Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-06-21
2001-11-27
Yamnitzky, Marie (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C428S704000, C313S504000, C313S506000
Reexamination Certificate
active
06322909
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an organic electroluminescent device (which will hereinafter be called “organic EL device” simply) used for a planar light source or display device, particularly to a red-light emitting organic EL device.
2. Description of the Related Art
The organic EL device is considered promising as a spontaneous emission type planar display device. The organic EL device is expected to find its utility for a full color display and is therefore under brisk development, because different from an inorganic EL device which requires AC drive and high voltage, it is free from such limitations and in addition, is presumed to facilitate multi-color emission owing to a variety of usable organic compounds. When the above-described organic EL device is applied to a full color display, it is necessary for the device to emit three primary colors, that is, red, green and blue.
A number of reports have been published on green light emission and reported as a green light emitter are, for example, a device using tris(8-quinolinol)aluminum [Applied Physics Letters, 51, 913(1987)] and a device using a diarylamine derivative (Japanese Patent Application Laid-Open No.53397/1996).
There are also many reports on a blue-color emitting device, for example, a device using a stilbene compound (Japanese Patent Application Laid-Open No. 295359/1993), a device using a triarylamine derivative (Japanese Patent Application Laid-Open No. 53955/1995), a device using a tetraaryldiamine derivative (Japanese Patent Application Laid-Open No. 48656/1996) and a device using a styrylated biphenyl compound (Japanese Patent Application Laid-Open No. 132080/1994). In addition, it has been reported that a distyrylarylene derivative used as a light emitting material exhibited brightness of at least 20000 cd/m
2
, luminous efficiency of 5 lm/W and half life of at least 5000 hours (special lecture at the 70th Spring Annual Meeting of The Chemical Society of Japan).
Concerning an organic EL device from which red light emission is available, red light emission is obtained by the conversion of the wavelength of blue light emission in a fluorescent dye layer according to Japanese Patent Application Laid-Open No. 152897/1991, while it is obtained by doping of a red fluorescent dye into an emitter layer from which green or blue light emission is available according to Japanese Patent Application Laid-Open Nos 272854/1995, 288184/1995 or 286033/1996. Neither is sufficient from the viewpoints of brightness and color purity. Japanese Patent Application Laid-Open No. 791/1991 discloses an organic EL device using a red fluorescent dye singly for an emitter layer. Although Japanese Patent Application Laid-Open No.93257/1994 discloses the use of a squarylium compound represented by the following structural formula:
as a dopant in an emitter layer, red light emission of the resulting device is not sufficient in color purity. Further improvement is necessary for the practical use of it.
The above-described method according to the conversion of the wavelength of blue light emission in a fluorescent dye layer is also accompanied with the problems that sufficient luminous efficiency cannot be attained because a quantum yield for color conversion of EL light emission through the fluorescent dye layer is limited; and the use of the layer inevitably raises the production cost.
In addition, the red-light emitting material exemplified in the above conventional example has a low quantum yield of fluorescence and even by an increase in the electric current flowing inside of the device, light is emitted only at brightness of about 1000 cd/m
2
so that it is not suited for practical use.
SUMMARY OF THE INVENTION
The present invention has been completed with a view to overcoming the above-described problems and an object of the present invention is to provide a red-light emitting organic EL device having high brightness, excellent color purity and excellent luminous life.
The above-described object can be accomplished by the means which will be described below. In the present invention, there is provided an organic EL device which comprises a pair of a cathode and an anode and, therebetween, at least one organic thin-film layer containing an emitter layer, wherein
(a) said at least one organic thin-film layer comprises a squarylium compound represented by the following structural formula (1):
wherein R1 to R3 each independently represents a hydrogen atom, a hydroxyl group, a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms, a nitro group, a cyano group, a substituted or unsubstituted alkoxy group having 1 to 5 carbon atoms, aryl group or a halogen atom, or by the following structural formula (2):
wherein R1 to R5 each independently represents a hydrogen atom, a hydroxyl group, a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms, a nitro group, a cyano group, a substituted or unsubstituted alkoxy group having 1 to 5 carbon atoms, aryl group or a halogen atom.
Also embraced is the above-described organic EL device wherein:
(b) said emitter layer comprises a green to yellow-light emitting material having an EL spectrum of 500 nm to 580 nm and a squarylium compound represented by the above-described structural formula (1) or (2);
(c) said emitter layer comprises a quinoline metal complex and a squarylium compound represented by the above-described structural formula (1) or (2); or
(d) said squarylium compound represented by the above-described structural formula (1) or (2) is contained in an amount ranging from 0.001 to 50 wt. % based on a host material.
The red-light emitting material according to the present invention is fluorescent, having an absorption band in a green color region and a high quantum yield in a red color region of 600 to 650 nm so that red color emission inevitable for application of an organic EL device to a color display can be attained at high brightness and high efficiency.
In addition, the material according to the present invention does not disturb transfer of carriers injected into the organic EL device, because owing to a high quantum yield, even a trace amount of it mixed in the emitter layer of the organic EL device permits light emission in a red-color region at a high brightness. Furthermore, the material according to the present invention can be easily formed into a thin film by the electric resistance heating type film formation method. The thin film thus formed is markedly stable and excellent in flatness, and no change in the film structure such as crystallization or formation of a coagulated state is recognized, which facilitates extension of the life of the organic EL device.
Thus, the organic EL device using the material of the present invention is effective as a red light emitting device for a color display device.
REFERENCES:
patent: 5525516 (1996-06-01), Krutak et al.
Baowen Zhang et al., “Photoluminescence and electroluminescence of squarylium cyanine dyes”, Synthetic Metals, vol. 91, pp. 237-241, Dec. 1997.
Michael Best & Friedrich LLC
NEC Corporation
Whitesel J. Warren
Yamnitzky Marie
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