Fluorescent metal complexes and dimers have been synthesized and used for organic light-emitting diodes. The metal complexes with conjugated molecular ligands have strong photoluminescence, and showed bright emission of over 10 000 cd m 22 in bilayer electroluminescent (EL) cells. The EL cell structure was [transparent anode/hole-transport layer/emitting layer/cathode]. The newly synthesized pyrazoline dimers are also highlȳ uorescent, and they have hole-transport tendency. Therefore, another type of structure [transparent anode/ emitting layer/electron-transport layer/cathode] was applied to get emission from the pyrazoline derivatives. As a result, one pyrazoline derivative showed bright blue EL of 1700 cd m 22 at the maximum when it was combined with an oxadiazole electron-transport material.

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.

Organic electroluminescent (EL) cells with azomethin-zinc complexes as an emitting layer showed bright blue emission. A cell structure of [indium-tin-oxide (ITO)/hole transport layer/emitting layer/MgIn] was employed. The color was blue, and emission peaks were 458∼470 nm. High luminance exceeding 1000 cd/m2 was obtained for the first time with an EL cell using a complex emitting material. The azomethin-zinc complexes exhibited good electron transport, as indicated by the high luminance obtained in two-layer EL cells.

Organic electroluminescent (EL) devices with 8-hydroxy-quinoline derivative-metal complexes (Znq2, Beq2, Mgq2, ZnMq2, BeMq2 and AlPrq3) as the emitter have been developed. The emissions of these devices are green or yellow. These devices have a luminance of more than 3000 cd/m2. In particular, the EL device with Znq2 has a luminance of 16200 cd/m2.

The red-emitting phosphorescent organic light-emitting device employing Bis[2-(2-benzothiazoyl)phenolato]zinc(II) (Zn(BTP)2) as a host material has been demonstrated. The device generates saturated red emission with Commission Internationale de l’Eclairage coordinates of (0.67, 0.33), characterized by a low driving voltage of 3.2V and high external quantum efficiency of 10.3% at 10mA∕cm2. Moreover, the efficiency and lifetime are improved by using 30% NPB-doped Zn(BTP)2 host in the emitting layer, where NPB is 4,4′-bis(N-phenyl-1-naphthyl-amino)biphenyl. The maximum external quantum efficiency of the device reaches 12.6% at 1.7mA∕cm2. The device has exhibited excellent stability. The half-luminance lifetime is 240h at 80mA∕cm2 (initial luminance of 6070cd∕m2). The relationship between the employed host material and device performance is discussed.