Hiroyoshi Onnagawa

High electron mobility in bathophenanthroline

We have measured electron mobility in vacuum-deposited films of 4,7-diphenyl-1,10phenanthroline (bathophenanthroline, or BPhen) using a time-of-flight technique. Electron transport was highly dispersive for BPhen with a dispersion parameter of a value 0.30. The electron mobility in excess of 10−4 cm2/V s has been observed at electric fields of the order of 105 V/cm with weakly dependent on the electric field. The characteristic energy of the distribution is obtained a value 0.09 eV. It is directly confirmed that the BPhen has superior electron-transport capability.

Carrier transport properties of organic materials for EL device operation

We have measured drift mobilities in vacuum deposited films of tris(8-quinolinolato)aluminum (Alq3), a triphenylamine derivative (TPD) and a naphtyl-substituted benzidine derivative (α-NPD) using a time-of-flight (TOF) technique. The hole mobilities of TPD and α-NPD are about two orders of magnitude higher than the electron mobility of Alq3. This implies that the current density versus applied voltage characteristics is dominated by the electron mobility of Alq3 rather than the hole mobility of TPD or α-NPD in double-layered devices ITO/TPD or α-NPD (50 nm)/Alq3 (50 nm)/MgAg.

Nondoped-type white organic electroluminescent devices utilizing complementary color and exciton diffusion

We fabricated nondoped white organic electroluminescent devices using vacuum-deposited thin films of blue-emitting 4,4′-bis[N-1-napthyl-N-phenyl-amino]biphenyl (α-NPD) and orange-emitting 4-(dicyanomethylene)-2-metyl-6-(p-dimethyl aminostyryl)-4H-pyran (DCM), a hole-blocking layer of 2-(4-biphenyl)-5-(p-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD) and electron-transporting tris(8-quinolinolato) aluminum (III). Excitons formed at the α-NPD/tBu-PBD interface sequentially transfer their energy to α-NPD via the Forster mechanism. The exciton is captured by an ultrathin DCM layer located within the pure α-NPD layer. The position of the DCM determines the device spectrum, and enables a white emission to be achieved. The spectrum is not sensitive to the voltage applied, and the devices show maximum luminance of about 1000 cd/m2.

Optical and Electrical Properties of CuInS2 Thin Films by Spray Pyrolysis

The optical, electrical and structural properties of CuInS2 thin films prepared by spray pyrolysis have been studied. The polycrystalline films produced by spraying a solution of Cu (CH3COO)2H2O, InCl3 and (NH2)2CS with the atomic ratio Cu:In:S=1:1:2.8~1:1:3.2 and annealing at 500°C for 2 hours were nearly stoichiometric, and the depth profiles of the atomic composition were flat except near the surface. The optical band gap at room temperature was 1.43 eV, and the polarity of the thermoelectromotive force showed p-type conduction at a sulfur concentration ratio to Cu[S]≥2.8. The activation energy of conductivity was 0.3 eV.

Organic Electroluminescent Devices Using a Mixed Single Layer

Electroluminescent devices with mixed single-layer organic materials that consist of hole transport meterial, electron transport material and dopant material are studied. High luminance could be obtained by optimizing the weight ratios of hole and electron transport materials.

Organic heterojunction phototransistor

We propose an organic heterojunction phototransistor stacked with organic layers of electron and hole transport materials. The materials used in this study are perylene derivative (td-PTC) as the ultrathin base layer and triphenylamine derivative (TPD) as the emitter and collector layers. The current versus voltage characteristics as a function of incident optical intensity are typical of a phototransistor with current saturation. The bipolar transistor operation is confirmed by a comparison with the diode structure of the TPD/td-PTC device and because of the base thickness dependence of the current. External quantum efficiency is 2.9% under blue light-emitting diode illumination. In the current response, the initial displacement current is superimposed. Obtained response times are 600 μs for both transient turn-on and turn-off currents.

Electrical properties of organic electroluminescent devices with aluminium alloy cathode

Abstract Organic electroluminescent devices with an aluminium alloy cathode, consisting of aluminium-lithium, aluminium-calcium or aluminium-magnesium, have been examined. From the temperature dependence of the current-voltage characteristics in metal/aluminium complex (Alq 3 )/metal structure devices, the carrier-injection mechanism in the higher current range is ascribed to thermionic (Schottky) emission and the carrier-conduction mechanism of the Alq 3 layer in the lower current range is observed to be three-dimensional variable-range hopping.

Dual drive and emission panel

We propose double-faced organic light-emitting diodes (OLEDs) of a dual drive and emission (DDE) panel. Two OLEDs with two transparent electrodes for data signals and an intermediate reflective electrode for the common scan signal are independently operated. The typical structure of a stacking device is transparent electrode (1)/organic layer (1)/interlayer electrode/organic layer (2)/transparent electrode (2) on a glass substrate. Symmetric bright emission could be obtained using AlNd as the interlayer electrode and MoO3 as the hole injection layer for the upper OLED. The proposed device is useful for emissive double-faced panels displaying different images.

Alignment of Nematic Liquid Crystal Molecules Using Nanometer-Sized Ultrafine Patterns By Electron Beam Exposure Method

We have investigated the alignment of nematic liquid crystal (NLC) molecules using nanometer-sized ultrafine groove, tilted, and dot patterns fabricated by the electron beam exposure method. In order to improve the wetting condition of NLCs, we sputtered silicon dioxide on a substrate after the fabrication of ultrafine patterns. For a homogeneous alignment cell with an ultrafine groove pattern, an azimuthal anchoring energy of 2.84×10-4 J/m2 was obtained. We also confirmed tilted and homeotropic alignments using tilted and dot patterns, respectively.

Relationship between Molecular Structure and Temperature Dependence of Threshold Voltage in Fluorinated Liquid Crystals

We evaluate the temperature dependence of threshold voltage and physical constants (anisotropy of dielectric constants and elastic constants) by using fluorinated phenylbicyclohexane liquid crystal compounds. It is found that the positions of fluorine substituents influence the dielectric constant and its temperature dependence. A systematic relationship between the fluorine substituents and physical constants was obtained.