Shigeki Naka

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.

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.

Enhanced carrier injection in pentacene thin-film transistors by inserting a MoO3-doped pentacene layer

We report on the enhanced carrier injection in pentacene thin-film transistors with a thin MoO3-doped pentacene layer between pentacene semiconductor and the source-drain electrodes. Device performance including drain current, field effect mobility, and threshed voltage are improved by employing a MoO3-doped pentacene thin layer. The barrier height at the Au/pentacene interface is lowered from 0.12 to 0.05 eV after inserting a MoO3-doped pentacene thin layer between them. The reduced barrier height is attributed to the formation of a good contact between MoO3-doped pentacene and Au owing to smoothed surface morphology of pentancene and suitable band bending by MoO3 doping.

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.

Bias and temperature dependent charge transport in solution-processed small molecular mixed single layer organic light emitting devices

The authors investigate bias and temperature dependent current density-voltage (J-V) characteristics in solution-processed small molecular mixed single layer organic light emitting devices using Au/MoO3 as hole and Al as electron injection electrode. Hole and electron injections are primarily ascribed to the Schottky thermionic emission mechanism. However, at high field of >9×105 V cm−1, hole transport is found to be bulk trapped corresponding to space charge limited current with an exponential distribution of traps. The bulk trap density, about 1018 cm−3 in mixed organic layer, is evaluated by the differential method.

Direct Comparison of Solution- and Vacuum-Processed Small Molecular Organic Light-Emitting Devices with a Mixed Single Layer

It will be interesting and valuable information can be achieved if a direct comparison between organic light emitting devices (OLEDs) fabricated by vacuum evaporated method (vac) and solution-based manufacturing processes (sol) was realized. Small molecular OLEDs with a mixed organic layer structure (MOLOLEDs) make it possible for direct comparison between devices with the same materials but fabricated by the two processing methods. This article shows a direct comparison of the luminescence characteristics, charge conduction, and device physics between MOLOLEDs fabricated by vac- and sol-processing techniques. It gives an elementary explain how the organic/metal interfaces influence the charge conduction and device performance.

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.