Hisakazu Takahashi

Red organic light-emitting diodes using an emitting assist dopant

We propose an emitting assist (EA) dopant system for obtaining organic light-emitting diodes (OLEDs) with pure red emission. The EA dopant (rubrene) did not itself emit but assisted the energy transfer from the host (Alq3) to the red emitting dopant (DCM2). The cell structure used was {indium tin oxide/hole injection layer [(20 nm), CuPc/hole transport layer (50 nm), NPB/emitting layer (40 nm), Alq3+DCM2 (2%)+rubrene (5 wt %)]/MgIn}. (CuPc: Copper (II) phthalocyanine, NPB: N, N′-Di(naphthalen-1-yl)-N, N′-diphenyl-benzidine, DCM2: 4-Dicyanomethylene- 2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-8-yl)vinyl]-4H-pyran). A stable red emission (chromaticity coordinates: x=0.64, y=0.36) was obtained in this cell within the luminance range of 100–4000 cd/m2. When the cell was not doped with rubrene, the emission color changed from red to orange as the luminance increased. The EA dopant system is a promising method for obtaining red OLEDs.

Green fluorescent organic light-emitting device with external quantum efficiency of nearly 10%

Green fluorescent organic light-emitting device (OLED) exhibiting a high external quantum efficiency of nearly 10% has been developed. The OLED consists of simple three organic layers, using NPB, 0.8% C545T doped TPBA, and DBzA as a hole-transporting layer, an emitting layer, and an electron-transporting layer, respectively, [fluorocarbon coated indium tin oxide/NPB (60 nm)/08% C545T doped TPBA (40 nm)/DBzA (20 nm)/LiF (1 nm/Al], where NPB is 4,4′-bis (N-phenyl-1-naphthylamino)biphenyl, C545T is 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]pyrano[6 7 8-ij]quinolizin-11-one, TPBA is 9,9′,10,10′-tetraphenyl-2,2′-bianthracene, and DBzA is 9,10-bis[4-(6-methylbenzothiazol-2-yl)phenyl]anthracene. The high external quantum efficiency is maintained in the wide range of current density of 2–100 mA∕cm2. The current efficiency and power efficiency of the OLED are also very high, 29.8 cd/A and 26.2 lm/W, respectively, at a current density of 20 mA/cm2. The OLED is promising for prac...

White-Light-Emitting Material for Organic Electroluminescent Devices

White emission is important for applying organic EL devices to full-color displays and backlighting. In order to obtain white emission, the use of a white-light-emitting material which shows the white emission by itself is advantageous for these applications because of its high reliability and productivity. A device structure of indium-tin-oxide (ITO, anode)/hole transport layer/emitting layer/MgIn alloy (cathode) was employed. Bis(2-(2-hydroxyphenyl)benzothiazolate)zinc ( Zn(BTZ)2) was used as the emitting material. Zn(BTZ)2 showed greenish-white emission with a broad electroluminescent spectrum (half-width: 157 nm, peak wavelength: 486, 524 nm). It exhibited a high luminance of 10,190 cd/m2 at the applied voltage of 8 V because Zn(BTZ)2 has a good electron-transport property. As such, Zn(BTZ)2 is expected to serve as a new white-light-emitting material for organic EL devices.

Dielectric Characteristics of (A1/21+·A1/23+)TiO3 Ceramics at Microwave Frequencies

We have investigated the dielectric characteristics at microwave frequencies of perovskites with the formula (A1/21+A1/23+)TiO3, (where A1+ represents an alkali metal from the lithium to potassium series and A3+ represents a member of the lanthanide series from lanthanum to lutetium). It was found for the first time that in the case of Li1+ substitution at the A1+ site, the larger the ionic radius of the element which substituted at the A3+ site, the higher the dielectric constant was and the greater the negative shift of the temperature coefficient of resonant frequency was. For the (1-x)(Li1/21+Sm1/23+)TiO3-x(Na1/21+Sm1/23+)TiO3 system at 3 GHz, high dielectric properties of er=81, Q=2050 and τf=+17ppm/ °C were obtained when x=0.4.

Microwave Dielectric Properties of CaO-Li2O-Ln2O3-TiO2 Ceramics

We have investigated the microwave dielectric properties of the Li2O-Ln2O3-TiO2 system and the CaO-Li2O-Ln2O3-TiO2 system (Ln: Lanthanide), both of which have a perovskite structure. The relationship between the kind of lanthanide element and dielectric properties at microwave frequencies was confirmed. It was also found that the ionic radius of lanthanide ions has a strong influence on the dielectric properties. Excellent dielectric properties of er=110, fQ value=4500 GHz and high temperature stability for the resonant frequency at 3 GHz were obtained with a composition of CaO:SrO:Li2O:Sm2O3:TiO2=15.0:1.0:9.0:12.0:63.0 (molar ratio). Also, band-pass filters made of this material were examined. The filters were greatly miniaturized to the size of about 2×5×4 mm3 (0.04 cm3).

Development of OLED with high stability and luminance efficiency by co-doping methods for full color displays

We propose co-doping systems in emission layers of red and green organic light-emitting diodes (OLEDs). The luminance-voltage, luminous and power efficiency-voltage characteristics, operational stability, and the energy bands of materials were measured. In red OLED devices, we propose an emitting assist (EA) dopant for better luminance efficiency and power efficiency with pure red emission and improved operational stability. The EA dopant (rubrene) did not emit itself but assisted the energy transfer from a host (Alq) to an emitting dopant (DCJTB). By doping rubrene, the luminance efficiency increased from 1.7 to 4.3 cd/A (from 0.6 to 1.9 lm/W) with chromaticity of (x=0.64, y=0.36) unchanged. An improved lifetime was also observed. In green OLED devices, we introduced hole transporting material (NPB) into an emission layer for better charge injection balance. The green devices with the emitting dopant (C545T) achieved the luminance efficiency of 8.5 cd/A compared with 6.9 cd/A without NPB. We studied the co-doping methods and use of this approach for active-matrix full color display. The power consumption of white emission at 100 cd/m/sup 2/ was reduced by 32%. The effectiveness of these co-doping methods was demonstrated for practical applications.

High efficiency red organic light-emitting devices using tetraphenyldibenzoperiflanthene-doped rubrene as an emitting layer

Red organic light-emitting devices (OLEDs) have been developed employing a novel fluorescent emitting layer, tetraphenyldibenzoperiflanthene-doped rubrene. The devices are characterized by low driving voltage below 4V at a current density of 20mA∕cm2 and high color purity with Commission Internationale de l’Eclairage coordinates of (0.66, 0.34). The OLED using the novel emitting layer in combination with the electron-transporting layer consisting of 9,10-bis[4-(6-methylbenzothiazol-2-yl)phenyl]anthracene exhibits a high power efficiency of 5.3lm∕W at a current density of 20mA∕cm2. The half-luminance lifetime of the red OLED is 223h at a current density of 80mA∕cm2 (initial luminance of 3570cd∕m2). Both the driving voltage and current efficiency of the device are significantly improved compared to a device using tris(8-quinolinolato)aluminum as an electron-transporting layer. The studies on charge transport for the host materials indicate that the high efficiency is attributed to the improved charge inject...

Organic light-emitting diodes using a gallium complex

A gallium complex (GaMq2Cl) consisting of two 2-methyl-8-hydroxyquinolines (Mq) and a chlorine was synthesized and used for the fabrication of organic light-emitting diodes (OLEDs). The photoluminescent peak of GaMq2Cl at 492 nm was as strong in intensity as that of tris(8-hydroxyquinolinato)aluminum (Alq3). The OLED using GaMq2Cl as an emitting material showed blue-green luminance of 10 490 cd/m2. When it was used as an electron transport material in a rubrene doped cell, an OLED with a high luminance of 27 700 cd/m2 was obtained. We found that GaMq2Cl also was useful as a host material.

ORGANIC LIGHT-EMITTING DIODES USING 3- OR 5-HYDROXYFLAVONE-METAL COMPLEXES

3- or 5-hydroxyflavone-metal complexes were synthesized and applied to organic light-emitting diodes (OLEDs) as an emitting material and an electron transport material. When bis(5-hydroxyflavonato)beryllium [Be(5Fla)2] was used as an emitting material, the OLED showed a luminance of 1900 cd/m2 with yellow. On the other hand, when used as an electron transport material in an organic three-layer cell, a high luminance of more than 10 000 cd/m2 was obtained. Furthermore, the turn-on voltage of the OLED could be lowered to 2.4 V and its lifetime was more than 1300 h. Be(5Fla)2 was found to have a good electron transporting capability and good stability.

Organic electroluminescent devices doped with condensed polycyclic aromatic compounds

Abstract Two dopants with condensed aromatic rings were examined. Hexabenzobenzene (coronene) and tri-peri-naphthylenebenzene (decacyclene) were respectively doped into blue organic electroluminescent (EL) cells. The coronene-doped cell exhibited a blue-green emission of 5300 cd/m2 at 17 V. The decacyclene-doped cell exhibited a green emission of 12 300 cd/m2 at 19 V. Lifetime of the decacyclene-doped cell was greatly improved.