Shining a light on LED efficiency

Researchers from the University of Utah have made a breakthrough in LED efficiency, resulting in a 41 to 63 per cent increase in energy conversion. What’s more, they achieved this using ‘plastic LED’s’ made from organic materials called electrically conducting polymers and oligomers.

Electric light bulbs may brighten up our lives but behind the bright exterior lays a tale of inefficiency because only a few percent of the electricity flowing into light bulbs comes out as visible light, with most of the rest emitted as heat.

Light-emitting diodes, however, are more efficient, last longer are being used increasingly in all classes of electronic equipment

Conventional LED’s are said to 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.

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 can be likened to Earth rotating on its axis. Because the electrons and holes spin in different directions physicists believed light would be emitted only one of every four times when an electron combined with a hole, said Vardeny.

Markus Wohlgennant, a University of Utah postdoctoral physicist, placed small pieces of 10 different plastics in a magnetic field at supercold temperatures, then used a laser, rather than electricity, to make the LED materials emit light.

By additionally 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.

The microwaves are said to randomise 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.

Microwaves, however, would be expensive and impractical to use to improve the efficiency of real LED’s. 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, randomising incoming electrical charges so the plastic LED materials are better at converting the electricity into light rather than heat.

‘In the future, white LED’s will replace incandescent light bulbs,’ said Vardeny. ‘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.

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. ‘We succeeded in fooling quantum mechanics,’ said Vardeny. ‘We did not break any laws of physics. We just fooled them.’

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