IN THE search for cheap, efficient sources of white light, LEDs have long been touted as offering the most potential. Now a new generation of white organic LEDs could become the source of choice for homes, offices and even computer displays.
LEDs are preferable for many applications because they convert electrical energy into photons so efficiently. While incandescent light bulbs convert only 5 per cent of the energy passing through them into light and compact fluorescent bulbs manage 20 per cent efficiency, LEDs routinely achieve 30 per cent or more.
The problem is that conventional LEDs produce light only at specific wavelengths, so manufacturers have had to employ two tricks to make white light. One is to use several LEDs that each emit a primary colour. When combined, these colours look white to the human eye. The other approach is to cover a blue LED in a phosphorescent chemical, or phosphor, that absorbs a portion of the emitted bluish light and re-emits it as amber. Again, we see the combination as white.
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“Conventional LEDs produce light only at specific wavelengths. Making white light from them is costly”
These solutions are relatively costly, though. Now a potentially cheaper option is emerging thanks to the development of organic dyes that emit blue and amber photons, and the ability to combine both in the light-emitting layer of an LED. The result is an organic LED (WOLED) that produces white light directly.
WOLEDs have not made it out of the lab yet, however. One problem is that high currents tend to break down the organic dyes they rely on and this dramatically reduces their lifetime compared with inorganic LEDs made of materials such as indium gallium arsenide. One way around this would be to find a way to achieve an acceptable brightness with as low a current as possible.
Now a group led by Dongge Ma at the Changchun Institute of Applied Chemistry at the Chinese Academy of Science has come up with a simple way of doing this: stacking two white-light-emitting layers in a single device so that they operate in series (Journal of Applied Physics, ). “The stacked structure allows higher brightness at lower current,” says Paul Burrows, an electronics engineer at Reata Research, a science and technology consultancy in Kennewick, Washington.
The new device does have a potential drawback, though. The stacked structure requires double the voltage, which might change the device’s efficiency, though whether it will is not yet clear.
Colin Humphreys, a materials scientist at the University of Cambridge, warns against too much optimism at this stage. “There has been a lot of hype over WOLEDs,” he says, but before they reach the market various technological hurdles must be overcome. “They could be a huge market or a very small market, and it is not obvious which it will be,” he says.
