More Colorful Displays with Quantum Dots
Nanocrystals improve the efficiency and color range of LCDs.
Liquid-crystal displays (LCDs) are everywhere, from cell phones and cameras to laptops and flat-screen TVs, but they’re still relatively inefficient and limited in color quality. At best, only about 7 percent of the backlight illumination makes it through roughly 20 layers of optics, electronics, and filters to reach a viewer’s eyes. And these filters ensure that the resulting colors are dull compared to some other displays, for example those made with organic light-emitting diodes (LED).
QD Vision, a startup based in Cambridge, MA, has developed a technology that it claims will improve the efficiency of LCDs by 40 percent. In addition, the company says it will provide purer colors, allowing the displays to produce high-dynamic range, which features better contrast between the darkest blacks and the whitest whites.
The technology, called a quantum light optic, will be sold to three of the five major LCD manufacturers and integrated into commercial displays by 2011, says Seth Coe Sullivan, founder and chief technology officer of QD Vision.
Quantum dots are at the heart of the quantum light optic technology. These dots are nanoscale crystals of semiconductors that emit pure colored light. When quantum dots are added to existing lighting technology, such as the light-emitting diodes found in LCD backlights or those found in light bulbs, they shine so brightly that they reduce the number of diodes needed to achieve the same overall brightness. Furthermore, since quantum dots shine at specific colors, they can be added to a white LED to improve the spectral properties of its light.
QD Vision, which was spun out of research at MIT, is already using quantum light optic technology to improve the efficiency of light-emitting diodes. The first lighting products based on the technology will be available by February 2010, says Sullivan.
Quantum dots can also improve efficiency and color purity of displays. While some larger LCDs are illuminated using cold-cathode fluorescent lights, most small displays are lit up with white LEDs. Manufacturers attach a strip of white LEDs to the edge of a plane of glass. This glass spreads out the light and, because of special texturing on its surface, it directs the light out toward a viewer.
The type of white LED that manufacturers use for the backlight consists of a mixture of semiconducting materials that produce blue light and a phosphor that produces yellow. Energy from the blue light causes the phosphor to glow, producing a yellowish white light. To produce multiple colors, for the red and green subpixels used in a display, manufacturers add color filters. These filters separate out red and green light from the white LED.
However, the phosphor light produced by most LEDs is made up of a smear of different colors, says Sullivan. “There’s a little red light in the yellow phosphor, and there’s a little bit of green light in the yellow phosphor,” he says. To let enough brightness through, color filters need to be spectrally broad, but this means they also let through a mixture of color, which hurts its quality.
When a quantum light optic is placed over a blue LED, it eliminates the need for the yellow phosphor altogether. The light from the blue LED excites electrons in the quantum dots and–depending on the size of the dots used–a specific color of light is produced. A cadmium selenide dot that’s six nanometers wide, for example, produces red light; a dot that’s four nanometers wide produces green light; and one that’s two nanometers wide produces blue light. So instead of a yellowish white light, the light consists of purer red, blue, and green.
With quantum dots, manufacturers can use LCD color filters that only let pure colors of light through. Sullivan claims that quantum dots can improve an LCD’s color gamut, or color range, by 80 to 100 percent.
Additionally, fewer LEDs are needed to achieve the same result. And the LEDs that are needed are simpler and cheaper than those with the yellow phosphor. “All of a sudden,” says Sullivan, “your TV is as good as your CRT [cathode-ray tube] was 10 years ago in terms of color, and there’s also a manufacturing cost savings to LCD makers, which is a big deal.”
Manufacturers are increasingly looking to use LEDs in the backlights of larger displays. The California Energy Commission recently announced that it is imposing new power consumption limits on high-definition television sets in the state, the biggest market for such sets in the U.S., by 2013. Since LEDs are more energy efficient than cold-cathode fluorescent lights, quantum dots could “be a good way to make LED backlights better, more efficient, and more cost-effective,” says Bruce Berkoff, chairman of the LCD TV Association.”
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