Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
1998-07-06
2001-04-10
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S503000, C428S917000
Reexamination Certificate
active
06215245
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to an organic electroluminescent light emitting device (which will hereinafter be often called an organic EL device for short) using an organic compound, and more particularly to an cathode for feeding electrons to a light emitting layer.
BACKGROUND TECHNIQUE
In recent years, organic EL devices have been under intensive investigation. One such organic EL device basically includes a transparent electrode (anode) of tin-doped indium oxide (ITO), etc. A thin film is formed on the transparent electrode by evaporating a hole transporting material such as tetraphenyldiamine (TPD). A light emitting layer of a fluorescent material such as an aluminum quinolinol complex (Alq
3
) is deposited on the hole transporting thin film. An electrode (cathode) is formed thereon from a metal having a low work function such as magnesium or Mg. This organic EL device attracts attentions because it can achieve a very high luminance ranging from several hundreds to tens of thousands cd/m
2
with a voltage of approximately 10 volts.
A cathode considered to be effective for such organic EL devices is made up of a material capable of injecting more electrons into the light emitting layer. In other words, the lower the work function of a material, the more suitable is the material as the cathode. Various materials having a low work function are available. For instance, the materials which are used as the cathodes of organic EL devices generally include MgAg, and AlLi disclosed in JP-A 4-233194. The reason is that the production process for organic EL devices relies mainly upon evaporation making use of resistance heating, and so the evaporation source used therewith is as a matter of course limited to one having a high vapor pressure at low temperatures. A cathode produced by such an evaporation process making use of resistance heating is poor in its adhesion to the interface between the cathode and the organic layer. Consequently, non-light emitting spots called dark spots occur on pixels just after production. The dark spots become large as the device is driven, and so become a leading factor that governs the service life of the device.
The aforesaid JP-A 4-233194 describes that alkali earth metals, and rare earth metals having a low work function are suitable for cathodes, and adds that alkali metals are excluded from electrode materials because they are too unstable for use as electrodes.
On the other hand, JP-A 4-212287 discloses that relatively stable alloys comprising alkali metal elements and other metals and having a low work function are usable as cathodes. More specifically, the cathode alloys disclosed therein are composed mainly of alkali metal elements such as Li, Na, and K, and more stable other metals such as Mg, Al, In, Sn, Zn, Ag, and Zr. Here, the alkali metal elements such as Li, Na, and K are stabilized by alloying with more stable metals such as Mg, Al, In, Sn, Zn, Ag, and Zr. However, the examples given therein reveal that alloying is carried out by co-evaporation. It is thus believed that the aforesaid stabilizing metals, too, are limited from the standpoint of vapor pressure. Furthermore, the organic EL element set forth therein fails to solve the aforesaid problem because the constituting films are all formed by evaporation.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to achieve an organic EL device comprising a cathode which is improved in terms of its interfacial adhesion to an organic layer and electron injection efficiency, and so makes a contribution to improvements in light emission properties and causes less damage to the organic layer, and which reduces the occurrence of dark spots and suffers from little or no degradation of performance.
Such an object is achieved by the inventions defined below as (1) to (
9).
(1) An organic EL device comprising a film form of cathode prepared by a sputtering technique, and formed of an alloy of sodium and/or potassium.
(2) The organic EL device according to (1), wherein said cathode has a concentration gradient in a thickness-wise direction thereof, said concentration gradient having a higher concentration of sodium and/or potassium on an interface side between said cathode and an organic layer.
(3) The organic EL device according to (1) or (2), wherein said cathode further comprises an aluminum alloy layer composed mainly of aluminum on a side thereof that is not opposite to said interface between said cathode and an organic layer.
(4) The organic EL device according to any one of (1) to (3), wherein said alloy is an alloy of sodium and/or potassium with at least one element selected from a group consisting of a transition metal element, aluminum, gallium, indium, and titanium.
(5) The organic EL device according to any one of (1) to (4), wherein said transition metal element is chromium, iron, cobalt, nickel, copper, zinc, rubidium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, lanthanum, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, polonium, and thorium.
(6) The organic EL device according to any one of (3) to (5), wherein said aluminum alloy layer having an aluminum content of at least 80 at %.
(7) A method of fabricating an organic EL device, wherein a film form of cathode as recited in any one of (1) to (6) is prepared by a sputtering technique under a film forming condition where a product of a film forming gas pressure and a substrate-to-target distance is 20 to 65 Pa.cm.
(8) The method according to (7), wherein at least one gas selected from a group consisting of argon, krypton, and xenon is used as a film forming gas.
(9) The method according to (7) or (8), wherein said sputtering technique is a DC sputtering process.
REFERENCES:
patent: 3173050 (1965-03-01), Gurnee
patent: 4885211 (1989-12-01), Tang et al.
patent: 5923119 (1999-07-01), Hara et al.
patent: 60-165771 (1985-08-01), None
patent: 2-15595 (1990-01-01), None
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Patel Ashok
TDK Corporation
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