Simple design of low-cost, organic white-light LED holds promise for widespread application

April 9, 2009--Researchers in China say they have developed a new type of light emitting diode (LED) made from inexpensive, plastic-like organic materials that can produce twice as much light as a normal LED--including white light for home and office lighting. "This work is important because it is the realization of rather high efficiency white emission by a tandem structure," says Dongge Ma, who led the research at the Chinese Academy of Sciences (Beijing).

April 9, 2009--Researchers in China say they have developed a new type of light emitting diode (LED) made from inexpensive, plastic-like organic materials. Designed with a simplified "tandem" structure, it can produce twice as much light as a normal LED--including white light for home and office lighting. "This work is important because it is the realization of rather high efficiency white emission by a tandem structure," says Dongge Ma, who led the research with his colleagues at the Changchun Institute of Applied Chemistry at the Chinese Academy of Sciences(Beijing).

LEDs are more environmentally friendly and much more efficient than other types of light bulbs, turning 20 to 50 percent or more, of input energy into light. (By comparison incandescent bulbs convert only about 5 percent. While compact fluorescent bulbs turn about 20 percent, they contain mercury vapor, an environmental toxin.) But making LEDs that consumers want to use to light their homes has been a challenge.

One of the main barriers to the widespread use of LED lights is the light itself. LEDs can easily be manufactured to produce light of a single color with applications such as traffic lights and auto brake lights. Indoor lighting though, requires "natural" white light. This quality is measured by the color-rendering index (CRI), which assigns a value based on the light source's ability to reproduce the true color of the object being lit. For reading light, a CRI value of 70 or more is optimal. LEDs can produce white light by combining a mixture of blue, green, and red light, or by sending colored light through a filter or a thin layer of phosphors -- chemicals that glow with several colors when excited. However, these solutions increase costs. To reach a larger market, scientists would like to make inexpensive LEDs that can produce white light on their own.

The Chinese researchers report important advances towards this goal. First, they built LEDs from organic, carbon-based materials, like plastic, rather than from more expensive semiconducting materials such as gallium, which also require more complicated manufacturing processes. Second, they demonstrated, for the first time, an organic white-light LED operating within only a single active layer, rather than several sophisticated layers. Moreover, by putting two of these single-layer LEDs together in a tandem unit, even higher efficiency is achieved.

In the latest issue of Journal of Applied Physics, published by the American Institute of Physics (AIP), the scientists report that their LED was able to achieve a CRI rating of nearly 70--almost good enough to read by. Progress in this area promises further reduction in the price of organic LEDs.

Roughly 20 percent of the electricity consumed worldwide is used to light homes, businesses, and other private and public spaces. Though this consumption represents a large drain on resources, it also presents a tremendous opportunity for savings. Improving the efficiency of commercially available light bulbs--even a little--could translate into dramatically lower energy usage if implemented widely.

For more information see the paper, "A high-performance tandem white organic LED combining highly effective white units and their interconnection layer," at the Journal of Applied Physics.

Posted by Barbara G. Goode, barbarag@pennwell.com, for Laser Focus World.

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