Producing white light isn’t an easy job, especially when you try to use highly efficient LEDs, which are semiconductors, and thus have a reduced energy consumption. Researchers from the National Renewable Energy Laboratory (NREL) have demonstrated they can produce white light by first making a yellow-green LED, and then combining it with other colored LEDs.
LEDs are devices that emit photons when you apply a voltage to them – surely you already saw them in various devices by now. Producing white light with LEDs, though, is difficult, since you have to join several colored ones having their intensities and color saturation properly calculated. To get white, you would need red, green and blue (RGB – just like in a TV set). Green LEDs are a problem because traditional manufacturing technologies reduced the overall luminosity by approximately 20 percent, by applying one or more phosphor materials to a blue LED. This was done to convert high energy blue spectrum light into lower-energy light, but with the luminosity trade-off.
For building more efficient green LEDs, NREL researchers used a technique they created for making highly-efficient multi-junction solar cells, not requiring any phosphors. The problem is that the semiconductors they used (indium and gallium nitride) have different-sized crystal lattice structures, and that isn’t good for the efficiency. They have to have the lattice structure’s layers equal in size to be efficient.
“If you try to do it in one shot, the whole thing will be defective,” says Angelo Mascarenhas, team leader for solid state spectroscopy in the Center for Basic Sciences at NREL. “You have to grow a sequence of layers in a step-wise fashion.”
Applying the same concepts to LEDs, Mascarenhas and colleagues combined aluminum gallium indium phosphide and gallium arsenide, two well-developed semiconductor materials, which yielded yellow-green. If they can now develop a blue-green LED, they can combine the two near-green diodes with existing red and blue LEDs that would yield high-quality white light with a color rendering index (CRI) above 90. This would be much better than the ratings in the 70s that conventional LEDs usually get. (Sunlight is the standard, with a CRI of 100.)
Just like soldiers have their battlefield, scientists argue and compete in the field of math, physics and experiments. Professor Eugene Fitzgerald, from the MIT, says he had already developed yellow-green LEDs using similar layering techniques, and is now trying to arsenide-phosphides further to get a pure green LED for a three-colored white light, instead of one made of four colors, like the NREL team used. We’ll see who will finish the cheapest and the best LED first.
