Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1997-08-20
2002-06-25
Yamnitzky, Marie (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C313S504000, C313S506000
Reexamination Certificate
active
06410166
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to organic electroluminescence device doping material (hereinafter, referred to as organic EL device doping material) suitable for doping a luminous layer of organic electroluminescence device (hereinafter referred to as organic EL device) and organic electroluminescence (EL) device having a luminous layer doped by the organic EL doping material.
2. Description of the Prior Art
An organic EL device has a thin film containing fluorescent organic compound sandwiched by a cathode and an anode. When electrons and electron holes are implanted for re-joining them, excitons are generated, and extinction of the excitons produces light emission (fluorescence, phosphorescence), which is utilized for display.
FIG. 6
shows an example of basic configuration of the organic EL device. This organic EL device is composed of a substrate
100
, an anode
101
, which is an indium tin oxide (ITO), a hole transporting layer
102
which is a derivative of triphenylamine, an organic luminous layer
103
which is tris (8-quinolinate) aluminum (III), and a cathode
104
which is an alloy of magnesium and silver. These components are stacked on one another in this order. Each of the organic layers has a thickness of about 50 nm. Each of the film-layers is formed by means of vacuum deposition. When voltage of 10 VDC is applied to this organic EL device, a green light emission of about 1000 cd/m
2
can be obtained. This light emission is taken out from the ITO side. This organic EL device has a short service life and brightness is reduced to half after about 100 hours.
As for the organic EL devices which were developed prior to the organic EL device illustrated in
FIG. 6
, luminescence brightness obtained when it is driven by several tens of volts was only several cd/m
2
. The reason why the aforementioned EL device of
FIG. 6
enables a high brightness is considered to be as follows.
1) The organic layer has a thickness as small as 100 nm for employing an organic material which is nearly an insulator in which the carrier movement is in the order of 10
−3
to 10
−5
cm/Vs.
2) A hole transporting layer is provided for isolating a function, thus enhancing rejoining in the luminous layer.
As for the reason why the organic EL device of
FIG. 6
has a short service life, the following points are to be taken into consideration.
1) Physical Change of the Organic Layers
A crystalline grain boundary is generated in the organic films, especially in the hole transporting layer
102
, which causes a short-circuiting.
2) Cathode
104
Oxidation/Peeling Out
As magnesium having a low work function is used, reaction is caused by humidity and oxygen in the device as well as humidity and oxygen in the air, thus generating oxide.
This significantly lowers electron implantation efficiency, and causes peeling out from the organic layer.
Afterwards, for obtaining multi-coloring the aforementioned organic EL device, a method was developed as follows. A coloring matter, or a pigment, such as coumarin and DCM was doped into the aforementioned organic luminous layer by several mol % so that these coloring matters generate electroluminescence. Because these coloring matters exhibit a high quantum yield of fluorescence, an external quantum yield was also improved. Such a doping for luminescence of a coloring matter is considered to be especially effective in the following cases.
1) In the band model, i.e., in the energy diagram, a HOMO level and a LUMO level of a dopant are found between the HOMO level and the LUMO level of the host material Alq,. This model is somehow applicable to organic cases.
2) The host material has a luminous spectrum which is mostly overlapped with the excitation spectrum of the dopant.
FIG. 7
is a schematic view of a structure of an organic EL device in which a pigment has been doped into an organic luminous layer
105
for multi-coloring. In this organic EL device, because Alq
3
has a comparatively low fluorescence quantum yield, a fluorescent pigment having a high fluorescence quantum yield is doped into Alq
3
by several percent mols., thus enhancing the device efficiency. Here, the dopant used may be an organic coloring matter or a pigment, for example, coumarin and DCM. Coumarin generates a blue-green light emission, whereas DCM generates an orange light emission. This EL device improves light emission efficiency as well as enables to realize multi-color emission. Even a material which causes concentration quenching can be used.
Recently, study is also made on an organic ED device using polyvinylcarbazole (PVK) as a host material. There is an example that a film was formed by way of dipping or a spin painting of PVK in which tetraphenylbutadiene (TPB), Nile red, and coumarin in solution are dispersed so as to form an electrode, and white luminescence was obtained. This case requires the same conditions as the organic EL device shown in
FIG. 7
, and should be soluble in a solvent.
A method has been suggested to obtain luminescence from a pigment by dispersing the pigment in polyvinylcarbazole (PVK).
FIG. 8
shows a basic structure of the device in this case. The device is composed of a substrate
200
, an anode
201
made of ITO, a luminous layer
202
made of PVK in which the pigment has been dispersed, and a cathode
203
made of Mg, Ag, and the like. These components are stacked on one another in this order. In this configuration, there is a single organic layer. It is also possible to provide an electron transporting layer between the luminous layer
202
and the cathode
203
As for the pigments for obtaining luminescence of colors. TPB, Coumarin 6, and Nile Red are used for obtaining blue color, green color, and red color, respectively. It is also possible to use these pigments simultaneously so as to obtain a white luminescence. The film of the organic layer is formed by the wet method such as dipping and spin coating. In the case of a single layer, the film thickness is 100 nm, whereas in the case of two layers, each of the layers is formed to have a thickness of about 50 nm.
The organic EL device has been studied in the direction of a multi-color type. The conventional dopant which has been doped in the organic luminous layer for the multi-color type of the organic EL device was a pigment. Pigments in general have a planar molecular structure and have a strong intermolecular force. Consequently, there is a problem that when a film is formed, a pigment easily agglutinates. For example, if a plenty of pigment is doped in Alq
3
and a film is formed, the resultant luminous layer is turbid, lacks in stability, and has a low voltage resistance. That is, when it is used as a device, it is readily destroyed. Moreover, the conventional dopant is a pigment, which has a small molecular weight and a low melting point. Therefore, the resultant organic luminous layer of a formed film has insufficient heat resistance.
SUMMARY OF THE INVENTION
In view of the above-described problems, the present invention provides an organic EL device doping material having a high melting point and a thermal stability, improving stability of a corresponding organic luminous layer when doped; and an organic EL device containing such a doping material in the organic luminous layer.
An organic electroluminescence device doping material includes metal chelate complex composed of ligands having N N′-bissalicylidene-2,3-diaminobenzofuran (SABF) skeleton.
An organic electroluminescence device doping material can be expressed by Chemical Formula 1,
wherein X is a central metal ion, and R
1
to R
2
, independently denote any one of hydrogen group, halogen group, alkyl group, cyano group, nitro group, ester group, amino group, mono- or disubstituted amino group, acylamino group, hydro group, alkoxy group, mercapto group, alkyloxy group, alkylthio group, aryloxy group, arylthio group, siloxy group, acyl group, cycloalkyl group, carbamoyl group, carboxylic group, sulfonate group, imide group, substituted or unsubstituted aliphati
Miyauchi Toshio
Takahashi Hisamitsu
Tanaka Satoshi
Tsuruoka Yoshihisa
Futaba Corporation
Yamnitzky Marie
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