Jong-Wook Park

Organic light emitting diodes fabricated with single wall carbon nanotubes dispersed in a hole conducting buffer : the role of carbon nanotubes in a hole conducting polymer

Abstract In order to investigate the role of single wall carbon nanotubes (SWNTs) in a hole conducting polymer, organic light emitting diodes (OLEDs) were fabricated with a conjugated emissive copolymer, poly(3,6- N -2-ethylhexyl carbazolyl cyanoterephthalidence) (PECCP) and SWNTs dispersed in a hole conducting buffer polymer, polyethylene dioxythiophene (PEDOT). Devices made with SWNTs dispersed in PEDOT and devices made without SWNTs in the PEDOT emit green light at 2.37xa0eV as expected for PECCP. However, we observed that the device made with SWNTs in the buffer layer shows a significant decrease in the electroluminescence (EL) as compared to that of the device without the SWNTs. In contrast, the photoluminescence (PL) from the same organic layer combination, excited from the PECCP side and measured through the PEDOT and the indium tin oxide glass, shows very little difference between the films with and without the SWNTs. The current–voltage ( I – V ) characteristics of OLEDs with SWNTs show a lower I – V power dependence ( I – V 2 ) near 1–2xa0V than that of the device without SWNTs which has a power dependence of I – V 5 . The EL and the I – V data together with the PL suggest an electronic interaction between the SWNTs and the host polymeric material, PEDOT. We propose that this electronic interaction originates from the hole trapping nature of SWNTs in a hole conducting polymer.

Blue organic light-emitting diodes with carbazole-based small molecules

Abstract We synthesized bis(3-N-ethylcarbazolyl) terephthalidene (BECP) and bis(3-N-ethylcarbazolyl) cyano-terephthalidene and characterized electroluminescence properties of these materials. Our device shows a strong blue emission at 472 nm with a luminance efficiency of 0.9 lm/W at a voltage, a current density, and a brightness of 8 V, 5.7 mA/cm2 and 130 cd/m2, respectively and shows typical rectifying diode characteristics in ITO/CuPc/TPD/BECP/Bu-PBD or Alq3/LiF/Al device.

AC electrical properties of conjugated polymers and theoretical high-frequency behavior of multilayer films

Abstract AC electrical properties of various conjugated polymers such as emeraldine base form of polyaniline, polyacetylene derivatives, and poly(1,4-phenylenevinylene) and its derivative are studied. Dielectric constant and conductivity are measured in the frequency range from 10xa0Hz to 2xa0MHz and in the temperature range from 78 to 300xa0K. Frequency dependence of ac dielectric constant follows Debye or Cole–Cole dielectric relaxation equation, and temperature dependence of relaxation time τ follows Arrhenius equation τ=τ 0 exp (E a /k B T) . Frequency dependence of ac conductivity shows a constant behavior in low-frequency regime (≤10xa0kHz) and a power-law behavior ( σ ∼ ω s ) in high-frequency regime (≥100xa0kHz). For the applications of multilayer films of conducting polymers such as electromagnetic interference (EMI) shielding or optical filter, we theoretically calculate the reflectance, absorbance, and EMI shielding efficiency in high-frequency range (≥1xa0GHz) based on plane wave theory.

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.

SYNTHESIS AND ELECTRO-OPTICAL PROPERTIES OF POLY(2-ETHYNYLPYRIDINIUMTOSYLATE) HAVING PROPAGYL SIDE CHAIN

Novel conjugated ionic polymer was prepared by the polymerization of 2-ethynylpyridine with propargyl tosylate in refluxing methyl alcohol. The polymerization proceeded well in homogeneous manner to give a relatively high yield of polymer. The resulting poly(2-ethynylpyridinium tosylate) having propargyl side chain [poly(EPT-P)] were hygroscopic and soluble in water, methyl alcohol, DMF, and DMSO. The inherent viscosities of the polymers were in the range of 0.08-0.29dL/g. Instrumental analyses using NMR, IR, and UV-visible spectroscopies and elemental analyses indicated that the resulting poly(EPT-P) have a conjugated ionic polymer backbone carrying N-propargyl-2-pyridinium tosylate. Thermal and electro-optical properties of the polymers were also studied.

Characteristics of organic electroluminescent devices using polypyrrole conducting layer

Abstract We observed EL properties based on PVK, poly(3,6-N-2-ethylhexyl carbazolyl cyanoterephthalidene) (PECCP) and Alq3 emitting material devices with doped PPy conducting layer (>5xa0S/cm) and undoped polyacetylene derivative layer (PEPB, 10−10–10−11xa0S/cm) on top of rough ITO (surface resistance: less than 7xa0Ω/□). PPy conducting layer improved an EL efficiency of ITO/TPD/Alq3/Al device, which has a rough ITO surface of more than 1000xa0A peak to peak level. In PVK, PECCP EML device, when PPy is layered on top of ITO, the turn-on voltage is lowered from 13 to 10xa0V and from 15 to 9xa0V, respectively. The conductivity of undoped PEPB film is too low to improve EL property though it has good smoothness.

Synthesis of Poly(2-ethynylpyridinum bromide) Containing Propargyl Side Chain and Its Photoluminescence Properties

Abstract Water-soluble conjugated ionic polymer was prepared by the polymerization of 2-ethynylpyridine with propargyl bromide in refluxing methanol. The polymerization proceeded well in homogeneous manner to give a high polymer yield (95%). The resulting poly(2-ethynylpyridinium bromide) having propargyl side chain was mostly brown powder and soluble in the polar solvents such as methanol, water, DMF, and DMSO. The inherent viscosities of the polymer were in the range of 0.10–0.21dL/g. This polymer exhibited good film surface by microscope, and characteristic UV-visible absorption band at 2.3 eV and red PL spectrum at 760nm corresponding to the photon energy of 1.63 eV.

Synthesis and luminescent properties of bithiazole derivatives

Abstract New blue photoluminescent materials 4,4 ′ -bis(1-naphthyl)-2,2 ′ -bithiazole (BNBT) and 4,4 ′ -bis(4-biphenyl)-2,2 ′ -bithiazole (BPBT) were synthesized by the facile reaction of 2-bromoacetoaromatic compound with dithioxamide. The resulting BNBT and BPBT having thiazole moiety were characterized by NMR, infrared (IR), UV/visible absorption and are well vacuum deposited for thin films. The BNBT and BPBT show blue photoluminescent (PL) spectra at 437 and 432 nm, respectively, and typical rectifying diode characteristics in ITO/BNBT or BPBT/Al device. The BNBT and BPBT are thermally stable up to 330°C and 360°C, respectively, under N 2 .

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 .

Characteristics of organic electroluminescent devices using polypyrrole conducting layer and undoped conjugated polymer layer

Abstract We observed EL properties based on poly(3,6-N-2-ethylhexyl carbazolyl cyanoterephthalidene) (PECCP) and PECCP/Alq3 emitting material devices with doped PPy conducting layer (>5 S/cm) on top of rough ITO (surface resistance: less than 7 Ω/□). The ITO/PECCP/Alq3/Al device shows relatively high luminance efficiency, (0.17 lm/W), because of the PECCP layer. The PPy conducting layer is working on improving I–V characteristics of PECCP/Alq3 small molecule EML and PECCP EML system on ITO which has a rough surface of more than 1000 A peak to peak level. In the PECCP EML device, when PPy is layered on top of ITO, the turn-on voltage is lowered from 15 to 9 V.