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Let there be light

April 12, 2006 By Mark Peplow This article courtesy of Nature News.

Organic LEDs use fluorescence to pump up efficiency.

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The traditional light bulb's days could be numbered, according to scientists who have taken an important step towards making white organic light-emitting diodes (OLEDs) commercially viable.

They expect that, for the wealthy at least, it could be just a few years before rooms are lit by gently glowing thin panels. The lights should be ultra efficient, saving on energy bills and helping to lower energy consumption.

Lighting accounts for about 22% of the electricity consumed in buildings in the United States, and 40% of that amount is eaten up by inefficient incandescent bulbs. "It's in society's best interest that we use less energy," says Stephen Forrest, an electrical engineer at Princeton University, New Jersey, and part of the team that developed the new LED reported in Nature1.

Traditional incandescent light bulbs are very effective at throwing out light, but a lot of the energy used to run them is converted into heat. Fluorescent lights, such as those seen in most offices, are much better, although the materials used to produce them are relatively expensive. Inorganic LEDs - those used in some Christmas tree lights and bicycle lamps, for example - are very bright point sources. They are good for producing visual effects or attracting the notice of oncoming cars, but they can't throw out enough light to illuminate an entire room.

The carbon-based phosphorescent polymers in OLEDs might be better suited to lighting large areas because they can be 'printed' on to surfaces, making lighting screens potentially easy and cheap to mass-produce. Such polymers are currently found in the tiny bright screens of some MP3 players and mobile phones. But they also aren't yet bright enough to light a room.

To boost the output of OLEDs, Forrest and colleagues have exploited the way electrons behave inside the devices.

Quick and slow

LEDs are typically made of a mix of phosphorescent materials that spit out blue, red and green light - together, these combine to form white light. But the phosphors that produce blue light aren't ideal. These materials tend to degrade relatively quickly, shortening a device's lifetime and making its light more yellow as the LED ages.

Forrest and his colleagues turned instead to a long-lived fluorescent material that churns out blue light. Both fluorescence and phosphorescence rely on energetic electrons spitting out light as they relax within an atom, but the two light-producing processes differ slightly in the way that the electrons relax. Electrons that are primed to participate in phosphorescence maintain their excited state for microseconds, whereas those more suited to fluorescence relax about a hundred times faster.

The cathode of any LED - the device that starts electricity flowing through the material - produces excited electrons such that about 75% are primed for phosphorescence and 25% for fluorescence.

Forrest's trick is simply to place the blue fluorescing material close to the cathode in his device, so that it is ready to catch all those quick-to-relax electrons on their way out of the starting gate. The red- and green-producing phosphors are a few nanometres farther along, in exactly the right place to catch the remaining excited electrons just as they are ready to relax.

Coincidentally, really white light is about 25% blue, says Forrest. So having 25% of the electrons revved up to fluoresce blue light is perfect. Theoretically, these LEDs could take advantage of all the electrons, possibly achieving 100% internal efficiency.

Long lasting

The blue fluorescent compounds used in Forrest's device last for around 10,000 hours. That is still not as good as the red and green phosphors but leagues ahead of blue phosphors. A conventional bulb may last about 1,000 hours, and a fluorescent lamp about 20,000 hours.

The main barrier to producing home lights this way is cost, Forrest says. "In terms of the technology, we could do this today. We just can't make it for a reasonable price. But we are marching down a path that will make it practical."

White OLEDs could eventually be mounted on glass or flexible plastics. The main problem then would be to ensure that no oxygen or moisture is allowed to leak through the plastic coatings to contaminate the light-emitting molecules inside. "It's a packaging problem," says Bernard Kippelen, an electrical engineer at the Georgia Institute of Technology, Atlanta. "But if you look back at the progress of similar technologies, I'm confident that these problems will be solved," he adds.

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References

  1. Sun Y., et al. Nature, 440. 908 - 912 (2006).

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