Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-07-21
2001-12-11
Yamnitzky, Marie (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C313S504000, C313S506000
Reexamination Certificate
active
06329085
ABSTRACT:
FIELD OF THE INVENTION
This invention relates, in general, to multicolor organic light emitting devices (OLED's) and, more particularly, to combinations of dopants and host compounds that enable the generation of a saturated red emission suitable for use in devices such as, for example, flat panel electronic displays.
BACKGROUND OF THE INVENTION
Electronic display devices are becoming an increasingly indispensable tool in modern society for the delivery of visual information. These devices find widespread utility in television sets, computer terminals, and in a host of related applications. No other type of technology offers comparable speed, versatility and potential for interactivity. Current electronic display technologies include, for example, cathode ray tubes (CRT's), plasma displays, light emitting diodes (LED's), thin film electroluminescent displays, and the like.
The most widely-used non-emissive technology for display devices makes use of the electro-optic properties of a class of organic molecules known as liquid crystals in fabricating liquid crystal displays (LCD's). LCD's operate fairly reliably, but are limited by relatively low contrast, low speed, and fade-out when viewed from oblique angles, as well as the requirement for high power backlighting. Active matrix displays, in a partial solution to these shortcomings, employ an array of transistors, each capable of activating a single liquid crystal pixel, thus improving contrast.
There is no doubt that flat panel display technology is of significant scientific and commercial interest. Consequently, it is the subject of extensive ongoing research. See Depp, S. W. and Howard, W. E., “Flat Panel Displays,”
Scientific American,
March 1993, pps. 90-97. According to Depp and Howard, by 1995, flat panel displays alone were expected to generate a market of between $4 and $5 billion. Key to the success of any potential display technology in this market is the ability to both provide a high resolution, full-color display at good light level and, at the same time, to be competitively priced.
Organic thin film materials represent a technical development that has demonstrated considerable progress in the fabrication of red, green and blue light emitting devices. These organic light emitting devices have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, et al.,
Laser Focus World,
Feburary 1995). Furthermore, since many of the organic thin films used in such devices are transparent in the visible spectral region, they potentially allow for the realization of a completely new type of display pixel in which the red (R), green (G), and blue (B) emission layers are placed in a vertically stacked geometry to provide a simple fabrication process, minimum R-G-8 pixel size, and maximum fill factor.
Disclosed in U.S. Pat. No. 5,294,869 to C. W. Tang and J. E. Littman is a concept for using separate, side-by-side red, green, and blue OLED's to make a full color display. However, it is believed by the inventors of the instant disclosure that such concepts have never been successfully realized in a practical device.
Such schemes suffer from a complex layer structure, and lack of known methods for damage-free, post-deposition patterning of organic layers at the resolution required for color displays. Others have alternatively suggested using an array of white OLED's (J. Kido, et al.,
Science
267, 1332 (1995)) backed by side-by-side R, G and B color filters deposited and patterned prior to OLED growth. However, such a design sacrifices at least 66% of the light from each white OLED, with the remainder being absorbed in the filter, also generating heat. Such a design suffers, therefore, from low efficiency and conditions of accelerated degradation. Alternative schemes based on microcavity filtering of a broad-spectrum OLED (A. Dodabalapur, et al.,
Appl. Phys. Lett.
64, 2486 (1994)) suffer from complex and expensive substrate patterning requirements and extremely limiting directionality of the resulting color pixels.
An example of a multicolor electroluminescent image display device employing organic compounds for light emitting pixels is disclosed in Tang et al., U.S. Pat. No. 5,294.870. This patent discloses a plurality of light emitting pixels which contain an organic medium for emitting blue light in subpixel regions. Fluorescent media are laterally spaced from the blue-emitting subpixel region. The fluorescent media absorb light emitted by the organic medium and, in turn, emit red and green light in different subpixel regions. The use of materials doped with fluorescent dyes to emit green or red on absorption of blue light from the blue subpixel region is less efficient than direct formation via green or red LED's. The reason is that the efficiency will be the product of (quantum efficiency for EL) and (quantum efficiency for fluorescence) and (1-transmittance). Thus, a drawback of this display and all displays of this type is that different laterally spaced subpixel regions are required for each color emitted.
Color-tunable OLED's potentially allow for full-color operation without the complex structures common to other types of devices. Published examples of tunable OLED's utilize a blend of either two polymers (M. Granstrom and O. Inganas,
Appl. Phys. Lett.
68, 147 (1996)) or a polymer doped with semiconductor nanocrystallites (B. O. Dabbousi, et al.,
Appl. Phys. Lett.
66, 1316 (1995); V. L. Colvin, et al.,
Nature
370, 354 (1994)). Each component of the blend emits radiation having a different spectral energy distribution. The color is tuned by varying the applied voltage. A higher voltage results in more emission from the higher bandgap polymer, which emits radiation toward the blue region of the-spectrum, while also resulting in higher overall brightness due to increased current injection into the device. Although tuning from orange to white has been demonstrated, incomplete quenching of the low-energy spectral emission appears to prohibit tuning completely into the blue. In addition, emission intensity can only be controlled by using pulsed current and reduced duty cycles. In a color display, therefore, prohibitively high drive voltages and very low duty cycles may be necessary for blue pixels. This necessitates a complex driver circuit, renders passive matrix operation extremely difficult, if not impossible, and is likely to accelerate degradation of the display.
A transparent organic light emitting device (TOLED) which represents a first step toward realizing high resolution, independently addressable stacked R-G-B pixels has been reported recently in the published international Pat. application No. WO 96/19792. This TOLED had greater than 71% transparency when turned off, and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode layer, and a Mg:Ag-ITO layer for electron injection. A device was disclosed in which the Mg:Ag-ITO electrode was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each device in the stack was independently addressable and emitted its own characteristic color through the transparent organic layers, the transparent contacts and the glass substrate allowing the entire device area to emit any combination of color that could be produced by varying the relative output of the two color-emitting layers.
Thus, for the specific device disclosed in WO 96/19792, which included a red-emitting layer and a blue-emitting layer, the color output produced by the pixel could be varied in color from deep red through blue.
It is herein believe that WO 96/19792 provided the first demonstration of an integrated OLED where both intensity and color could be independently varied by using external current sour
Burrows Paul
Forrest Stephen R.
Thompson Mark E.
Kenyon & Kenyon
The Trustees of Princeton University
Yamnitzky Marie
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