Ho-Sik Lee

Synthesis and electroluminescent properties of bis(3-N-ethylcarbazolyl)cyanoterephthalidene

A new blue electroluminescent material analog to poly(3,6-N-2-ethylhexylcarbazolyl cyanotere-phthalidene) (PECCP) was synthesized by a facile reaction of 9-ethyl–3-carbazole carboxy aldehyde with 1,4-phenylenediacetonitrile. The resulting bis(3-N-ethylcarbazolyl)cyanoterephthalidene (BECCP) having cyano (electron acceptor part) and carbazole moiety (electron donor part) was characterized by NMR, IR, UV and was vacuum-deposited into thin film of smooth surface. BECCP shows blue PL and EL spectra at 480 nm corresponding to 2.58 eV and typical rectifying diode characteristics in ITO/BECCP/Al devices in both forward and reverse bias. The turn on voltage are +8 V and 4 V when the BECCP layer are 600 and 300 A thick, respectively.

Electrical properties of porphyrin-based switching devices

Abstract Molecular switching devices were reported utilizing Langmuir–Blodgett (LB) monolayer films containing the 5,10,15,20-Tetrakis-Octadecyloxymethylphenyl-Porphyrin-Zn(II) as a redox-active component. From current–voltage (I–V) characteristics, it was found that the devices exhibit outstanding switching diode and tunneling diode behavior at room temperature, which seems to be due to the asymmetric device structure. These electrical properties of the devices may be applicable to active components for the memory and/or logic circuits in the future.

Temperature dependent electrical properties in ITO/TPD/Alq3/Al organic light-emitting diodes

Temperature dependence of current-voltage-luminance characteristics in organic light-emitting diodes are studied in a device structure of ITO/TPD/Alq 3 /Al to understand an electrical conduction mechanism. The current-voltage-luminance characteristics are measured in the temperature range of 8K∼300K, and analyzed them using a hopping model with exponential trap distribution and Fowler-Nordheim tunneling. At low temperatures below 150K, the Fowler-Nordheim tunneling conduction mechanism is dominant. And we have obtained a zero field barrier height to be about 0.6∼0.8eV.

Study of Langmuir-Blodgett films of a conjugated poly(3,6-N-2-ethylhexyl carbazolyl cyanotere-phthalidene)

Green light-emitting material, poly(3,6-N-2-ethylhexyl carbazolyl cyanotere-phthalidene)(PECCP), was synthesized which has an electron-donor and electron-acceptor moiety in a repeated unit of polymer. We have made PECCP Langmuir-Blodgett(LB) films and they exhibit fluorescence. Organic light-emitting diodes with a single-layer structure (ITO/emitting layer/Al) have been fabricated with the PECCP LB films as an emitting material. We have observed green photoluminescent and electroluminescent spectra at λ max = 536nm corresponding to 2.34eV.

Electrical properties of C22-quinolinium(TCNQ) and polyamic acid alkylamine salts Langmuir–Blodgett films

Abstract Electrical properties of C22-quinolinium(TCNQ) and polyamic acid alkylamine salts Langmuir–Blodgett (LB) films have been investigated through a study of surface pressure (π)–area (A) isotherms, current–voltage(I–V) characteristics, frequency-dependent dielectric responses, and thermally-stimulated currents (TSC). Through-plane I–V characteristics of C22-quinolinium(TCNQ) LB films show an anomalous behavior of breakdown near the electric-field strength of 106 V/cm. Frequency-dependent dielectric constants of the thermally annealed C22-quinolinium(TCNQ) LB films show that there are two characteristic dispersions in a frequency range of 10–13 Hz; one is a dispersion occurring near 1 MHz coming from the orientational polarization of the molecules and the other is probably due to an interfacial polarization below 1 kHz when the annealing temperature is above 80°C. The annealing temperature-dependent dielectric constants of the C22-quinolinium(TCNQ) LB films show that there are two maximum; one at near 80°C and the other at near 180°C. The first peak seems to be related to a disorder of alkyl chains. In the I–V characteristics of Z-type polyamic acid alkylamine salts (PAAS) LB films measured along the through-plane at room temperature, ohmic conduction has been observed below around 105 V/cm, which is similar to that of C22-quinolinium(TCNQ) LB films. Non-ohmic conduction has been observed above around 105 V/cm and the conduction mechanism can be explained by Schottky effect. Frequency-dependent dielectric responses of the PAAS LB film show that there is a dispersion at 104 Hz when the annealing temperature reaches about 100°C. Measurements of thermally-stimulated currents of those LB films were performed from room temperature to about 250°C and the temperature was increased at a rate of 0.2°C/s linearly. TSC peaks are observed at about 70°C in the arachidic acid LB films, and at about 70 and 160°C in the PAAS LB films. They indicate that the peak at 70°C is resulted from the alkyl group, and the other peak at 160°C is probably due to C O group in PAAS molecules.

Current-Voltage Characteristics of Organic Light-Emitting Diodes Depending on the Application of Forward-Reverse Bias Voltage

We have investigated current-voltage(I-V) characteristics of organic light-emitting diodes based on TPD/Alq 3 organics depending on the application of forward-reverse bias voltage. Luminance-voltage characteristics and luminous efficiency were measured at the same time when the I-V characteristics were measured. We have observed that the I-V characteristics were measured. We have observed that the I-V characteristics shows a current maxima at low voltage, which is not related to the emission from Alq 3 .

Synthesis and properties of organic light-emitting diodes using new emissive materials

Abstract We have synthesized the new blue electroluminescent material, Bis(3-N-ethylcarbazolyl)cyanoisophthalidene(BECCIP), and characterized its properties by UV/visible absorption, photoluminescent(PL) and electroluminecent(EL) spectrum. This material is well vacuum-deposited for thin film and has clear surfaced thin film property. The BECCIP shows blue PL and EL spectra at around Δmax =485nm.

Properties of Photovoltaic Cell using ZnPc/C60 Double Layer Devices

It has been a long time since organic solar cells were expected as a low-cost energy-conversion device. Although practical use of them has not been achieved, technological progress continues. Morphology of the materials, organic/inorganic interface, metal cathodes, molecular packing and structural properties of the donor and acceptor layers are essential for photovoltaic response. We have fabricated solar cell devices based on zinc-phthalocyanine(ZnPc) as donor(D) and fullerene as electron acceptor(A) with doped charge transport layers, and BCP and as an exciton blocking layer(EBL). We have measured the photovoltaic characteristics of the solar cell devices using the Xe lamp as a light source. We were use of layer leads to external power conversion efficiency was at illumination intensity . Also we confirmed the optimum thickness ratio of the DA hetero-junction is about 1:2.

Characteristics of organic light-emitting diodes using PECCP Langmuir-Blodgett film as an emissive layer

Abstract We have synthesized green light-emitting material, poly(3,6-N−2-ethylhexyl carbazolyl cyanotere-phthalidene)(PECCP), which has an electron-donor and electron-acceptor moiety in a repeated unit of polymer. Organic light-emitting diodes were made with PECCP Langmuir-Blodgett(LB) films in a sandwich structure between indium-tin-oxide(ITO) and aluminum electrodes. Z-type LB films were manufactured using a Kuhn type to use for an emissive layer. We have observed green photoluminescent and electroluminescent spectra at λmax = 536 nm corresponding to 2.31 eV.

Characteristics of Organic Electroluminescent Devices using Alq3 and TPD Materials with Buffer Layers

Abstract Organic electroluminescent devices were fabricated using Alq3 (8-hydroxyquinolinate aluminum) and TPD (N,N′-diphenyl-N,N′-bis (3-methylphenyl)-[1-1′-biphenyl]-4,4′-diamine). We investigated current-voltage characteristics of the EL devices having buffer layer of PEI (Poly ether imide) blended with TPD, and conducting polymer PEDT (3,4-pyrazino-3′,4′-ethylenedithio-2,2′,5,5′-tetrathiafulvalenium). We measured UV/visible absorption spectrum. and PL (Photoluminescence) spectrum.