Stock material or miscellaneous articles – Composite – Of inorganic material
Patent
1996-08-28
1998-11-10
Nold, Charles
Stock material or miscellaneous articles
Composite
Of inorganic material
428917, 313506, H05B 3300
Patent
active
058341307
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an organic electroluminescent device.
BACKGROUND ART
It has been considered that light emission of an organic electroluminescent device is caused by the phenomenon where holes and electrons injected from electrodes are combined within a luminescent layer to generate excitons, and the excitons excite molecules of luminescent materials constituting the luminescent layer. By employing fluorescent dye as a luminescent material, there can be obtained an emission spectrum as an electroluminescence, being equivalent to the photoluminescence of the dye molecules.
Tang and Vanslyke recently proposed a device comprising two layers of a hole-transport layer and an electron-transport luminescent layer, which efficiently emits green light at a lower voltage (about 10 V) compared with the conventional organic electroluminescent devices having
Lett., 51 (1987) 913!. This device is composed of an anode, a hole-transport layer, an electron-transport luminescent layer and a cathode, all of which are formed in this order on a glass substrate.
In this device, the hole-transport layer not only allows holes to be injected from the anode into the electron-transport luminescent layer, but prevents electrons injected from the cathode from escaping into the anode without combining with the holes, confining the electrons in the electron-transport luminescent layer. This electron confinement facilitates the combination of the holes and the electrons compared with conventional single-layer structure devices, resulting in substantial decrease in drive voltage.
Saito et al. showed that not only an electron-transport layer but a hole-transport layer can be a luminescent layer in a two-layer structure 1489!.
Saito et al. also proposed an organic electroluminescent device having a three-layer structure wherein an organic luminescent layer is interposed S. Tokito, T. Tsutsui and S. Saito; Jpn. J. Appl. Phys., 27 (1988) L269!.
The two-layer structure device of Saito et al. is composed of an anode, a hole-transport luminescent layer, an electron-transport layer and a cathode, all of which are formed in this order on a glass substrate. In contrast to the device of Tang et al., the electron-transport layer not only allows electrons to be injected from the cathode into the hole-transport luminescent layer, but prevents the holes injected from the anode from escaping into the cathode without combining with the electrons, confining the holes in the hole-transport luminescent layer. This hole confinement leads to substantial decrease in drive voltage, as in the device of Tang et al.
The three-layer structure device of Saito et al. was attained by a further improvement for the device of Tang et al. The above device is composed of an anode, a hole-transport layer, a luminescent layer, an electron-transport layer and a cathode, all of which are formed in this order on a glass substrate. The hole-transport layer confines electrons in the luminescent layer, and the electron-transport layer confines holes in the luminescent layer. As a result, the combination of the electrons and the holes within the luminescent layer is more efficient than the two-layer structure device. Further, the electron-transport layer and the hole-transport layer prevent the excitons generated by the above combination from escaping into either of the anode and the cathode. Therefore, the three-layer structure device of Saito et al. may further increase the luminous efficiency.
Examples of the hole-transport materials which constitute these organic electroluminescent devices include aromatic tertiary amines such as triphenylamine. Examples of the electron-transport materials are oxadiazoles. Examples of the luminescent materials are tetraphenybutadiene derivatives, tris(8-quinolinolato)aluminum (III) complex, distyrylbenzene derivatives and distyrylbiphenyl derivatives.
Advantages of the above organic electroluminescent devices are that they can emit light of high luminance at a lower voltage than conventional electroluminescent
REFERENCES:
Appled Physics Letter (1994), vol. 64, No. 7, Feb. 14, 1994, Junji Kido et al. pp. 815-817.
Applied Physics Letter (1993), Part 2, Jul. 1, 1993, vol. 32, No. 7A, Junji Kido et al. pp. 917-920.
Applied Physics Letter (1993), vol. 63, No. 19, Nov. 8, 1993, Jinji Kido et al. pp. 2627-2629.
Nold Charles
Sumitomo Electric Industries Ltd.
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