Cheaper LED shines the right light

By Eric Smalley, Technology Research News

Most Internet users connect to the Internet through dial-up modems, which are several orders of magnitude too slow to accommodate applications like downloading movies or videoconferencing.

Even digital subscriber line (DSL) or cable connections, which increase download speeds from 56 thousand bits per second to 1.5 million or so, would require six hours to download two hours of the type of compressed full-motion video found on a DVD. Full motion video uses 30 frames, or separate pictures per second, which adds up to about 4 gigabytes of data for that two-hour movie.

One way to deliver on the full-speed hype of the Internet is to extend the global network's fiber-optic backbones all the way to individual homes. Fiber lines can carry much more information than telephone and cable wires because light can be pulsed much more quickly than electricity. The ones and zeros of digital information are represented by the presence or absence of a pulse of light or electrical current.

Before speedy fiber optic lines reach the average house, the technology will have to get considerably cheaper. "If these components will have to be at every house, the price of each component will have to be very low," said Nir Tessler, a senior lecturer of electrical engineering at the Israel Institute of Technology.

Tessler is one of a team of researchers at the Israel Institute of Technology and Hebrew University in Israel that has found a way to mix plastic and microscopic semiconducting crystals to make material that could lower the cost of fiber-optic communications. The material forms a light emitting diode (LED) that emits wavelengths of light in the infrared range used by existing fiber lines.

Most LEDs are made from small chips of semiconductor crystal, which is also the type of material used to make computer chips.

Organic LEDs, which are made from polymers, or plastics, are cheaper than those made from semiconductor materials, but emit only visible light waves, which range from 0.4 to 0.7 microns. Telecommunications over fiber optics uses 1.3 micron waves of near-infrared light.

Attempts to make near-infrared organic LEDs have run up against an efficiency problem: they use a lot of power because the devices are not efficient at converting electricity into light. The researchers got around this problem by mixing their polymer with more efficient microscopic semiconductor crystals. The nanocrystals have a core of indium arsenide surrounded by zinc selenium.

Key to the new material's potential utility is that the nanocrystal-laced plastic can be spread into a thin film to make LEDs, which is a cheaper manufacturing process than etching layers of semiconductor crystals with light and chemicals to produce today's LEDs.

The researchers' device converts electricity to light with an efficiency of between 1.5 and 3 percent, which is much lower than the 90 percent efficiency possible with semiconductor LEDs, but it has the potential to improve to usable levels.

"Their claim of about three percent internal efficiency is certainly impressive for a first attempt using an organic technology," said William Gillin, a lecturer in experimental physics at the University of London. "A cheap device that operates in the 1.3 micron band would certainly be useful for the fabrication of optical amplifiers for [telecommunications]," he added.

The researchers are working on a second generation of more efficient devices and are extending the wavelength range, said Tessler. The LEDs could be used in practical applications in four to eight years, he said.

Tessler's research colleagues were Vlad Medvedev of Technion and Miri Kazes, ShiHai Kan and Uri Banin of Hebrew University. They published the research in the February 22, 2002 issue of the journal Science. The research was funded by Israel Institute of Technology, Hebrew University and the Israel Science Foundation.

Timeline:   4-8 years
Funding:   University, Government
TRN Categories:   Optical Computing, Optoelectronics and Photonics; Materials Science and Engineering; Telecommunications
Story Type:   News
Related Elements:  Technical paper, "Efficient Near-Infrared Polymer Nanocrystal Light-Emitting Diodes," Science, February 22, 2002


February 27, 2002

Page One

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Cheaper LED shines the right light

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