Joon-Ung Lee

Kwangwoon University, Sŏul, Seoul, South Korea

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Publications (19)0 Total impact

  • Joon Ho Ahn, Joon-Ung Lee, Tae Wan Kim
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    ABSTRACT: Temperature-dependent impedance characteristics of ITO/Alq3/Al organic light-emitting diodes were studied in the frequency range from 40 to 108Hz, and the temperature was varied from 10 to 300K. At each temperature, the frequency-dependent complex impedance was measured under discrete bias voltages from −6 to +20V, and the voltage-dependent impedance was measured at 102Hz, 103Hz, 104Hz, and 105Hz. A Cole–Cole plot shows that there is one relaxation, and a parallel capacitor–resistor network in series with a contact resistance could be considered as an equivalent electrical circuit to this device. As the temperature decreases, a radius in the Cole–Cole plot increases, which indicates an increase of resistance of the device.
    Current Applied Physics - CURR APPL PHYS. 01/2007; 7(5):509-512.
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    Yong-Cheul Oh, Joon-Ung Lee
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    ABSTRACT: We have investigated dielectric properties depending on bias voltage in organic light-emitting diodes using 8-hydroxyquinoline aluminum as an electron transport and emissive material. We analyzed the dielectric properties of organic light-emitting diodes using impedance of characteristics. Impedance characteristics was measured complex impedance Z and phase in the frequency range of 40 Hz to Hz. We obtained complex electrical conductivity, dielectric constant, and loss tangent of the device at room temperature. From these analyses, we are able to interpret a conduction mechanism and dielectric properties contributed by an interfacial and orientational polarization.
    Journal of the Korean Institute of Electrical and Electronic Material Engineers. 01/2005; 18(11).
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    ABSTRACT: According to the trend for electric power equipment of high capacity and reduction of its size, the needs for the new high performance electric equipments become more and more important. On of the possible solution is high temperature superconducting (HTS) power application. Following the successful development of practical HTS wires, there have been renewed activities in developing superconducting power equipment. HTS equipments have to be operated in a coolant such as liquid nitrogen () or cooled by conduction-cooling method such as using Gifford-McMahon (G-M) cryocooler to maintain the temperature below critical level. In this paper, the dielectric strength of some insulating materials, such as unfilled epoxy, filled epoxy, and polyimide in was analyzed. Epoxy is a good insulating material but fragile at cryogenic temperature. The filled epoxy composite not only compensates for this fragile property but enhances its dielectric strength.
    Transactions on Electrical and Electronic Materials. 01/2005; 6(6).
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    Dong-Hoe Chung, Joon-Ung Lee
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    ABSTRACT: 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.
    Transactions on Electrical and Electronic Materials. 01/2004; 5(1).
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    ABSTRACT: Frequency-dependent response of organic light-emitting diodes was studied in a device structure of ITO/Alq3 (60 nm)/Al. Frequency was varied from 40 Hz to 1 MHz and the bias voltage was varied from −5 to +13 V. We have interpreted the frequency-dependent response in terms of equivalent circuit model of parallel combination of resistance Rp and capacitance Cp. From the frequency-dependent response, we are able to obtain characteristic crossover frequency ω0 separating resistive and capacitive response. And we found that there is an effect of interfacial polarization in the frequency range well below the crossover frequency when the applied voltage is lower than turn-on voltage.
    Current Applied Physics - CURR APPL PHYS. 01/2004; 4(5):543-546.
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    ABSTRACT: 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.
    Properties and Applications of Dielectric Materials, 2003. Proceedings of the 7th International Conference on; 07/2003
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    ABSTRACT: ZnO varistors have an excellent non-linearity and a large surge-energy absorption capability. For these reasons, the ZnO varistors are widely used to protect electrical/electronic circuits from an abnormal surge and/or noise signal. In order to obtain the low-voltage varistor with randomly distributed large seed grain within bulk, the ZnO varistors are made by a new three-composition seed grain method. From the I-V characteristics of the fabricated varistors, the knee voltage of varistors with the seed grain 5wt% is 5 V. We may also present a carrier oscillation properties for the low-voltage varistor fabricated by the new method of three-composition seed grain.
    Journal of the Korean Institute of Electrical and Electronic Material Engineers. 01/1999; 12(11).
  • Jong-Bae Wang, Joon-Ung Lee
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    ABSTRACT: The changes and stability of the structure in polymer which was varied with the ratio of additives addition and the postcuring heat-treatment have been explained through the estimation of the distributed relaxation times in the dielectric properties and the contribution of the ions at low-frequency and high-temperature regions. In the dielectric properties, the a peak related with Tg of epoxy network appeared near 130[°C], the conduction loss due to thermal dissociation of hardener above 150[°C] started off in the low frequency side below 330[Hz]. The anomalous dielectric behaviours of polymers at low frequencies and high temperature regions have been interpreted in terms of diffusion of impurity ions
    Properties and Applications of Dielectric Materials, 1997., Proceedings of the 5th International Conference on; 06/1997
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    ABSTRACT: The dielectric properties of (Sr.Ca)TiO<sub>3</sub>-based boundary layer (BL) capacitor materials were investigated. (Sr.Ca)TiO<sub>3</sub>-based compositions were fired at 1350°C in a N<sub>2</sub> atmosphere to get the based semiconductive ceramics. The Nb<sub>2</sub>O<sub>5</sub> was chosen for semiconductor of grain. The metal oxides of CuO were printed on the semiconductive ceramics, and those were fired at 1200°C for the subsequent grain boundary diffusion. The grain size of the semiconductive ceramics was about 20 μm and the apparent permittivity of the resulting material varied between 2×10<sup>4</sup> and 3×10<sup>4</sup>
    Properties and Applications of Dielectric Materials, 1997., Proceedings of the 5th International Conference on; 01/1997
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    ABSTRACT: In this study, we may be presented the oscillation properties for the low-voltage varistor fabricated by a new method of three-composition seed grain, in order to analyze the behavior of carriers at the its equivalent circuit model. The oscillation phenomena of carriers appeared from current-voltage characteristics under knee voltage is shown by the transient flow of nontrapped carriers group in the trap level of intergranular layer, surface state and/or depletion layer. In particularly, current oscillation phenomena is hardly shown in the high electric field. It is that the injected carriers from both electrodes are directly from the conduction band of forward biased ZnO grain through the intergranular layer into the reverse biased ZnO grain, because the trap level in the electric field above the knee voltage is mostly filled
    Properties and Applications of Dielectric Materials, 1997., Proceedings of the 5th International Conference on; 01/1997
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    ABSTRACT: A calculation using the FEM (Finite Element Method) is employed for the design of electromagnetic devices and products made of composite polymer materials. Applications include the insulating materials of high voltage machines or cables, and sensor materials for the low frequency range. In this study, an acoustic sensor composed of a polymer electret membrane is designed using FEM. This sensor shows a good response in the low frequency range
    01/1996;
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    ABSTRACT: 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 β peak near 10[°C] into two peaks of β<sub>1</sub>(10°C) and β<sub>2</sub>(20°C) and increased magnitude of TSC spectra between α and β peak, which would be closely related with the process of thermal oxidation and separation of hardener's 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
    Electrets, 1994. (ISE 8), 8th International Symposium on; 10/1994
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    ABSTRACT: In order to estimate space charging process in the corona charging apparatus which has been used to make polymer electret cells, the electrical properties of 30[μm] thickness polypropylene films are obtained from TSC measurement after corona charging between copper knife electrode and aluminium cylinder electrode with the voltages of -8, -7, -6, and -5 [kV], respectively. The electrostatic contour and the electric field vector are calculated using finite element method with the electrical properties obtained from TSC spectra analysis. The edge effect around the edge of the knife electrode affects the electrostatic contour on the surface of the specimen and the electric field concentration inside the polymer. As a result, the uneven charging state in the electret cell due to the design error is calculated, and the optimal design of corona charging apparatus appropriate to various specimens is made practicable
    Properties and Applications of Dielectric Materials, 1994., Proceedings of the 4th International Conference on; 08/1994
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    ABSTRACT: The dielectric properties and the microstructure of (Sr.Ca)TiO<sub>3</sub>-based boundary layer (BL) capacitor materials were investigated. (Sr.Ca)TiO<sub>3</sub>-based compositions were fired at 1350°C in a N<sub>2</sub> atmosphere to get the base semiconducting ceramics. The metal oxides of CuO were printed on the semiconducting ceramics and fired at 1200°C for the subsequent grain boundary diffusion. The grain size of the semiconducting ceramics was 20~40 μm and the apparent permittivity of the resulting material varied between 2×10<sup>4</sup> and 3×10<sup>4</sup>
    Properties and Applications of Dielectric Materials, 1994., Proceedings of the 4th International Conference on; 08/1994
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    ABSTRACT: Thermally stimulated current spectroscopy was applied to investigate the influence of structural change and interfacial state on the electrical properties of epoxy composites cured with acid-anhydride. A overabundantly added hardener separated a β peak near 10[°C] into two peaks of β<sub>1</sub>(10°C) and β<sub>2</sub>(20°C). Filling with an inorganic filler increased the magnitude of TSC between the α and β peaks and lowered T <sub>g</sub>. Silan coupling agents on the filler improved interfacial deformation, decreasing TSC
    Properties and Applications of Dielectric Materials, 1994., Proceedings of the 4th International Conference on; 08/1994
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    ABSTRACT: The authors present the results of an investigation of the behavior of carriers for nonirradiated and irradiated OPP film by 1 and 5 Mrad. The investigation of electrical properties was performed using the thermally stimulated current (TSC) technique. The first peak of the TSC spectrum appeared due to the carbonyl group `C=0' in the amorphous phase of the polymer. The second peak is attributed to the process of detrapping of ions from the thermal excitation in the crystalline phase. The dipole moment obtained from the first peak was in the range of 0.7-1.7 Debye. The hopping length of the carriers at the condition of the second peak is several tens of Å. The activation energy obtained from the first peak was 0.2-0.7 eV and that obtained from the second peak was 0.3-1.1 eV
    Properties and Applications of Dielectric Materials, 1991., Proceedings of the 3rd International Conference on; 08/1991
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    ABSTRACT: To contribute more information on the conduction mechanism in ZnO varistors, a study of ZnO varistors with TiO<sub>2</sub> additives was conducted. The properties of ZnO varistors are found to depend on the grain structure of ZnO. The mechanism of nonlinear conduction of ZnO may be explained on the basis of the grain structure. The TSC (thermally stimulated current) spectrum was measured to study the trapped carriers in ZnO varistors. Four peaks of TSC may be distinguished on the spectrum of TSC. The origin of the first, second, and third TSC peaks was attributed to the detrapping of the trapped electrons in the shallow trap, donor level, and deep trap, respectively. The origin of the fourth peak was attributed to the process of depolarization of the donor ions in the depleted layer. Adding TiO<sub>2</sub> to ZnO varistors caused the leakage current to increase
    Properties and Applications of Dielectric Materials, 1991., Proceedings of the 3rd International Conference on; 08/1991
  • Joon-Ung Lee, Jin-Woong Hong, S. Grzybowski
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    ABSTRACT: First Page of the Article
    Electrical Insulation and Dielectric Phenomena, 1991. CEIDP. 1991 Annual Report. Conference on; 02/1991
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    ABSTRACT: Oscillation phenomena accompanied by negative resistance were studied experimentally in oriented polypropylene (OPP). The negative resistance and current oscillation in OPP were measured according to changes in temperature and field. It is shown that the negative resistance can be interpreted based on J.F. Gibbons' (1967) formulation. The mobility can be calculated from the oscillation current
    Electrical Insulation and Dielectric Phenomena, 1990. Annual Report., Conference on; 11/1990

Publication Stats

3 Citations

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Institutions

  • 1990–2007
    • Kwangwoon University
      • Department of Electrical Engineering
      Sŏul, Seoul, South Korea
  • 1997
    • Daebul University
      South Korea
  • 1991
    • University of Seoul
      • School of Electrical and Computer Engineering
      Seoul, Seoul, South Korea