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Date: June 27, 1997
Research Team Advances
Display Technology for Television
and Computers
PRINCETON, N.J. -- A June 27 Science article reports that a team of five scientists at Princeton University and the University of Southern California (USC) has developed an independently controlled, three-color organic light-emitting device. This advance adds finely tuned color to a technology developed by the same team last year, which is expected to permit such applications as high-definition televisions with flat displays that hang on a wall like a painting and laptop computers with bright displays that consume considerably less energy.
The new technology uses a vertically stacked pixel architecture that allows for independent tuning of color, grayscale, and intensity. Each color element -- the primary colors red, blue, and green -- can be continuously and independently varied, allowing the device to emit any mixture of the constituent colors. By contrast, the standard cathode-ray tube currently used in televisions and computer screens uses requires pixels comprising side-by-side red, green, and blue phosphors; the eye achieves full color by fusing the primary shades. The stacked design should allow for the creation of bright displays with more intense, true color and higher resolution than possible with the traditional side-by-side phosphors: "The vertically stacked pixel offers the minimum possible R-G-B pixel size and maximum fill factor (defined as the percentage of the display occupied by light-emitting pixels)," say the authors in the Science article.
This stacked organic light-emitting device (SOLED) consists of 12 sequential layers of metal oxide, amorphous organic, crystalline organic and metal thin films deposited by a combination of thermal evaporation and radio frequency sputtering.
Last year, the same team announced the development of a transparent organic light-emitting device (TOLED), the technology that provides the building blocks for the new design. A TOLED emits light like a cathode-ray tube, but is made out of thin organic films and is therefore especially good for making flat displays. The new design stacks red, green and blue TOLEDs and allows them to be independently tuned, such that the shades of differently colored light passing through the device can create a full-color spectrum on the screen. Each of the three colors is sufficiently bright to allow flat-panel video display applications.
"The real achievement here is a manipulation of materials that only appears possible with organics," said Stephen Forrest, who is James S. McDonnell Distinguished University Professor of Electrical Engineering at Princeton. "Organics do not have to be crystalline to be deposited on a substrate. This fact allows extraordinarily thin layering and enables us to capture their transparency to radiation."
The display market is currently estimated at $30 billion annually worldwide, comprised mostly of cathode-ray tube and liquid-crystal display (LCD) technologies. TOLEDs are seen as a replacement technology for LCDs; TOLEDs are emissive and energy efficient, whereas LCDs display by blocking emitted light, and so use more energy. Monochromatic TOLEDs are currently in manufacture for such uses as car stereo displays. The next developmental stage for the SOLED technology will be to refine the materials, as well as the engineering of the stack, so as to introduce imaging and improve color definition.
In addition to Forrest, the authors of the Science paper include two Princeton graduate students in electrical engineering, Vladimir Bulovic and Zilan Shen, research scholar Paul Burrows, and Mark E. Thompson, an associate professor of chemistry at USC.
Funding for the research came from Universal Display Corp., DARPA and the Air Force Office of Scientific Research. Transfer of the TOLED technology to industry is expedited by Princeton's Advanced Technology Center for Photonics and Optoelectronic Materials, which receives support from the New Jersey Commission on Science and Technology.