Freeing light shines promise on energy-efficient lighting

Jul 15, 2008

(PhysOrg.com) -- The latest bright idea in energy-efficient lighting for homes and offices uses big science in nano-small packages to dim the future Edison's light bulb.

In the August issue of Nature Photonics, available online, scientists at the University of Michigan and Princeton University announce a discovery that pushes more appealing white light from organic light-emitting devices.

More white light is the holy grail of the next generation of lighting. The innovation in the paper "Enhanced Light Out-Coupling of Organic Light-Emitting Devices Using Embedded Low-Index Grids" describes a way to deliver significantly more bright light from a watt than incandescent bulbs.

"Our demonstration here shows that OLEDs are a very exciting technology for use in interior illumination," said Stephen Forrest, U-M professor of electrical engineering and physics and vice president for research. "We hope that white emitting OLEDs will play a major role in the world of energy conservation."

Forrest and co-author Yuri Sun, visiting U-M from Princeton University, have wrestled with a classic problem in the new generation of lighting called white organic light-emitting devices, or WOLED: Freeing the light generated, but mostly trapped, inside the device.

A lighting primer: Incandescent light bulbs give off light as a by-product of heat. The light is appealing, but inefficient, putting out 15 lumens of light for every watt or electricity.

The best fluorescent tube lights put out some 90 lumens of light per watt, but the light can be harsh, the fixtures are expensive, and the tubes lose their efficiency with age. And they rely on many environmentally unfriendly substances such as mercury.

WOLEDs show promise of providing a light that's much easier to manipulate, while being long lasting and able to provide in different shapes, from panels to bulbs and more. WOLEDs generate white light by using electricity to send an electron into nanometer thick layers of organic materials that serve as semiconductors. These carbon-based materials are dyes, the ones used in photographic prints and car paint, so they are very inexpensive, and can be put on plastic sheets or metal foils, further reducing costs.

The excited electron in these layers casts bright white light. The bad news, Forrest said, has been that some 60 percent of it is trapped inside the layers, much the way light under water reflects back into the pool, making the water surface seem like a mirror when viewed from underneath.

The Nature Photonics paper describes a tandem system of organic grids and micro lenses that guide the light out of the thin layers and into the air. The grids refract the trapped light, bouncing it into a layer of dome-shaped lenses that then pull the light out.

This process---all of which is packed into a lighting sandwich roughly the thickness of a sheet of paper---was shown to emit approximately 70 lumens from a single watt of power.

More light out means getting more bang for the electricity buck, a crucial question since 22 percent of the U.S. electricity consumption is lighting.

"If you can change the light efficiency by just a few percentage points, there's a few less coal plants you'll need," Forrest said.

Reducing the amount of coal-generated electricity and finding more efficient ways to power appliances and lighting is one of the focuses of U-M's Michigan Memorial Phoenix Energy Institute, and the WOLED work is one example of how science can open new doors in conservation, said Gary Was, institute director.

"That energy efficient lighting can be made from the same materials as car paint and that they can be made in such thin, formable sheets boggles the mind," Was said. "This is one of many exciting creations that research is giving us in the pursuit of energy efficiency. This is also the kind of innovation that is required in the drive for energy sustainability.

Forrest said WOLED work isn't done yet. The fun part, he said, is that WOLEDs can be framed in different forms.

"Plugging into a wall at low voltage, putting it on a flexible metal foil, or on plastic that won't break when you drop it," Forrest said. "This is what makes it so fun because it's such a unique lighting source."

The research was funded by the U.S. Department of Energy through a subcontract from the University of Southern California and by Universal Display Corp.

Forrest is part of the Michigan Memorial Phoenix Energy Institute, which develops, coordinates and promotes multidisciplinary energy research and education at U-M. He also is on the scientific advisory board of Universal Display Corp.

The next challenge, he said, is to reduce the cost, which currently is too high to be commercially competitive.

"You have to be able to do this dirt cheap, Forrest said. "People don't spend much for their light bulbs."

Provided by University of Michigan

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nano999
2 / 5 (9) Jul 15, 2008
This is real nice but how long is it going to take to get this technology into retail stores? 5, 10, or more years? You can't even buy LEDs (ok, maybe there is 1 or 2 light gadgets)at China-Mart yet for Dogs sake. Hopefully, CFLs will just be a passing fad.
agg
2 / 5 (5) Jul 15, 2008
organic? gasp, that must be good. Alert Al Gore.
Arikin
4.6 / 5 (5) Jul 16, 2008
70 lumens per watt is wonderful. Market it now! Please :-) Even at that level it saves so much energy. Later improvements will be easier because the basic ingredients are already being massed produced.
DGBEACH
4.3 / 5 (3) Jul 16, 2008
Hopefully they have also found a way to get around OLED's limited lifespans, which still aren't close to those of the present hi-power LEDs (100K hrs).

And nano999, there have been LED lamps around for many years, though mostly only online. The Pool lighting industry has used them quite a bit. They're just alot more expensive.
Lord_jag
3.7 / 5 (3) Jul 16, 2008
All the elevators in my building use LED lighting. Most traffic lights are LED's now as they don't burn out very often. Many turn signals in cars are now LED's for the same reason. Especially when embedded in the wing of a car.

Most hardware stores have flashlights with LED's for lights. It makes the batteries last FAR FAR longer.

70 lumens is okay. I'm sure it's just a matter of time before they find a non-reflective surface to unlock all the light. Once they pass CFL's they will be everywhere.
Mayday
4 / 5 (1) Jul 16, 2008
I don't understand where the high expense is. Everything descibed sounds like it should manufacture well at high volume. Could it be worse than these horrible florescent bulbs being shoved down our throats?

Hey, there's a swell idea: Let's save the environment by making our light bulbs with mercury! :-/ And even better: Let's make them flicker at a brain-dulling rate and be non-dimable!

But seriously, here's my deal-breaker: Are OLEDs dimmable? My proposition is that we consume so much energy on lighting primarily because in most indoor situations we are simply over-lit. My favorite is the number of situations you can see everyday where lights are glaring over a scene where there are in fact no people present. Yet we'll turn up the A.C to create genuine discomfort instead of going after more obvious savings. :-D

They need to get this OLED material in front of some talented industrial designers. Fun stuff.

SLam_to
4 / 5 (1) Jul 16, 2008
OLEDs and LEDs are dimmable. However some are not dimmable by the standard dimmer switches you have for incadecent lighting. It's not necessarily an issue with the individual OLEDs, but the circuit that may be used to drive a large cluster of them.
rubberman
4 / 5 (1) Jul 16, 2008
The unfortunate reality is, like everything else in our society, money has the last say.Production of these for purchase by the general public would be stalled by major light bulb manufacturers until they find a way to make them last the "correct" amount of time to ensure repeat business. LED's have been around for 60 years but never mass produced (until very recently)to replace incandescent or fluorescent because a 12 year duty cycle means very little in the way of repeat business. Sorry for the negativity folks but i've been involved in lighting design for several years so I look very forward to the day when this technology is readily available for usage. Sadly, the reason those fluorescent tubes are rammed down our throats is due to their lifespan (long compared to incandescent but short compared to LED or RF).But thanks to global climate change, sales are through the roof....
rubberman
not rated yet Jul 16, 2008
As a footnote, I'm curious about the term "white light", are they referring to a broad spectrum light source or are they referring to CRI....