Bright Future

New materials and devices bring white LEDs closer to home

July 12, 2005 at 10:46 am

Even though 125 years have passed since Thomas Edison invented the incandescent lightbulb, his basic technology is still the main source of light in most homes today. That may change over the next decade, however, in favor of light-emitting diodes, or LEDs. These devices, long used in digital clocks, consist of a semiconductor chip typically made of a gallium-based material. The chip emits light when a voltage is applied.

LIGHT MOTIF. One way to make a white-light emitting diode (LED) is to start with a blue LED chip (left) and add a luminescent phosphor. Researchers have found that positioning the phosphor just above the chip (right) dramatically increases the light emitted. Schubert

Unlike incandescent lightbulbs, LEDs don’t waste electricity heating a filament to 2,000°C. Instead, they channel a larger proportion of power directly into light. A LED can last for up to 100,000 hours compared with the 1,000-hour lifetime of a typical incandescent lightbulb and the 10,000-hour lifetime of a typical fluorescent lightbulb. And unlike fluorescent lights, LEDs contain no mercury.

Over the past decade, both colored and white LEDs have found their way into numerous commercial products, including traffic lights, flashlights, and many architectural features. “The progress has been quite phenomenal,” says electrical engineer E. Fred Schubert of Rensselaer Polytechnic Institute in Troy, N.Y.

However, penetrating the general-illumination market, which is dominated by incandescent and fluorescent lighting, will require brighter, white LEDs that can produce the same quality of light as conventional light sources do. Today, light from white LEDs is typically bluer than other lights are.

By one measure of efficiency—the proportion of electricity that’s converted into visible light—white LEDs have already surpassed incandescent lightbulbs and have matched some fluorescent lights. However, a number of technical barriers need to be overcome before white LEDs can meet most home and office needs, says Schubert.

For instance, current white LED chips are relatively small, about 1 millimeter square, and scaling them up comes at a substantial cost. Larger chips tend to be less efficient than smaller devices because large LEDs scatter a greater proportion of their light sideways and so emit less light directly toward the viewer.

Optical options

Manufacturers use several methods to make white LEDs. One tack combines three or more different-colored LEDs, such as blue, green, and red, in a single unit. The combined wavelengths add up to white light. Unfortunately, these devices are relatively complicated to design and package.

A simpler approach places a luminescent material on top of a blue-emitting semiconductor. That coating, called a phosphor, contains rare earth atoms that convert some of the blue light into yellow. The combination of yellow and blue creates white light. Phosphors are, however, relatively inefficient at capturing and reemitting light. Furthermore, because the coating lies directly on the LED chip, some of the phosphor’s light is emitted back toward the semiconductor chip, where it’s absorbed.

Schubert and his colleagues at the Samsung Advanced Institute of Technology in Suwon, South Korea, have designed a LED that overcomes this loss of light. Rather than putting the phosphor directly on the chip, the researchers place the material at a short distance above it. The chip and phosphor are placed inside a silver, light-reflecting cup. In this configuration, some of the light from the phosphor that’s emitted back toward the chip hits the sides of the cup and is reflected toward the viewer.

In the June 13 Applied Physics Letters, the researchers report that their scheme could boost the phosphor’s efficiency by up to 50 percent.

Another approach that’s still in the early stages of development relies on nanoscale crystals called quantum dots. These semiconductor particles are being developed for use in solar cells and medical-imaging devices (SN: 1/22/05, p. 53: Available to subscribers at Infrared Vision: New material may enhance plastic solar cells; 2/15/03, p. 107: NanoLights! Camera! Action!). Quantum dots are especially promising for making white light because they can generate a wide range of colors. By varying the size of the dots, researchers can tune the crystals to emit different wavelengths.

From ultraviolet to infrared, “you can pick any wavelength you want,” says chemist Victor Klimov of Los Alamos (N.M.) National Laboratory.

The challenge of this approach to LEDs lies in delivering current to the quantum dots to trigger their light emission. When researchers have coated dots with an electrically conducting polymer, the material has degraded. Now, Klimov and his colleagues have developed a way to encapsulate quantum dots directly in a semiconductor chip that can energize the dots.

In the June Nano Letters, the Los Alamos team reports fabricating an LED from quantum dots of two sizes. The device therefore emits two colors, orange and red. In newer work, the researchers have created a three-dot LED that can emit red, blue, and green. “Right now, the quality of the white light is not great,” says Klimov. “But it’s a start.”

Valuable variety

Although researchers are finding ways to further improve LED technology, it may be a while before white LEDs light up homes and offices. In the meantime, some versions of the technology are showing up in other products, says Mike Krames of Lumileds Lighting in San Jose, Calif., one of the leading manufacturers of LEDs. “There are many other more-near-term applications that are very exciting,” he says.

For instance, last August, Sony released a liquid crystal–display television that uses Lumileds’ LEDs instead of a fluorescent lamp behind the screen for backlighting. That television produces colors more vivid than usual.

Although the product is currently available only in Japan, it could appear on the U.S. market by the end of the year, says Krames.

Lumileds is also working with automakers to incorporate white LEDs into their vehicles’ headlights.

Given the myriad potential applications of white LEDs, it’s unlikely that any one technology will reign in the end. “If you look at the field of lightbulbs, you literally find hundreds or thousands of different bulbs,” says Schubert. “They’re all for some specific application. And we see a similar situation with LEDs.”