Koji Noda

Metal oxides as a hole-injecting layer for an organic electroluminescent device

We demonstrate that by using thin films of metal oxides, such as vanadium oxide (VOx), molybdenum oxide (MoOx) and ruthenium oxide (RuOx), as a hole-injecting layer for an organic electroluminescent (EL) device consisting of N,N-diphenyl-N,N-bis(3-methylphenyl1)1,1-biphenyl-4,4 diamine (TPD) and tris-(8-quinolinolato) aluminium (Alq), the EL device performance can be significantly improved. The `operating voltage of the device is reduced with respect to a device with a well known indium - tin-oxide (ITO) electrode for hole injection. We consider that the improvement of the operating voltage is attributable to the lower energy barrier for hole injection at the metal oxide/TPD interface.

Thermal stability in oligomeric triphenylamine/tris(8-quinolinolato) aluminum electroluminescent devices

Thermal stability of the electroluminescent (EL) devices using various hole-transporting materials based on triphenylamine, and a typical emitting material, tris(8-quinolinolato) aluminum has been systematically studied. The thermal stability of the EL devices is clearly seen to depend on the glass transition temperature (Tg) of the hole-transporting material. The highest thermal stability up to 155u2009°C is obtained in the device using the pentamer of triphenylamine. It has been found that the linear linkage of triphenylamine is useful to attain high Tg rather than the branch linkage.

New 9-fluorene-type trispirocyclic compounds for thermally stable hole transport materials in OLEDs

A novel trispirocyclic hydrocarbon having three 9-fluorene moieties around the core of truxene (5) was prepared readily via coupling of truxenone with 2-bromobiphenyl; 5 was a high melting (>500 °C) solid. For the application of 5 to an effective hole transport material (HTM) in the OLED, a triphenylamine derivative carrying six diphenylamino groups at the 2- and 7-positions of each 9-fluorene moiety (6) was designed in order to get high thermal stability as an improved material of the TPD type HTM. The synthesis of 6 was easily achieved using 4,4′-bis(diphenylamino)-2-bromobiphenyl (9). The trispirocyclic 6 was found to show a glass transition temperature as high as 170 °C. It effects the formation of its stable cation radical upon electrooxidation in solution, and amorphous thin films in solid. A multi-layered EL device for 6 as an HTM using Alq3 as an electron transporting emitter showed good EL characteristics such as the maximum luminance of 37u2006000 cd m−2 at 14 V. Thus, the hexakis(diphenylamino)substituted trispirocycle 6 n (TX-F6S) can be used as an efficient and thermally stable HTM in OLEDs.

Organic light-emitting diodes using novel metal–chelate complexes

Abstract Electroluminescent (EL) properties of novel metal–chelate complexes have been investigated in the organic light-emitting diodes (OLEDs) comprised of the hole transport layer (HTL) and emitting layer (EML). The metal–chelate complexes have either polycyclic aromatic or fused ring ligands, which coordinate to zinc or aluminum. Various EL emission colors over the range of blue to yellow were obtained. The OLEDs using the complexes with polycyclic aromatic ligands showed desirable blue emission with a quantum efficiency of 1.5–1.7% at a luminance of 300 cd/m 2 and CIE color coordinates of around (0.17, 0.16). By perylene-doping into the blue-emitting complex, the improvement of EL efficiency was observed. On the other hand, the complex with fused ring ligands gave a quantum efficiency of 0.8% at yellow emission.

Organic electroluminescent devices using alkaline-earth fluorides as an electron injection layer

Abstract We investigated the properties of organic electroluminescent (EL) devices using alkaline-earth fluorides as an electron injection layer, and found that the alkaline-earth fluorides are desirable materials to reduce the turn-on voltage of the EL devices with Al cathode. In the devices using the fluorides as the electron injection layer, we found that the turn-on voltage can be associated with electronegativity of metal ion of the fluoride; the fluoride with a low electronegativity metal ion results in lowering the barrier height at the organic/Al interface and enhanced the electron injection.

STRONGLY DIRECTED SINGLE MODE EMISSION FROM ORGANIC ELECTROLUMINESCENT DIODE WITH A MICROCAVITY

The strongly directed spontaneous emission along the optical axis of an organic electroluminescent (EL) diode with a typical emitting material, tris(8‐quinolinolato) aluminum, is realized with a planner microcavity structure. The structure of the microcavity EL diode is designed to have a resonance condition in which the total optical length is 3/2λ and the resonance wavelength λ is located at shorter wavelength side of a natural emission spectrum of a noncavity EL diode.

Highly efficient blue-green emission from organic light-emitting diodes using dibenzochrysene derivatives

We have fabricated highly efficient organic light-emitting diodes (OLEDs) using new hole-transporting emissive materials based on dibenzochrysene. Hole drift mobilities of the dibenzochrysene derivatives were measured in the vacuum-deposited films and found to be 5×10−4–2×10−3u200acm2/Vu200as (at 5×105u200aV/cm). The OLEDs consist of an emitting layer of the dibenzochrysene derivative and an electron-transport layer of tris(8-quinolinolato) aluminum. Emission colors of the OLEDs were blue-green and their spectra were consistent with the photoluminescence spectra with a peak wavelength of 490 nm. High external quantum efficiency of 2% was obtained at a luminance of 300 cd/m2, and good durability in a continuous operation at room temperature and high temperatures was achieved.

Influence of hole transporting material on device performance in organic light-emitting diode

Abstract We have studied the influence of hole transporting material on the electroluminescence characteristics in two-layer devices based on tris(8-quinolinolato) aluminum. Five hole transporting materials including two novel materials were used. No difference in turn-on voltages for light emission has been seen in the devices fabricated on indium-tin-oxide treated by argon/oxygen plasma, and a high luminance of 10000 cd/m2 was achieved at an operating voltage around 10 V. However, the photometric efficiency depended on the hole transporting material. High photometric efficiency of 6.1 cd/A and high luminous efficiency of 3.6 lm/W at a luminance of 300 cd/m2 were obtained in one of the devices.

Electrical properties of Ta–Sn–O films on indium tin oxide electrodes

We have found that Ta-Sn-O films prepared on indium tin oxide (ITO) electrodes by magnetron co-sputtering of Ta 2 O 5 and SnO 2 have much higher breakdown field strength than the Ta 2 O 5 films on the ITO electrodes. The highly insulating Ta-Sn-O films were obtained in the Sn concentration range of 3 to 40 at%. The figure of merit, which was defined by the multiplication of the breakdown field strength by the relative dielectric constant, of the Ta-Sn-O films was found to become a maximum in Sn concentration of about 3 at%. The experimental results of temperature dependence of the leakage currents indicated that the conduction mechanisms at room temperature changed from Poole-Frenkel type to Fowler-Nordheim tunneling type by adding SnO 2 into Ta 2 O 5 films.

Improving the sensitivity and selectivity of alcohol sensors based on organic thin-film transistors by using chemically-modified dielectric interfaces

Organic thin-film transistors (OTFTs) with chemically-modified dielectric interfaces are proposed as exhaled-alcohol sensors. Although sensors based on OTFTs have high sensitivity to moisture and would therefore be unsuitable for use as exhaled-gas sensors, it was found that a chemically-modified dielectric interfaces play a significant role in the sensitivity and selectivity of ethanol/moisture. OTFTs fabricated on a surface with low surface free energy, such as (tridecafluoro-1,1,2,2,-tetrahydrooctyl) trimethoxysilane (FTS), octadecylmethoxysilane (OTS) and hexamethyldisilazen (HMDS)-treated surfaces, have low sensitivity to moisture. OTFTs fabricated on a surface with a high ratio of dispersive to polar components of the surface free energy, such as HMDS and OTS-treated surfaces, have high selectivity of ethanol to moisture.