Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2000-08-11
2003-06-17
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C313S504000, C313S506000, C313S503000
Reexamination Certificate
active
06579629
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to organic light-emitting diode devices and methods for making such devices, which use a sputtered cathode layer.
BACKGROUND OF THE INVENTION
Organic electroluminescent device (OEL device), alternately known as organic light emitting diode (OLED), is useful in flat-panel display applications. This light-emissive device is attractive because it can be designed to produce red, green, and blue colors with high luminance efficiency; it is operable with a low driving voltage of the order of a few volts and viewable from oblique angles. These unique attributes are derived from a basic OLED structure comprising of a multilayer stack of organic thin films sandwiched between an anode and a cathode. Tang et al in U.S. Pat. Nos. 4,769,292 and 4,885,211 has disclosed such a structure. A common OLED structure includes of a bi-layer organic stack of a hole-transport layer and an electron transport layer, typically of the order of a few hundred angstroms thick for each layer. The anode material is usually an optically transparent indium-tin oxide (ITO) glass, which also serves as the substrate for the OLED. The cathode is usually a metallic thin film.
In the fabrication of OLED vapor deposition is used. Using this method, the organic layers are deposited in thin-film form onto the ITO glass substrate in a vacuum chamber, followed by the deposition of the cathode layer. Among the deposition methods for the cathode, vacuum deposition using resistive heating or electron-beam heating has been found to be most suitable because it does not cause damage to the organic layers. However, it would be highly desirable to avoid these methods for fabrication of the cathode layer. This is because they are inefficient processes. In order to realize low cost manufacturing one must adopt and develop a proven robust high throughput industrial process specific to OLEDs fabrication. Sputtering has been used as such a method of choice for thin film deposition in many industries. Conformal, dense, and adherent coatings, short cycle time, low maintenance of coating chamber, efficient use of materials are among few of the benefits of sputtering.
The fabrication of the OLED cathode layer employing high-energy deposition process such as sputtering is not commonly practiced because of the potential damage inflicted on the organic layers, and thus degradation of the OLED performance. Sputter deposition takes place in a complex environment that comprises of energetic neutrals, electrons, positive and negative ions and emissions from the excited states that can degrade the organic layers upon which the cathode is deposited.
Liao et. al (Appl. Phys. Lett. 75,1619 [1999]) investigated using x-ray and ultraviolet photoelectron spectroscopies the damages induced on Alq surfaces by 100 eV Ar+irradiation. It is revealed from core level electron density curves that some N—Al and C—O—Al bonds in Alq molecules were broken. The valance band structure is also tremendously changed implying the formation of a metal-like conducting surface. It is suggested that this would cause nonradiative quenching in OLEDs when electrons are injected into the Alq layer from the cathode and also would results in electrical shorts.
During sputter deposition of cathode the Alq surface is subjected to high doses of Ar
+
bombardments at several hundred volts. As shown by Hung et. (J. Appl. Phys. 86, 4607 [1999]) that a dose only of9×10
14
/cm
2
altered the valance band structure. However, sputtering a cathode on Alq surface in Ar atmosphere would degrade the device performance.
Sputtering damage is somewhat controllable, at least to some extent, by properly selecting the deposition parameters. In the European patent applications EP 0 876 086 A2, EP 0 880 305 Al, and EP 0 880 307 A2, Nakaya et al. of TDK Corporation disclose a method of depositing a cathode by a sputtering technique. After depositing all organic layers, with vacuum still kept, the devices was transferred from the evaporation to a sputtering system wherein the cathode layer was deposited directly on the emission layer. The cathode was an Al alloy comprised of 0.1-20 a % Li that additionally contained at least one of Cu, Mg and Zr in small amounts and in some cases had a protective overcoat. The OLED devices thus prepared using no buffer layer were claimed to have good adhesion at the organic layer/electrode interface, low drive voltage, high efficiency and exhibited a slower rate of development of dark spot. Grothe et al. in patent application DE 198 07 370 C1 also disclose a sputtered cathode of an Al:Li alloy which had relatively high Li content and having one or more additional elements chosen from Mn, Pb, Pd, Si, Sn, Zn, Zr, Cu and SiC. In all of those examples no buffer was used, yet electroluminescent was produced at lower voltage. Some sputtering damage was possibly controlled by employing a low deposition rate. It is easily anticipated that by lowering sputtering power the damage inflicted on the organic layers can be reduced. At low power, however, the deposition rate can be impracticably low and the advantages of sputtering are reduced or even neutralized.
To realize high speed sputtering a plasma resistant coating on electron transport/emissive layer may necessary. It is known that a layer containing robust molecules can be effective in significantly reducing the damage inflicted on to emissive and other underlying layers during cathode sputtering deposition. The buffer layer, however, in addition to being resistant to plasma, must not interfere with the operation of the device and must preserve the device performance as much as possible. Hung et. Al (J. Appl. Phys. 86, 4607 [1999]) disclosed the application a cathode buffer layers that permitted high-energy deposition of a cathode. The cathode contained a dopant, e.g. Li, which was believed to diffuse through the buffer layer and provided an electron injecting layer between the organic light emitting structure and the buffer layer. In the patent application EP 0 982 783 A2 Nakaya et al. disclose a cathode of Al:Li alloy. The cathode was prepared by sputtering using a buffer layer constructed of a porphyrin or napthacene compound that was deposed between the emission and the cathode. The device containing the sputtered electrode exhibited low drive voltage, high efficiency and retarted dark spot growth.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an OLED device structure, which has reduced sputtering damage.
The above object was achieved by using sputtering as the method for deposition of the OLED cathode and an OLED device comprising:
a) a substrate;
b) an anode formed of a conductive material over the substrate;
c) an emissive layer having an electroluminescent material provided over the anode;
d) a buffer structure including at least two layers, a first buffer layer provided over the emissive layer and containing an alkaline halide and a second buffer layer provided over the first buffer layer and containing phtlalocyanine; and
e) a sputtered cathode layer having an alloy containing an alkaline metal provided over the buffer structure.
An advantage of the present invention is that cathode sputtering damage can be reduced in OLED devices and displays. The method permits high and uniform deposition rate and is suitable for high process throughput and large-area substrates.
The buffer structure in accordance with the invention having two buffer layers exhibited substantially superior performance in comparison to that of devices having only one buffer layer but otherwise identical in structure.
Another advantage of the present invention is that OLED devices produced by the sputtering deposition method are efficient and operable with a low drive voltage.
REFERENCES:
patent: 4720432 (1988-01-01), VanSlyke et al.
patent: 4769292 (1988-09-01), Tang et al.
patent: 4885211 (1989-12-01), Tang et al.
patent: 5645948 (1997-07-01), Shi et al.
patent: 5776623 (1998-07-01), Hung et al.
patent: 593
Madathil Joseph K.
Raychaudhuri Pranab K.
Tang Ching W.
Garrett Dawn
Kelly Cynthia H.
Owens Raymond L.
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