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Blue LED Improves Light Output, Efficiency

Post Time:Mar 19,2009Classify:Glass QuotationView:393

A new type of blue LED that improves light output by 18%, and the amount of electricity the LED converts into light by 22% has been developed by researchers at Rensselaer Polytechnic Institute in New York of the US, in collaboration with Samsung Electro-Mechanics.

Dropping the Droop

The Rensselaer-Samsung device is a polarization-matched LED. The researchers said the key to the improved LED was achieving a notable reduction in efficiency droop - a phenomenon that provokes LEDs to be their most efficient when they receive low-density electric currents, but to lose their efficiency as higher density currents are fed into the device. The cause of droop is not fully understood, but studies have indicated that electron leakage likely plays a key role.

"This drop is under the spotlight since today's high-brightness LEDs are operated at current densities far beyond where efficiency peaks," said E Fred Schubert, professor of future chips at Rensselaer and leader of the LED project. "This challenge has been a stumbling block, because reducing the current densities to values where LEDs are more efficient is unacceptable."

The answer, he said, is the new LED, with a redesigned active region that is polarization-matched, and that brings LEDs closer to being able to run efficiently at high current densities. His team discovered that the active region of LEDs contained materials with mismatched polarization, which likely caused electron leakage, resulting in the loss of efficiency.

The team greatly reduced the contrast in the material polarization by using a new quantum-barrier design by replacing the conventional gallium indium nitride/gallium nitride layer in the LED multi-quantum-well active region with gallium indium nitride/gallium indium nitride. This results in reduced electron leakage, lower efficiency droop, enhanced light-output power, a lower forward voltage, a smaller diode ideality factor, decreased wavelength shift, and no sub-threshold turn on.

Curved Photodetector

Meanwhile, a flexible light-sensitive material holds promise to correct today's distorted cell-phone photos, and replace the need for heavy telephoto lenses for high-end digital cameras, according to Zhenqiang Ma, associate professor at the University of Wisconsin-Madison, who with his team developed the new material.

When a device records an image, light passes through a lens and lands on a photodetector. The lens bends the light and curves the focusing plane. In a digital camera, the point where the focusing plane meets the flat sensor will be in focus, but the image gets more distorted the further it is from the focus point. High-end digital cameras incorporate multiple panes of glass to refract light and flatten the focusing plane. The trade-off is that such lens systems are large, bulky and expensive.

"If you can make a curved plane, you just need one lens," said Ma. His group can create curved photodetectors with specially fabricated nanomembranes made of 250nm flexible membranes of single-crystal germanium, a material that is very sensitive to light and that often is used in high-end imaging sensors. The germanium membranes, with selectively ion-implantation doped regions, were released from a germanium-on-insulator substrate and integrated with a 175um thick polyethylene terephthalate substrate using dry printing. The membranes can be applied to any polymer substrate, such as a flexible piece of plastic.
 

Source: Nikkei Electronics AsiaAuthor: shangyi

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