ten compared to that of Al cathode only device. Our observation shows that cathode is important in improving the efficiency of the organic light-emitting diodes.

Dynamic Response of Organic Right-emitting Diodes in ITO/Alq 3 Structure

Dynamic response of organic light-emitting diodes were analyzed in (100 nm)/Al device structure with a variation of voltage an frequency. At low frequency region, complex impedance is mostly governed by resistive component, and at high frequency region by capacitive component. Also, we have evaluated resistance, capacitance and permittivity of devices.