Researchers have created the first light-emitting diode (LED) made of polymers that can be treated with everyday chemical techniques to emit distinct patterns of light. The development opens the door for the evolution of plastic LEDs from humble indicator lights on electronic devices into increasingly sharp and sophisticated displays -- even LED-based computer monitors or television screens. The new LED was profiled in a recent cover article of the journal Advanced Materials.
To make the LED, researchers at the University of Rochester and Hewlett Packard's Solid State Technology Lab in Palo Alto, Calif., put to work on polymers the same microlithographic technique used to emblazon silicon circuitry -- essentially using silicon techniques to make the plastics that might someday supplant silicon as the material heart of electronics. It's the first time that this method of silicon circuit production has crossed over to polymer devices, and signals another step in plastic's emergence as a serious candidate for use in the electronics of the future.
"With this work, we've cracked the problem of how to produce patterns of light with a single LED," says lead researcher Guillermo Bazan, an associate professor of chemistry at Rochester. "And since we've used the same technique that has already been used to mass-produce many millions of integrated circuits, we know that it's a very easy and efficient way to process materials."
LEDs are optoelectronic devices able to convert electricity into light. Such devices are found in digital displays all around us -- in alarm clocks, dashboard displays, kitchen appliances, and machines ranging from calculators to CD players and photocopiers. Controlling precisely how they shine is crucial to their performance.
"The control we've exerted over the spatial arrangement of illumination should play an important role in broadening the applications of LEDs," Bazan says. "Further development of this technology will bring about the levels of definition and sophistication necessary if plastic is to be used to make the tiny glowing pixels of a television or computer monitor. Current LEDs just aren't small enough to provide the kind of resolution you'd want for a TV screen or computer monitor."
A plastic-based television or computer screen would be a significant advance in compactness and cost over the cathode ray tubes now found in most computers and televisions, says Bazan.
Bazan, Rochester graduate student Michelle Renak, and HP researcher Daniel Roitman made their patterned LED by implanting a poly(paracyclophene) film with a compound called triphenylsulfonium trifluoromethanesulfonate, a photoacid generator that produces triflic acid when exposed to light. When the film reacts with the acid, it's converted to poly(p-phenylenevinylene), which has the light-emitting properties characteristic of an LED and is one of the best-conducting plastics known.
Scientists produce patterns of light by placing a "mask" -- essentially a cover with a pattern of holes cut into it -- over the light-sensitive polymer mix before it's exposed to light. When the light shines through the mask, the triflic acid produced remains localized in the spots that were illuminated, and distinct patterns of light-emitting regions are created. So precise is the process that Bazan's LED emits individual pinpoints of light only five thousandths of a millimeter in width.
"Some people have tried to pattern both polymer and non-polymer LEDs before, but their techniques were prohibitively complex," Bazan says. "Ours is the first group to create the kind of polymer mixes needed for patterning that takes advantage of technology already widely in use."
Bazan and his collaborators have moved on to working on an LED that can emit pinpoints of different colors of light. The microlithographic technique might also be used someday to make polymer microchips by patterning conducting grids on polymers, much as silicon chips are now made.
The work is sponsored by the U.S. Office of Naval Research.