Tae Wan Kim

Improvement of efficiency in organic light-emitting diodes using PVK and CuPc buffer layer

Abstract We have seen the effects of buffer layer in organic light-emitting diodes using poly(vinylcarbazole) (PVK) and copper phthalocyanine (CuPc) in a device structure of ITO/buffer/TPD/Alq3/Al. Polymer PVK buffer layer was made using spin casting method and CuPc layer was made using thermal evaporation. Current–voltage characteristics, luminance–voltage characteristics and external quantum efficiency were measured at room temperature with a thickness variation of PVK and CuPc layer. We have obtained an improvement of the external quantum efficiency by a factor of four when the PVK buffer layer is used. And we have also obtained an improvement of efficiency by a factor of two when the CuPc buffer layer is used. Our analysis shows that the PVK and CuPc layer are good candidates for hole-injection buffer layer. A reason for the improvement of the efficiency is that the PVK layer functions as a hole blocking and the CuPc layer as a hole injection supporter.

Synthesis and luminescent properties of bithiazole and dithiafulvene derivatives

New luminescent materials, 4,4-Bis(1-naphthyl)-2,2-bithiazole(BNBT), 2-Benzylidene-4-phenyl-1,3-dithiole(BPDT) and Poly (phenylene-dithiafulvene)(PPDF) were synthesized. The BNBT and BPDT give blue photoluminescent spectra at 437nm and 445nm, respectively and show typical rectifying diode characteristics in ITO/ BNBT or Dithiafulvene Derivatives /Al device.

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...

I–V and π−A characteristics of ultrathin Langmuir-Blodgett films

Abstract The deposition conditions and electrical properties of C 22 -quinolium(TCNQ) Langmuir-Blodgett (LB) films have been studied. The optimum conditions of film deposition were examined by a study of surface pressure-area (π− A ) isotherms with variations of pH, temperature, barrier moving speed, dipping speed, spreading amount of solution etc. The LB films were deposited on a slide glass substrate and tested with several tools. The π− A isotherm shows that there might be two phase transitions near surface pressures of 30 and 45 mN m −1 . It is thought that the phase just below 30 mN m −1 is a liquid phase and that above 30 mN m −1 is a solid phase. The LB films were made under the following conditions; surface pressure, 45 mN m −1 ; 25°C; pH 5.6; spreading amount 60–70 A 2 per molecule; and dipping speed, 4 mm min −1 . The electrical conductivities and conduction mechanism were investigated by measuring current-voltage ( I–V ) characteristics along the vertical and horizontal direction of the film. We observed an anisotropy in conductivity between the vertical and horizontal direction of the film. The conductivity along the film surface direction is about 10 −7 S cm −1 and that of the perpendicular direction is about 10 −14 S cm −1 at room temperature. We saw non-ohmic conducting behaviour in the presence of high electric field along the vertical direction. When the electric field is high in this direction, a breakdown occurs in the I–V characteristics. We analysed the I–V characteristics in detail near the breakdown region by changing the type of applied voltage (step or pulse) as well as the temperature.

Layer Thickness-dependent Electrical and Optical Properties of Bottom- and Top-emission Organic Light-emitting Diodes

We have studied organic layer-thickness dependent electrical and optical properties of bottom- and top-emission devices. Bottom-emission device was made in a structure of ITO(170 nm)/TPD(x nm)/(y nm)/LiF(0.5 nm)/Al(100 nm), and a top-emission device in a structure of glass/Al(100 nm)/TPD(x nm)/(y nm)/LiF(0.5 nm)/Al(25 nm). A hole-transport layer of TPD (N,N`-diphenyl-N,N`-di(m-tolyl)-benzidine) was thermally deposited in a range of 35 nm and 65 nm, and an emissive layer of (tris-(8-hydroxyquinoline) aluminum) was successively deposited in a range of 50 nm and 100 nm. Thickness ratio between the hole-transport layer and the emissive layer was maintained to be 2:3, and a whole layer thickness was made to be in a range of 85 and 165 nm. From the current density-luminance-voltage characteristics of the bottom-emission devices, a proper thickness of the organic layer (55 nm thick TPD and 85 nm thick layer) was able to be determined. From the view-angle dependent emission spectrum of the bottom-emission device, the peak wavelength of the spectrum does not shift as the view angle increases. However, for the top-emission device, there is a blue shift in peak wavelength as the view angle increases when the total layer thickness is thicker than 140 nm. This blue shift is thought to be due to a microcavity effect in organic light-emitting diodes.

Effect of Deposition Rate of Organic Layer on Electrical and Optical Characteristics of OLEDs

We investigated the effects of deposition rate on the electrical characteristics and optical characteristics of organic light-emitting diodes(OLEDs) in the ITO(indium-tin-oxide)/N,N′-diphenyl-N,N′-bis(3-methyphenyl)-1,1′-biphenyl-4,4′-diamine(TPD)/tris(8-hydroxyquinoline) aluminum (Alq3)/Al device. We measured current density, luminous flux and luminance characteristics of devices by varying deposition rates of TPD and Alq3. It has been found that optimal deposition rate of TPD and Alq3 was 1.5 A˚/s. AFM measurement shows that surface roughness of the deposited film was the lowest when the deposition rate was 1.5 A˚/s.

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 .

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.

π-A isotherms and electrical properties of polyamic acid alkylamine salts (PAAS) Langmuir-Blodgett films

Abstract Deposition conditions, surface morphology, and electrical properties of polyamic acid alkylamine salts (PAAS) Langmuir-Blodgett (LB) films have been investigated through a study of surface pressure-area π-A isotherms, AFM (atomic force microscopy), and current-voltage characteristics. To obtain the optimum conditions of film deposition, the π-A isotherms were examined by varying temperature, barrier moving speed, dipping speed, spreading amount of solution etc. The Z-type LB films were made at the surface pressure of 5 mN m−1 and 25 mN m−1 for the AFM study; the former surface pressure forms the gas phase and the latter one forms the solid phase. The LB films made in the gas phase show domains with a size of about 200 A diameter and 70 A height. However, the LB films made in the solid phase show a very smooth surface with 2 A surface roughness. In the current-voltage characteristics measured along the perpendicular direction of the films, ohmic conduction has been observed below ≈ 105 V cm−1 and the calculated electrical conductivity is about 10−13 S cm−1. Nonohmic conduction has been observed above ≈ 105 V cm−1 and the conduction mechanism can be explained by the Schottky effect.

Thermal annealing effects of C22-Quinolium(TCNQ) Langmuir-Blodgett films☆

Abstract Thermal annealing effects of C 22 -Quinolium(TCNQ) LB films were investigated through a study of frequency-dependent dielectric properties. Thermal annealing was performed in the range of 20∼240 °C. Thickness measurement by ellipsometry shows that there is a thickness drop near 100 °C, which is supposed to be due to a softening of the alkyl chains. The film thickness above 120 °C becomes ∼ 20 % of the room-temperature value. Frequency-dependent dielectric properties show that there are two characteristics dispersions; one is a dispersion occuring ∼ 1 MHz coming from the electric polarization of the molecules. The other one is an interfacial polarization effect below ∼ 1 kHz when the annealing temperature is above 80 °C.