9 December 2009—The next big thing in solid-state lighting may be exceedingly tiny—the quantum dot. Researchers from around the world gathered at the Materials Research Society fall meeting in Boston last week to discuss the progress they're making in using quantum dots to enhance the color and efficiency of light-emitting diodes (LEDs).
Quantum dots are nanometer-size bits of semiconductor material, such as cadmium selenide, that fluoresce when excited by photons or electrons. By choosing a certain material and a certain size, researchers can precisely tune the wavelength of light emitted. In general, a dot that's 2 nanometers in diameter emits blue light, a 4-nm dot emits green, and a 6-nm dot emits red.
Seth Coe-Sullivan, chief technology officer of QD Vision, a Watertown, Mass., start-up working on quantum dots for use in lighting and displays, says the advantage of quantum dots lies in the ability to pick a desired color without losing efficiency. Today's white-light LEDs consist of a blue-emitting LED coated with a phosphor that is excited by the LED and emits a yellow or orange light. The combination of blue and yellow produces a cold white light lacking in red photons, so human skin, among other things, looks unnatural under it. There are phosphors that can produce color closer to that of an incandescent light, but they come with a 30 percent drop-off in energy efficiency.
By contrast, Coe-Sullivan says, QD Vision has produced an optic coated with a thin film of quantum dots and fits over a blue LED lamp. With the optic, the lamp produces light with a color temperature of 2700 kelvins—about the same as that of an incandescent bulb. It has a color-rendering index (a measure of how "natural" colors appear under it) of 90, compared with 95 for an incandescent bulb and less than 75 for most white LEDs. It also produces 65 lumens per watt, a vast improvement over the 15 lm/W of incandescent bulbs and on par with compact fluorescent bulbs. QD Vision has started shipping that optic to two lighting manufacturers; the lamps should be on store shelves by January.
"The main benefit of the quantum dot is you're able to get a really efficient lightbulb with a high-quality color rendering index," says Vladimir Bulovic, a professor of electrical engineering and leader of the Organic and Nanostructured Electronics Laboratory at MIT. He says the QD Vision optic represents the first practical optoelectronic device based on this technology. Coe-Sullivan did his Ph.D. work in Bulovic's lab, and Bulovic is a founder of QD Vision.