Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-01-02
2004-04-13
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C428S704000, C313S504000, C313S506000, C252S301160, C252S301260, C257S040000, C257S103000
Reexamination Certificate
active
06720090
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to organic light emitting diode devices and more particularly to the design of the composition of the organic layers for improvements in luminance efficiency.
BACKGROUND OF THE INVENTION
Organic light emitting diodes (OLED), also known as organic electroluminescent (EL) devices, are a class of electronic devices that emit light in response to an electrical current applied to the device. The structure of an OLED device generally includes an anode, an organic EL medium, and a cathode. The term, organic EL medium, herein refers to organic materials or layers of organic materials disposed between the anode and the cathode in the OLED device. The organic EL medium may include low molecular weight compounds, high molecular weight polymers, oligimers of low molecular weight compounds, or biomaterials, in the form of a thin film or a bulk solid. The medium can be amorphous or crystalline. Organic electroluminescent media of various structures have been described in the prior art. Dresner, in RCA Review, 30, 322 (1969), described a medium comprising a single layer of anthracene film. Tang et al, in Applied Physics Letters, 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and commonly-assigned U.S. Pat. No. 4,769,292, reported an EL medium with a multi-layer structure of organic thin films, and demonstrated highly efficient OLED devices using such a medium. In some OLED device structures the multi-layer EL medium includes a hole transport layer adjacent to the anode, an electron transport layer adjacent to the cathode, and disposed in between these two layers, a luminescent layer. Furthermore, in some preferred device structures, the luminescent layer is constructed of a doped organic film comprising an organic material as the host and a small concentration of a fluorescent compound as the dopant. Improvements in EL efficiency and chromaticity have been obtained in these doped OLED devices by selecting an appropriate dopant-host composition. Often, the dopant, being the dominant emissive center, is selected to produce the desirable EL colors. Examples of the doped luminescent layer reported by Tang et al. in commonly-assigned U.S. Pat. No. 4,769,292 and by Chen et. al in commonly-assigned U.S. Pat. No. 5,908,581 are: tris(8-quinolinol)aluminum (AlQ) host doped with coumarin dyes for green emitting OLEDs; and AlQ doped with 4-dicyanomethylene-4H-pyrans (DCMs) for orange-red emitting OLEDs. In addition to enhancements in color and luminance efficiency, there are other significant benefits in using a doped luminescent layer. Shi et al., in commonly-assigned U.S. Pat. No. 5,593,788, disclosed that the use of a doped luminescent layer significantly improved the stability of OLED devices. In Shi's disclosure, a long operational life was obtained in an OLED device by using a quinacridone compound as the dopant in an AlQ host. The quinacridone dopant produced a green emission with a high luminance efficiency. Bryan et al., in commonly-assigned U.S. Pat. No. 5,141,671, disclosed a luminescent layer containing perylene or a perylene derivative as a dopant in a blue emitting host. They showed that a blue emitting OLED device with an improved blue hue and an improved operational stability was obtained. In both disclosures, the incorporation of selected fluorescent dopants in the luminescent layer is found to improve substantially the overall OLED device performance parameters.
The most common formulation of the doped luminescent layer includes only a single dopant in a host matrix. However, in a few instances, incorporation of more than one dopant in the luminescent layer was found to be beneficial in improving the hue. One such instance was reported by Hamada et al. in Applied Phys. Lett. 75, 1682 (1999). Using a luminescent layer containing rubrene, a yellow emitting dopant, and DCJ 4-(dicyanomethylene)-2-methyl-6-[2-(4-julolidyl)ethenyl]-4H-pyran, a red emitting dopant, in an AlQ host, Hamada et al. were able to produce a red emitting OLED device with excellent chromaticity. Furthermore, the red hue from the DCJ dopant remains essentially unchanged regardless of the intensity of light output from the OLED. In contrast, with only DCJ dopant in the AlQ host, the color of the light emitted from the dopant DCJ was noticeably blue-shifted, producing a less than desirable orange, rather than red, hue. With the dual dopant system, rubrene functions as a co-dopant in mediating energy transfer from the AlQ host to the DCJ emitter. In either single dopant or dual dopant systems, it has been noted that the luminance efficiency tends to decrease with increasing brightness, i.e. with increasing current density. With red-emitting dopants, the hue usually shifts towards orange with increasing current density.
Although EL efficiency, color, and stability have been improved significantly using doped luminescent layers of various compositions, the problem of decreasing EL efficiency with increasing light output or drive current density persists, particularly in red-emitting OLED devices.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide OLED devices with improved luminance efficiency that is essentially independent of the intensity of the light output from the OLED.
It is another object of the present invention to provide specifically red OLED devices with an improved luminance efficiency that is essentially independent of the intensity of the light output.
It is a further object of the present invention to provide specifically red OLED devices with chromaticity essentially independent of the light output.
These objects are achieved in an organic light emitting device comprising:
(a) a substrate;
(b) an anode and a cathode disposed over the substrate;
(c) a luminescent layer disposed between the anode and the cathode wherein the luminescent layer includes a host and at least one dopant;
(d) the host of the luminescent layer being selected to include a solid organic material comprising a mixture of at least two components wherein;
(i) the first component of the mixture is an organic compound that is capable of transporting both electrons and holes and that is substantially non-polar; and
(ii) the second component of the mixture is an organic compound that is more polar than the first component; and
(e) the dopant of the luminescent layer being selected to produce light from the light emitting device.
An advantage of the present invention is that, with an appropriate selection of the first and second host components and the dopants in the luminescent layer, OLED devices with high luminance efficiencies are produced.
Another advantage of the present invention is that it provides OLED devices with a luminance efficiency which is relatively independent of the intensity of the light output from the OLED.
Another advantage of the present invention is that it provides an OLED device with a chromaticity essentially independent of the intensity of the light output from the OLED.
Another advantage of the present invention is that it provides a red emitting OLED with excellent efficiency and chromaticity.
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patent: 4769292 (1988-09-01), Tang et al.
patent: 5141671 (1992-08-01), Bryan et al.
patent: 5281489 (1994-01-01), Mori et al.
patent: 5593788 (1997-01-01), Shi et al.
patent: 5776622 (1998-07-01), Hung et al.
patent: 5908581 (1999-06-01), Chen et al.
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patent: 6020078 (2000-02-01), Chen et al.
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patent: 2002/0048688 (2002-04-01), Fukuoka et al.
patent: 1 162 674 (2001-12-01), None
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patent: WO 99/53724 (1999-10-01), None
“Double Injection Electroluminescence in Anthracene” by J. Dresner, RCA Review, 30,322 (1969) no month.
Shi Jianmin
Tang Ching W.
Young Ralph H.
Eastman Kodak Company
Jones Deborah
Owens Raymond L.
Xu Ling X
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