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Researchers Predict Multicolor LED World

Photonics Spectra
Jun 1997
Kathleen G. Tatterson

ROCHESTER, N.Y. -- Dreams of ultrathin displays for computer and television screens could become a reality, thanks to a plastic light-emitting diode that emits light in several colors.
LEDs are everywhere; clock radios, microwave timers, dashboard controls and other everyday items use the devices for display and decoration. Conventional diodes, however, are not suited for larger applications such as flat-panel displays and television sets, because they are limited to a single color. Manufacturers using standard GaAs or GaN devices would have to devise complex combinations of layers to produce full-color displays. Moreover, their inability to produce green and blue light efficiently limits image quality.
Samson Jenekhe and his group at the University of Rochester in New York envision replacing bulky screens with thin arrays of their efficient multicolor polymer diodes. "These things are feasible because we have made the range of materials possible," Jenekhe said.
He said one application for the diodes could be the 10 million traffic lights blinking nationwide, which now use heavy, inefficient 120-V white light bulbs that glow behind red, yellow and green glass covers. "It will be a long time coming, but the potential is there," he added.
Polymer LEDs present other advantages in performance as well as production. They require a mere 3 V of electricity for start-up and shine at least as brightly as a conventional television screen. Also, since manufacturers can make plastic LEDs at room temperature, the devices are much cheaper to produce than their conventional counterparts, which require a higher-temperature environment.

Layers of plastic
Jenekhe's team accomplished this by matching electrons with electron holes, generating light. It built layers of polyquinoline and polyphenylenevinylene polymers from 25 to 130 nm thick to supply a steady stream of electrons and holes. The researchers control the color of the light emitted by simply changing the voltage applied, typically between 3 and 20 V.
This is not the first time scientists have made light-emitting diodes out of plastic. Several institutions, such as Philips Research of Eindhoven, The Netherlands; Uniax Corp. of Santa Barbara, Calif.; and Cambridge University in the UK are working to develop and commercialize plastic LEDs, but the devices can transmit in only one color.
Research Corporation Technologies of Tucson, Ariz., is a not-for-profit organization that handles technology transfer, patents and licensing agreements for research universities. Eugene Cochran, associate of technology transfer, is negotiating with several companies that are interested in the group's work. Cochran said major chemical firms and signage and lighting companies have expressed interest in producing, packaging and distributing the technology. He hopes to achieve a licensing agreement in the next two years.



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