Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-10-02
2003-03-18
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C313S504000, C313S506000, C427S066000
Reexamination Certificate
active
06534202
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an organic electroluminescent device and, more particularly, to a thin film device having a light-emitting layer of an organic compound which emits light upon electric field application.
2. Background Art
Conventional thin film electroluminescent (EL) devices generally comprise an inorganic material, such as a group II-VI compound semiconductor, e.g., ZnS, CaS or SrS, doped with Mn or a rare earth element (e.g., Eu, Ce, Th or Sm) as a luminescence center. EL devices made of these inorganic materials have such disadvantages as (1) necessity of alternating current drive (50 to 1000 Hz), (2) a high driving voltage (up to 200 V), (3) difficulty of full color light emission (particularly in blue), and (4) high cost of peripheral driving circuits.
To eliminate these disadvantages, EL devices using an organic thin film have recently been developed. In particular, in order to raise luminescence efficiency, the kind of an electrode has been optimized for improving efficiency in carrier injection from an electrode. Further, an organic electroluminescent device having a hole transport layer comprising an aromatic diamine and a luminescent layer comprising an aluminum complex of 8-hydroxyquinoline has been developed (
Appl. Phys. Lett.,
vol. 51, p. 913 (1987)). Thus, organic EL devices have shown great improvements on luminescence efficiency over conventional ones comprising single crystals of anthracene, etc. to gain characteristics approaching the level meeting practical use.
In addition to the electroluminescent devices using the above-described low-molecular-weight materials, those using high-molecular-weight materials such as poly(p-phenylenevinylene), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], and poly(3-alkylthiophene), and those using high-molecular-weight materials, such as polyvinylcarbazole, mixed with low-molecular light-emitting materials and electron transfer materials have also been developed.
Under these circumstances, the outstanding objects relating to organic electroluminescent devices include improvement in driving stability and reduction of driving voltage.
That is, it is a great problem for a light source, such as a backlight of facsimiles, copiers and liquid crystal displays, that an organic electroluminescent device requires a high driving voltage and has low driving stability including heat resistance. This is especially undesirable for such display devices as full color flat panel displays.
Factors of driving instability of organic electroluminescent devices include reduction of luminescent brightness, voltage increase in constant current drive, and development of non-light-emitting parts (dark spots). While there are a number of causes of these instability factors, deterioration of the cathode material, particularly the interface at the light-emitting side of the cathode seems to be a chief cause. In an organic electroluminescent device a metal of low work function, such as a magnesium alloy or calcium, is usually used as a cathode material in order to facilitate electron injection from the cathode into the organic layer. Such a metal is susceptible to oxidation with moisture in air, which is a large factor of driving instability. An electrode made of a low work function metal, while effective in lowering the driving voltage, needs improvements to overcome the above-mentioned instability.
On the other hand, a cathode comprising aluminum containing 0.01 to 0.1 part by weight of metallic lithium has been proposed (an unexamined published Japanese patent application 5-121172). Formation of this cathode requires strict control on the metallic lithium content. However, it is technically difficult to form a cathode layer of an aluminum-lithium alloy having a desired composition by binary vacuum deposition using aluminum and metallic lithium as independent deposition sources. It is conceivable to form a cathode by electron beam deposition or sputtering using a previously prepared pellet or target of an aluminum-lithium alloy having a desired composition. This method, however, involves a practical problem that the composition of the aluminum-lithium alloy deposition source will vary as film formation is repeated due to the differences between lithium and aluminum in vapor pressure or sputtering efficiency. Besides, use of lithium is disadvantageous in that metallic lithium atoms are apt to diffuse into the adjoining organic layer, causing extinction of luminescence and that lithium atoms are so sensitive to moisture that a device having a lithium-containing cathode strictly demands high accuracy of sealing.
A cathode comprising an aluminum alloy containing 6 mol % or more of lithium is also disclosed (an unexamined published Japanese patent application 4-212287). With this cathode, too, a device requires a strict protective film on account of the above-mentioned instability of metallic lithium atoms and cannot get rid of the instability due to diffusion of lithium atoms.
A cathode made of aluminum metal mixed with an alkali metal fluoride has been reported (
Appl. Phys. Lett.
, vol. 73, p. 1185 (1998)), which gives no considerations for device stability.
A two-layered cathode having Li
2
O and Al in independent layers has been proposed (
IEEE Transactions on Electron Devices
, vol. 44, No. 8, pp. 1245-1248 (1997)). In this technique, however, because a very thin film of 0.5 to 1.5 nm is used as a cathode interfacial layer, it appears that the film may fail to completely cover the organic layer, and reproducibility seems insufficient. Additionally Li
2
O has poor adhesion to an organic layer as compared with Al and may cause formation of dark spots.
Thus, cathode materials heretofore proposed for organic electroluminescent devices include aluminum metal alloyed with lithium or mixed with a lithium compound as stated, but none of them is effective in improving driving stability and reducing a driving voltage, or they involve a practical problem in the process for production.
SUMMARY OF THE INVENTION
An object of the present invention is to settle the above-described problems and to provide an organic electroluminescent device which can be driven at a low voltage with a high luminescence efficiency, maintains stable luminescence characteristics for an extended period of time, and exhibits excellent resistance to heat and weather and a process for easily producing such an organic electroluminescent device without requiring strict condition control.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The organic electroluminescent device according to the present invention comprises a substrate having a luminescent layer provided between an anode and a cathode, which is characterized in that the cathode comprises a metallic material, an alkali metal, and an oxygen atom.
The present inventors have conducted extensive studies, seeking for an organic electroluminescent device that exhibits excellent resistance to heat and weather, emits light with high brightness at a low voltage, retains stable luminescence characteristics in driving, and can be produced under a broad range of process conditions. As a result, they have found that the above objects are accomplished by making the cathode of a metallic material containing an alkali metal and an oxygen atom, and completed the present invention.
In the present invention, incorporation of an alkali metal into a cathode makes it possible to lower the work function of the cathode thereby to reduce the energy barrier of the cathode interface for electron injection. As a result, there is produced an effect on lowering the driving voltage of the device. Further, existence of oxygen atoms suppresses diffusion of alkali metal atoms such as lithium atoms into an adjacent layer. Because it already contains oxygen, the cathode is chemically stabilized against the external environment, such as an oxidizing environment. In other words, part of oxygen introduced into the cathode is bonded to the metallic material and/or the alkali metal to for
Sato Yoshiharu
Tanamura Mitsuru
Garrett Dawn
Kelly Cynthia H.
Mitsubishi Chemical Corporation
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