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.
Reineke and his colleagues were able to get such good results because of several design modifications and refinements to their device. One involves reducing its operating voltage by doping the organic material that connects the light-emitting material to its metallic contacts. “The efficiency of the device is highly reduced if it is near a metal contact” because of a phenomenon called quenching, says Reineke.
Another trick was to make the outer surfaces of the device from types of glass that have optical properties that more closely match those of the device substrate. Otherwise, much of the emitted light is reflected and either reabsorbed or lost through heat. “In conventional structures, about 80 percent of the light is lost,” says Reineke.
The most novel aspect of this new OLED, however, is the organization of different light-emitting materials within the device. Three materials are used–one each for emitting blue, green, and red light–along with a host matrix material in between. Reineke’s trick was to choose a matrix material with a high “spin state” that matched that of the blue and to sandwich the blue material between the green and red, as if it were part of the separating host matrix material.
“The matrix and the blue state are nearly identical,” Reineke says. This means that any electron-hole pairs (excitons) escaping the red or green material will have to pass through the blue, increasing the chances that they will be converted into photons.
“They do a nice job of tailoring the LED layers to get good quantum efficiencies,” says Kazlas. “It shows the promise of OLEDs, but from an industry perspective, OLEDs still have a long way to go.”
Indeed, a major drawback of OLEDs is their longevity. Although companies like Philips are able to make devices with life spans equivalent to fluorescent bulbs–in excess of 10,000 hours–materials that yield higher efficiencies tend not to last so long. “Our devices have lifetimes of just a few hours,” says Reineke.
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