Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2000-09-29
2003-07-08
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S212000, C428S213000, C428S917000, C313S502000, C313S506000, C427S066000
Reexamination Certificate
active
06589673
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic electroluminescent device (hereinafter, referred also to as an “organic EL device”), and a group of organic EL devices.
2. Description of the Related Art
Recently, attention is been focused on organic electroluminescent devices having a light-emitting or luminescent layer formed from a specific organic compound due such organic electroluminescent devices being able to achieve a large area display device operable at a low driving voltage. To produce an EL device having high efficiency, Tang et al., as is reported in Appl. Phys. Lett., 51, 913 (1987), have succeeded in providing an EL device having a structure in which organic compound layers having different carrier transporting properties are laminated to thereby introduce holes and electrons with a good balance from an anode and a cathode, respectively. In addition, since the thickness of the organic compound layers is less than or equal to 2,000 Å, the EL device can exhibit a high luminance and efficiency sufficient for practical use, i.e., a luminance of 1,000 cd/m
2
and an external quantum efficiency of 1% at an applied voltage of not more than 10 volts.
In the above-described high efficiency EL device, Tang et al. used magnesium (Mg) having a low work function in combination with the organic compound which is considered to be fundamentally an electrically insulating material, in order to reduce an energy barrier which may cause a problem during injection of electrons from a metal-made electrode. However, since the magnesium is liable to be oxidized and is unstable, and also exhibits only a poor adhesion to a surface of the organic layers, magnesium was used upon alloying, i.e., by the co-deposition of the magnesium with silver (Ag) which is relatively stable and exhibits good adhesion to a surface of the organic layers.
On the other hand, researchers of Toppan Printing Co. (cf, 51st periodical meeting, Society of Applied Physics, Preprint 28a-PB-4, p.1040) and those of Pioneer Co. (cf, 54th periodical meeting, Society of Applied Physics, Preprint 29p-ZC-15, p.1127) have had developments in the usage of lithium (Li), which has an even lower work function than that of Mg, and alloying lithium (Li) with an aluminum (Al) to obtain a stabilized cathode, thereby embodying a lower driving voltage and a higher emitting luminance in comparison with EL devices using magnesium alloys.
In addition, as is reported in IEEE Trans. Electron Devices, 40, 1342 (1993), the inventors of the present application have found that a two-layered cathode produced by depositing lithium (Li) alone with a very small thickness of about 10 Å on an organic compound layer, followed by laminating silver (Ag) to the thus deposited Li layer is effective to attain a low driving voltage in EL devices.
Using the above EL devices, as is disclosed in Japanese Unexamined Patent Publication (Kokai) No.63-264692, if a thickness of the organic compound layer is controlled to not more than 1 &mgr;m (substantially 0.2 &mgr;m or less), it becomes possible to operate the devices at a low voltage which is acceptable for practical use, even if an organic compound which is basically an electrically insulating material is used in the formation of the organic compound layer.
Further, the applicant of this application, as is disclosed in Japanese Unexamined Patent Publication (Kokai) No.10-270171, has discovered that if a metal showing a low work function such as an alkali metal, an alkali earth metal and transition metals including a rare earth metal, and an organic electron-accepting compound are mixed in the predetermined ratio through co-deposition to form an electron injection layer, the resulting EL device can be operated at a low driving voltage regardless of the work function of the cathode. In this EL device, a donor (electron-donating) dopant substance, i.e., metal, capable of acting as a reducing agent for the organic compound is previously doped into a layer of the organic compound to be contacted with the cathode, and thus the organic compound is retained as a molecule in the reduced form; namely, the molecule of the organic compound has electrons accepted or injected therein. As a result, an energy barrier in the electron injection from a cathode to an organic compound layer is reduced, thereby ensuring a low-voltage driving of the EL devices in comparison to the prior art EL devices. Moreover, in the formation of the cathode, it is possible to use any stable metals which are conventionally used as a wiring material such as aluminum (Al). Accordingly, if a suitable combination of the organic compound and the metal is applied to the metal doping layer, an increase of the driving voltage can be prevented in contrast to the prior art layer constituted from only an organic compound, and such effects can be obtained even if a layer thickness of the metal doping layer is increased to a level in the orders of micrometers. Namely, in this EL device, a dependency of the driving voltage upon layer thickness of the metal doping layer can be removed.
Referring again to the above-described EL device developed by Tang et al., an indium-tin-oxide (ITO) is coated as an anode electrode over the glass substrate. However, the use of the ITO anode electrode in the device taught by Tang et al. to obtain a good contact near to ohmic contact is considered to be made due to unexpected luck, because, in the hole injection to the organic compound, the ITO electrode has been frequently used as a transparent anode electrode made of metal oxide to satisfy the requirement for the emission of light in the planar area, and the ITO electrode can exhibit a relatively large work function of not more than 5.0 eV.
Further, in the EL device taught by Tang et al., a layer of copper phthalocyanine (hereinafter, CuPc) having a thickness of not more than 200 Å is inserted between the anode and the hole-transporting organic compound layer to further improve the contact efficiency of the anode interface region, thereby enabling the operation of the device at a low voltage.
Furthermore, the researchers of Pioneer Co., Ltd., have obtained similar effects by using a starburst type arylamine compounds, proposed by Shirota et al., of Osaka University.
CuPc compounds and starburst type arylamine compounds have characteristics that show a work function smaller than that of ITO and a relatively high mobility of the hole charge; and thus they can improve the stability of the EL devices during continuous drive, as a function of improved interfacial contact, in addition to low-voltage driving.
On the other hand, the applicant of this application and others, as is disclosed in Japanese Unexamined Patent Publication (Kokai) No.10-49771, have discovered that, if a Lewis acid compound and an organic hole-transporting compound are mixed in a predetermined ratio using a co-deposition method to form a hole injection layer, the resulting EL device can be operated at a low driving voltage regardless of the work function of the anode. In this EL device, a Lewis acid compound capable of acting as an oxidation agent for the organic compound is previously doped into a layer of the organic compound to be contacted with the anode, and thus the organic compound is retained as a molecule in an oxidized form. As a result, an energy barrier in the hole injection from an anode to an organic compound layer is reduced, thereby ensuring a low-voltage driving of the EL devices in comparison to the prior art EL devices. Accordingly, if a suitable combination of the organic compound and the Lewis acid compound is applied to the hole injection layer, an increase of the driving voltage can be avoided in contrast to the prior art layer constituted from only an organic compound, and such effects can be obtained even if a layer thickness of the hole injection layer is increased to a level in the order of micrometers. Namely, in this EL device, a dependency of the driving voltage upon the layer thickness of the hole injection layer
Endoh Jun
Kido Junji
Mori Kohichi
Yokoi Akira
Garrett Dawn
Kelly Cynthia H.
Kido Junji
McCormick Paulding & Huber LLP
LandOfFree
Organic electroluminescent device, group of organic... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Organic electroluminescent device, group of organic..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Organic electroluminescent device, group of organic... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3081370