IEEE Spectrum :: Quantum Dots Enhance LED Lighting

Exerpted from BY Neil Savage // Decemberwatt, a vast improvement over the 15 lm/W of
2009incandescent bulbs and double the efficiency of
9 December 2009—The next big thing incompact fluorescent bulbs. QD Vision has started
solid-state lighting may be exceedingly tiny—theshipping that optic to two lighting manufacturers;
quantum dot. Researchers from around the worldthe lamps should be on store shelves by January.
gathered at the Materials Research Society fall"The main benefit of the quantum dot is you're
meeting in Boston last week to discuss theable to get a really efficient lightbulb with a
progress they're making in using quantum dots tohigh-quality color rendering index," says Vladimir
enhance the color and efficiency of light-emittingBulovic, a professor of electrical engineering and
diodes (LEDs).leader of the Organic and Nanostructured
Quantum dots are nanometer-size bits ofElectronics Laboratory at MIT. He says the QD
semiconductor material, such as cadmium selenide,Vision optic represents the first practical
that fluoresce when excited by photons oroptoelectronic device based on this technology.
electrons. By choosing a certain material and aCoe-Sullivan did his Ph.D. work in Bulovic's lab, and
certain size, researchers can precisely tune theBulovic is a founder of QD Vision.
wavelength of light emitted. In general, a dotBulovic and other researchers are working on
that's 2 nanometers in diameter emits blue light, acreating quantum-dot LEDs that are electrically
4-nm dot emits green, and a 6-nm dot emits red.pumped, thus eliminating the need for a
Seth Coe-Sullivan, chief technology officer of QDgallium-nitride LED as a photon source. But the
Vision, a Watertown, Mass., start-up working onelectroluminescent LEDs produced so far in
quantum dots for use in lighting and displays, sayslaboratories are still in their early stages. To make
the advantage of quantum dots lies in the abilitysuch devices, researchers closely pack the
to pick a desired color without losing efficiency.quantum dots in an organic thin film that acts as a
Today's white-light LEDs consist of a blue-emittingtransport layer for electrons. But doing so reduces
LED coated with a phosphor that is excited bythe luminescent efficiency of the dots from more
the LED and emits a yellow or orange light. Thethan 90 percent to about 15 percent.
combination of blue and yellow produces a coldThe first practical use of electrically pumped
white light lacking in red photons, so human skin,quantum-dot LEDs will likely be in displays, such as
among other things, looks unnatural under it.computer monitors, where Bulovic says they'll
There are phosphors that can produce colorprovide more saturated color than organic LEDs
closer to that of an incandescent light, but theydo but still retain that technology's ease of
come with a 30 percent drop-off in energymanufacturing and flexibility. Electrically pumped
efficiency.quantum-dot solid-state lighting will take longer,
By contrast, Coe-Sullivan says, QD Vision hasbecause reaching the brightness required for
produced an optic coated with a thin film ofgeneral illumination requires driving the LEDs at
quantum dots and fits over a blue LED lamp. Withhigher currents, which reduces their lifetime. But
the optic, the lamp produces light with a colorBulovic is sure that researchers will eventually
temperature of 2700 kelvins—about the samereach that goal. One way of extending the lifetime
as that of an incandescent bulb. It has awould be to replace some of the organic materials
color-rendering index (a measure of how "natural"in the LEDs with metal oxides or chalcogenides,
colors appear under it) of 90, compared with 95which won't degrade with exposure to air and
for an incandescent bulb and less than 75 formoisture.
most white LEDs. It also produces 65 lumens per