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New polymers shine in rainbow patterns.

Scientists manipulating the chemical makeup of polymers have expanded the potential of plastics for making lightemitting diodes in a rainbow of colors. These materials could lead to billboardsize, mutlicolor electronic displays, higher resolution in portable computer screens and new devices for guiding light.

Polymer processing can be fine-tuned to alter the ability of semiconducting polymers to emit light then subjected to an electrical current, Paul L. Burn and his colleagues from the University of Cambridge in England report in the March 5 Nature.

These phsycists and chemists first discovered electroluminescent polymers in late 1990. Now they've created several new materials that work up to 30 times better than their original one and as well as the inorganic materials currently used in light-emitting diodes, they add.

The British team uses a conjugated polymer--every other carbon atom in the polymer's extensive backbone chain connects via a double bond, thereby creating a route along which electrons can travel. They make the polymer from building blocks, called monomers, that are poorly conjugated; their second bonds instead usually connect to side groups of atoms. By knocking off these side groups, chemists cause more double connections to form.

Burn and his colleagues put different side groups on the building blocks. By kicking off some or all of these side groups while joining the building blocks and by varying the ratio of monomers, they adjust the degree and spacing of conjugation, and consequently the color of light emitted. "The secret is in the conversion," says Andrew B. Holmes, a synthetic organic chemist who works with Burn. Polymers with longer conjugated sections emit redder lihgt, while those with lots of side groups tend to glow yellow.

Physicist Richard H. Friend and his colleagues at the Cavendish Laboratory in Cambridge have fashioned this new material into prototype light-emitting diodes with patterns of colors.

"They've made a combination of [chemical] structures and used them in a creative way. It saves steps in patterning," comments physicist-chemist Alan J. Heeger, who develops electroluminescent polymers at the University of California, Santa Barbara.

To achieve electroluminescence, researchers place a thin film of these polymers between two electrodes. One electrode sends electrons into the film, creating negative charges that traverse the polymer. The other electrode pulls electrons out of the film, leaving positive charges ("holes") that drift toward the negative side, Holmes explains. If an electron encounters a hole, the polymer gets excited briefly and then emits light as it relaxes again.

The British researchers' newfound chemical finesse allows them to be more effective in getting opposite charges together. They use building blocks that slow down positive charges, says Holmes. Also, interruptions in conjugation along the polymer's chain can block migration. Thus, more electrons meet holes, and the polymer becomes more efficient at emitting light-in theory, up to 25 percent efficient, compared with less that 1 percent now.

So far, the British group has created polymers that emit yellow-green, orangered or blue-green light, and Austrian scientists recently made a different polymer glow blue, says Holmes.

"Things are looking most promising," he says. "We think our polymer can be made up in as large an area as you need, and I think there's lots of opportunity for higher resolution."
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Title Annotation:semiconducting polymers
Author:Pennisi, E.
Publication:Science News
Date:Mar 14, 1992
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