Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
1998-03-16
2001-05-01
Yamnitzky, Marie (Department: 1774)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S690000, C428S917000, C313S502000, C313S504000, C313S506000, C257S102000, C257S103000
Reexamination Certificate
active
06224966
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an organic electroluminescent (EL) device, and more precisely, to an organic EL device having a long life and high light-emitting efficiency.
BACKGROUND OF THE INVENTION
As being self-luminescent, EL devices have high visibility. In addition, they have high impact resistance as being completely solid devices. Therefore, the use of EL devices in various displays as light emitters is being widely noticed.
EL devices are grouped into inorganic EL devices in which are used inorganic compounds as light-emitting materials, and organic EL devices in which are used light-emitting organic compounds. Of those, organic EL devices have been being much studied and expected as light emitters in the coming generations, since they require a greatly reduced level of voltage, they can be easily small-sized, they consume small electric power, and they can emit light in a mode of plane emission.
Known are various structures of organic EL devices having a basic constitution of anode/organic light-emitting layer/cathode and optionally provided with a hole injection and transportation layer and an electron injection layer, such as anode/hole injection and transportation layer/organic light-emitting layer/cathode, and anode/hole injection and transportation layer/organic light-emitting layer/electron injection and transportation layer/cathode, etc.
In those, the function of the hole injection and transportation layer is to inject holes thereinto from the anode and to transport them to the organic light-emitting layer, while that of the electron injection and transportation layer is to inject electrons thereinto from the cathode and to transport them to the organic light-emitting layer. The function of the organic light-emitting layer is to receive holes and electrons and to emit fluorescence through recombination of those holes and electrons.
Various structures of such organic EL devices have heretofore been studied in order to improve their light-emitting efficiency and to prolong their life. For example, proposed was the disposition of a plurality of organic light-emitting layers in EL devices to thereby improve the light-emitting efficiency of the devices. Japanese Patent Application Laid-Open (JP-A) No. 326146/1993 discloses an organic EL device having two organic light-emitting layers. However, the organic EL device disclosed, in which light being emitted from two layers is taken out, is problematic in that its light-emitting efficiency is low, or is in the order of 1 candela/ampere, and that its life is short, or is in the order of 1000 hours (initial luminance: 100 candela/m
2
). JP-A No. 326146/1993 discloses nothing relating to a technique of doping organic light-emitting layers with fluorescent substances.
The technique of doping organic light-emitting layers with fluorescent substances is known. For example, JP-A No. 65958/1995 which provides an organic EL device discloses a technique of doping both the organic hole transportation layer and the organic light-emitting layer with a fluorescent substance to thereby prolong the life of the device. However, the organic EL device disclosed is still problematic in that its emission half-life in continuous driving operation is short. JP-A No. 65958/1995 discloses nothing relating to the improvement in the light-emitting efficiency of the device through doping with fluorescent substances.
JP-A No. 213172/1996 discloses an organic EL device having two organic light-emitting layers of which one is doped with a fluorescent substance. However, the organic EL device disclosed is problematic in that its overall light-emitting efficiency is low as the light-emitting efficiency of the non-doped organic light-emitting layer is low.
SUMMARY OF THE INVENTION
Given this situation, the object of the invention is to provide an organic EL device having a long life and high light-emitting efficiency.
We, the present inventors have assiduously studied in order to obtain an organic EL device having those favorable properties, and, as a result, have found that the intended object of the invention is attained by an organic EL device having a plurality of organic light-emitting layers in which all plural organic light-emitting layers are doped with a fluorescent substance of the same type or the same color. The present invention has been completed on the basis of this finding.
Specifically, the invention provides the following:
[1] An organic EL device comprising a plurality of organic compound layers that include at least two or more organic light-emitting layers and are sandwiched between a pair of electrodes of anode and cathode, wherein two or more these organic light-emitting layers are all doped with a fluorescent substance of the same type or the same color that constitutes the organic light-emitting layers.
[2] The organic EL device of [1], wherein every organic host substance constituting each organic light-emitting layer has an electron affinity of not smaller than 2.6 eV.
[3] The organic EL device of [1] or [2], wherein the ionization potential of the host substance constituting each organic light-emitting layer satisfies the following requirement (1):
Ip(1)<Ip(2)<. . . Ip(
n
) (1)
wherein Ip(1) indicates the ionization potential of the organic host substance constituting the first organic light-emitting layer as counted from the side of the anode;
Ip(2) indicates the ionization potential of the organic host substance constituting the second organic light-emitting layer as counted from the side of the anode;
Ip(n) indicates the ionization potential of the organic host substance constituting the n'th organic light-emitting layer as counted from the side of the anode.
[4] The organic EL device of any one of [1] to [3], wherein the electron affinity of the host substance constituting each organic light-emitting layer satisfies the following requirement (2):
Ea(1)<Ea(2)<. . . Ea(
n
) (2)
wherein Ea(1) indicates the electron affinity of the organic host substance constituting the first organic light-emitting layer as counted from the side of the anode;
Ea(2) indicates the electron affinity of the organic host substance constituting the second organic light-emitting layer as counted from the side of the anode;
Ea(n) indicates the electron affinity of the organic host substance constituting the n'th organic light-emitting layer as counted from the side of the anode.
[5] The organic EL device of any one of [1] to [4], wherein the difference in the ionization potential between the organic host substances constituting the adjacent organic light-emitting layers is not smaller than 0.2 eV.
[6] The organic EL device of any one of [1] to [5], wherein the difference in the electron affinity between the organic host substances constituting the adjacent organic light-emitting layers is not larger than 0.2 eV.
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patent: 5593788 (1997-01-01), Shi et al.
patent: 5601903 (1997-02-01), Fujii et al.
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patent: 5693428 (1997-12-01), Fujii et al.
patent: 5773929 (1998-06-01), Shi et al.
patent: 0 553 950 (1993-08-01), None
Higashi Hisahiro
Hosokawa Chishio
Nakamura Hiroaki
Sakai Toshio
Idemitsu Kosan Co. Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Yamnitzky Marie
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