University of Rochester

New Polymer Diode is the First to Emit Light of Many Colors

April 16, 1997

Materials scientists have developed a new polymer-based light-emitting diode (LED) that can change color with the ease of a chameleon. The device, the first plastic material and the first LED able to emit light of multiple colors, could make light- emitting plastics viable in a host of devices, including computer screens, television monitors, fluorescent lighting and even traffic lights. The work signals an era when plastic actually forms the electronic soul of modern devices and is no longer relegated to the outer shell.

Lead researcher Samson Jenekhe of the University of Rochester is presenting the work at a meeting of the American Chemical Society this week in San Francisco. The team, which also includes several of Jenekhe's colleagues at the University and a scientist at Xerox Corporation, published a paper on the work in the February issue of Chemistry of Materials.

LEDs convert electricity to light and are found in digital displays all around us: alarm clocks, dashboard displays, kitchen appliances, and machines ranging from calculators to CD players and photocopiers. But today's LEDs, made of materials like gallium nitride or gallium arsenide, are limited to single colors, making them impractical for the biggest application of all: flat-panel computer and television screens. Jenekhe's multi- color polymers raise the prospect of replacing today's bulky screens with much thinner and more efficient arrays of LEDs.

"The holy grail of the whole field of LED design is a flat- panel display for use in future television screens and computer monitors," says Jenekhe, a professor of chemical engineering, chemistry, and materials science at Rochester. "An LED that can readily change color is a big step toward replacing current technology."

The plastic LED outshines the performance of traditional materials in several ways, Jenekhe says. Most importantly, just one layer of the devices can create full-color images; scientists using today's devices would have to devise complex combinations of layers to produce full color. In addition, today's LEDs have a difficult time efficiently producing green and blue light, limiting image quality.

Other advantages of the plastic over current LED technology: It requires just three volts of electricity for start-up and is at least as bright as a current television screen. Since these plastics can be made at room temperature, the LEDs are also far cheaper to produce than conventional LEDs, which must be made at high temperatures.

Key to the work was the team's ability to play matchmaker by bringing together electrons and electron holes, which produce light when they meet. Jenekhe's group was able to construct layers of polymers just millionths of a centimeter thick and position them just right to supply a steady stream of electrons and holes. Several researchers have tried to make such LEDs, and some, including a group at Cambridge University, have succeeded in producing a device capable of emitting light, but only of a single color. Jenekhe's LEDs, made out of polyphenylenevinylene and polyquinoline, emit colors ranging from red to yellow to green and even blue, depending on the voltage applied to them. By combining the light from several plastic LEDs, Jenekhe has even produced white light, making possible LED-based fluorescent lighting.

Another potential application, Jenekhe says, is in the approximately 10 million traffic lights nationwide. A large, lightweight polymer LED could replace the heavy, inefficient 120- volt white bulbs that glow behind the colored glass covers of traffic lights everywhere. Because the LEDs consume so little electricity, this could result in substantial power savings.

The research is supported by the University's National Science Foundation Center for Photoinduced Charge Transfer and the Office of Naval Research. Jenekhe's collaborators in the research include Yongli Gao, associate professor of physics and astronomy at the University, Xerox researcher Bing R. Hsieh, and Rochester graduate students Xuejun Zhang, X. Linda Chen, and Vi- En Choong.

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