January 22, 2001 – Only a few percent of the electricity flowing into light bulbs comes out as visible light, with most of the rest emitted as heat. Devices called light-emitting diodes (LEDs) are more efficient and longer-lived. That is why the red and green lights of LEDs are increasingly common on home electronic equipment, automobile brake lights, traffic signals and even billboards on New York’s Times Square. White LEDs eventually are expected to replace light bulbs.

Conventional LEDs convert about 10 percent of incoming electricity into light. Physicists have believed that no more than 25 percent of the energy flowing into an LED could be emitted as light, with the other 75 percent radiating as heat, said University of Utah physics chairman Z. Valy Vardeny.

But in the Jan. 25 issue of the British journal Nature, Vardeny and colleagues report developing a test that indicates 41 percent to 63 percent of the energy can be converted into light using “plastic LEDs” made from organic materials called electrically conducting polymers and oligomers.

The findings mean it should be possible to “make more efficient light emitters for lasers, for displays, for room lighting, for computer screens, for TV screens,” Vardeny said.

Vardeny conducted the study with University of Utah postdoctoral physicist Markus Wohlgenannt; Sumit Mazumdar at the University of Arizona; and S. Ramasesha and Kunj Tandon at the Indian Institute of Science in Bangalore, India.

LEDs produce light when incoming negative and positive electrical charges - called electrons and “holes” - are attracted to each other and combine. The electrons and holes have a physical property called a “spin” that is somewhat like planet Earth rotating on its axis. Because the electrons and holes have different spins, such as “up” and “down,” physicists believed light would be emitted only one of every four times when an electron combined with a hole, Vardeny said.

In the Nature study, Wohlgennant placed small pieces of 10 different plastics in a magnetic field at supercold temperatures. He used a laser (instead of electricity) to make the LED materials emit light. By also bombarding the plastic materials with microwaves, Vardeny and Wohlgennant showed some of the materials - particularly those that emit red and blue-violet light - could emit more light than they would otherwise.

They used the new testing method to calculate that some plastic LED materials should be able to convert 41 percent to 63 percent of incoming electricity into light.

Vardeny and Wohlgenannt believe they understand the increased efficiency of light production: The microwaves randomize the spins of the incoming positive and negative electrical charges so they combine more quickly, making it possible for light to be emitted by more than 25 percent of the combined negative-positive charges.

It would be expensive and impractical to use microwaves to improve the efficiency of real LEDs. So Vardeny said the university is seeking a patent on a method of “doping” light-emitting plastics with iron compounds and chemicals that have the same effect as microwaves, namely, randomizing incoming electrical charges so the plastic LED materials are better at converting the electricity into light rather than heat. Many companies already make LEDs for various kinds of lights and displays, and Vardeny hopes they will be interested in paying the University of Utah for rights to the new method of improving the efficiency of plastic LEDs.

Plastic LEDs represent a new wave of devices that are more efficient at emitting light, less expensive and safer to manufacture than conventional LEDs, which are made with traditional semiconductors such as gallium compounds, Vardeny said.

“In the future, white LEDs will replace [incandescent] light bulbs,” he said. “Because they are more efficient, they also last longer. Rather than replacing bulbs at home every 1,000 hours, you will replace them every 100,000 hours,” or once every 11 years.

Alan Heeger, a physicist at the University of California, Santa Barbara, praised the Utah study as “nice work” for two reasons:

• n The findings inspire optimism that the proportion of electricity that can be converted into light using plastic LEDs “can be significantly greater than 25 percent.”
• n The method developed by Wohlgennant and Vardeny can be used as a test to predict which polymers and oligomers are most efficient at converting electricity into light.

Vardeny said physical laws implied that only one of every four interactions between negative and positive charges in an LED material should produce light, thus the previously presumed 25 percent maximum efficiency for converting energy to light. In the new study, “we succeeded in fooling quantum mechanics,” he said. “We did not break any laws of physics. We just fooled them.”

By analogy, Vardeny said the law of gravity says objects should fall to the ground. But we appear to fool the law of gravity when other forces are used, for example, to make airplanes fly or hot air balloons float.

For background on conventional LEDS, as opposed to the “plastic LEDs” Vardeny studies, see pages 62 -67 of the February 2001 issue of Scientific American magazine.