Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
1998-07-28
2001-01-09
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S504000, C428S690000
Reexamination Certificate
active
06172459
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to organic light-emitting devices. More particularly, this invention relates to devices which have an electron-injecting layer providing an interface between an light-emitting structure and an organic cathode buffer layer.
BACKGROUND OF THE INVENTION
Organic light-emitting devices, also referred to as organic electroluminescent (EL) devices or as organic internal junction light-emitting devices, contain spaced electrodes separated by an organic light-emitting structure (also referred to as an organic EL medium) which emits electromagnetic radiation, typically light, in response to the application of an electrical potential difference across the electrodes. The organic light-emitting structure must not only be capable of producing light efficiently, but must be capable of fabrication in a continuous form (i.e., must be free of pinholes and particle defects) and must be sufficiently stable to facilitate fabrication and to support operation.
Initially organic EL devices were fabricated using single crystals of organic materials, as illustrated by Mehl et al U.S. Pat. No. 3,530,325, and Williams U.S. Pat. No. 3,621,321. Single organic crystal EL devices were relatively difficult to fabricate and further did not readily lend themselves to thin film constructions.
In recent years, preferred organic EL devices have been constructed employing thin film deposition techniques. Using an anode as a device support, the organic electroluminescent medium has been deposited as one or a combination of thin films followed by the deposition of a cathode, also formed as a thin film deposition. Thus, starting with the anode structure, it is possible to form the entire active structure of an organic EL device by thin film deposition techniques. As employed herein, the term “thin film” refers to layer thicknesses of less than 5 &mgr;m, with layer thicknesses of less than about 2 &mgr;m being typical. Examples of organic EL devices containing organic electroluminescent medium and cathode constructions formed by thin film deposition techniques are provided by Tang U.S. Pat. No. 4,356,429, VanSlyke et al U.S. Pat. Nos. 4,539,507 and 4,720,432, and Tang et al U.S. Pat. No. 4,769,292.
While the art has encountered little difficulty in constructing fully acceptable stable anodes for internal junction organic EL devices, cathode construction has been a matter of extended investigation. In selecting a cathode metal, a balance must be struck between metals having the highest electron-injecting efficiencies and those having the highest levels of stability. The highest electron-injecting efficiencies are obtained with alkali metals, which are too unstable for convenient use, while metals having the highest stabilities show limited electron injection efficiencies and are, in fact, better suited for anode construction.
Tang U.S. Pat. No. 4,356,429 teaches to form cathodes of organic EL devices of metals such as indium, silver, tin, and aluminum. VanSlyke et al U.S. Pat. No. 4,539,507 teaches to form the cathodes of organic EL devices of metals such as silver, tin, lead, magnesium, maganese, and aluminum. Tang et al U.S. Pat. No. 4,885,211 teaches to form the cathodes of organic EL devices of a combination of metals, with at least 50 percent (atomic basis) of the cathode being accounted for by a metal having a work function of less than 4.0 eV. VanSlyke U.S. Pat. No. 5,047,607 teaches the use of a cathode containing a plurality of metals, at least one of which is a low work function metal other than an alkali metal. Overlying the cathode is a protective layer comprised of a mixture of at least one organic component of the organic EL medium and at least one metal having a work function in a range of from 4.0 to 4.5 eV, and capable of being oxidized in the presence of ambient moisture.
While it has been contemplated to form cathodes over the organic EL medium from the combination of lower work function (<4.0 eV) electron-injecting metals and higher work function (>4.0 eV) more stable metals by conventional vapor deposition or by high energy sputter deposition or electron beam deposition, high energy deposition has not evolved thus far as a practical approach to form cathodes. It has been observed that electron bombardment and/or ion bombardment of the organic EL medium during sputter deposition or electron beam deposition of a cathode introduces damage into the EL medium. The damage is evidenced by substantially degraded electroluminescence performance of a device when compared to the electroluminescence performance of a device having a cathode formed by conventional thermal vapor deposition.
Thus, although cathodes formed over organic light-emitting structures by sputter deposition or electron beam deposition offer potential advantages of improved adhesion and step coverage, such advantages have not been realized due to the damaging effects related to high energy deposition.
Similarly, although cathodes made of a high-work-function metal are thermodynamically stable, such advantages have not been realized due to their limited electron injection efficiencies.
SUMMARY OF THE INVENTION
It is an object of the present invention to minimize or eliminate damage introduced into an organic light-emitting structure or into an organic EL medium during high energy deposition of a cathode or cathodes and to utilize metals or metal oxides with a high-work function to form an effective cathode.
It is another object of the present invention to provide an organic cathode buffer layer as a protection layer against damage of the organic light-emitting structure during high energy deposition of a cathode over the buffer layer and to provide an electron-injecting layer underlying the cathode buffer layer to improve device performance.
It is a still further object of the invention to provide an organic light-emitting device which has an organic light-emitting structure formed over an anode, an electron-injecting layer formed over the light-emitting structure, an organic cathode buffer layer disposed over the electron-injecting layer, and a cathode disposed over the cathode buffer layer.
These objects are achieved in an organic light-emitting device comprising:
a) a substrate;
b) an anode disposed over the substrate;
c) an organic light-emitting structure disposed over the anode;
d) a cathode buffer layer disposed over the organic light-emitting structure and of a material selected to permit high energy deposition of a cathode;
e) a cathode disposed over the cathode buffer layer; and
f) an interfacial layer provided with an electron-injecting material between the organic light-emitting structure and the cathode buffer layer.
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Hung Liang S.
Raychaudhuri Pranab K.
Tang Ching W.
Eastman Kodak Company
Guharay Karabi
Owens Raymond L.
Patel Nimeshkumar D.
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