Ultra-Efficient Organic LEDs  

Technology Review has an update on cheap, energy efficient OLED lighting - Ultra-Efficient Organic LEDs.

An organic light-emitting diode (OLED) developed in Germany has the potential to produce the same quality of white light as incandescent bulbs but with power efficiencies considerably better than even fluorescent lighting.

The prototype OLED could emerge as an ultra-efficient light source for displays and general lighting, says Sebastian Reineke, who led the research at the Institute for Applied Photophysics, in Dresden, Germany. The long-term goal is to fabricate the device using conventional low-cost roll-to-roll printing.

In recent years, many countries have begun looking to switch from incandescent lighting to compact fluorescent bulbs because the latter are so much more energy efficient. There has also been a lot of interest in using light-emitting diodes (LEDs) for displays and general lighting, again because of the potential energy savings they offer.

But with both fluorescent and LED lighting, the quality of white light produced has always left something to be desired. Fluorescent lighting can make people appear unhealthy because less red light is emitted, while most white LEDs on the market today have a bluish quality, making them appear cold.

In contrast, OLEDs can be made from a wide range of materials, so achieving good-quality white light is less challenging, says Reineke. It has not been the quality of light that has let OLEDs down but rather their efficiencies. Fluorescent lighting typically operates at around 60 to 70 lumens per watt, while incandescent bulbs operate at about 10 to 17 lumens per watt. In contrast, says Reineke, the best reported power efficiency of an OLED until now was 44 lumens per watt.

In this week's issue of the journal Nature, Reineke and his colleagues report a novel structural design for an OLED that exhibits efficiencies of 90 lumens per watt and shows potential to go up to 124 lumens per watt.

"These efficiencies are very compelling," says Peter Kazlas, director of device development for QD Vision, a company based in Cambridge, MA, that's developing quantum-dot-based LED lighting.

"OLEDs have the potential to grow into a really very energy-efficient light source," adds Kristin Knappstein, business-development manager at Philips Lighting, in Aachen, Germany. Her company already has an OLED lighting product on the market called Lumiblade. "In production, we achieve levels of between 15 and 20 lumens per watt," she says, adding that the ultimate potential is for the technology to reach efficiencies as high as 150 lumens per watt.

Seth Coe-Sullivan flicks the switches on two desk lamps, and even from across the conference room, it's immediately obvious which light the chief technology officer of QD Vision is there to brag about. The light coming from the lamp on the left is a harsh bluish white. The lamp on the right casts a warmer, more yellow glow. Coe-Sullivan holds a hand under each lamp. The hand under the bluish light looks pale and sickly; the other looks darker and healthier. The harsher light lacks wavelengths in the red end of the spectrum, so there's no light to illuminate the reddish tinge that blood provides to human skin.

QD Vision, based in Watertown, MA, is promoting a new LED-based lamp that it made with Nexxus Lighting of Charlotte, NC. Nexxus makes a lamp designed to screw into standard sockets used in recessed ceiling lighting. It consists of an array of white-light LEDs encircled by fins that remove excess heat. QD Vision adds an optic--a plastic cover with a special coating that snaps into place over the LEDs.

It's that coating that makes the difference in the quality of the light. It consists of quantum dots--tiny bits of semiconductor material just a few nanometers in diameter. When excited by a light source--in this case, the LEDs--quantum dots radiate light in a wavelength that varies according to the size of the dot: a two-nanometer dot gives off blue light, a four-nanometer dot emits green, and a six-nanometer dot produces red. The company makes the dots in controlled sizes, then mixes them in the right ratio to get the desired color.

This color-tailoring ability solves one of the major problems with using LEDs for general lighting applications. LEDs are appealing because they last for years, use perhaps 20 percent of the electricity of a standard incandescent bulb, and are highly efficient at converting electricity into visible light instead of into heat. But to make white light, you either have to mix together LEDs of different colors or use a blue LED coated with a phosphor that emits yellow light to produce a whitish mix. The problem with the phosphors is that they don't emit evenly across the visible spectrum. They tend to have gaps in the green section and even more so in the red, leading to the harsher, bluish light. "You can't precisely tailor phosphors anywhere in the visible spectrum," says Dan Button, QD Vision's CEO.