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New Nanocrystal Material Could Lead to Efficient LEDs

31 January 2018

Nanocrystals are the key to more efficient LEDs, according to researchers from the U.S. Naval Research Laboratory (NRL) Center for Computational Material Science.

Researchers at the US Naval Research Laboratory Center for Computational Materials Science, working with an international team of physicists, show that cesium lead halide perovskites nanocrystals (CsPbX3) -- which atomic structure is shown on rear screen -- emit light much faster than conventional light emitting materials. This property is enabling more efficient lasers and LEDs with larger emission power at lower energy use, as well as faster switching for communication and sensors. Pictured standing (l-r) are NRL researchers Dr. Alex Efros and Dr. Noam Bernstein; sitting (l-r) are Dr. John Michopoulos and Dr. John Lyons. (Source: US Naval Research Laboratory/James Marshall)Researchers at the US Naval Research Laboratory Center for Computational Materials Science, working with an international team of physicists, show that cesium lead halide perovskites nanocrystals (CsPbX3) -- which atomic structure is shown on rear screen -- emit light much faster than conventional light emitting materials. This property is enabling more efficient lasers and LEDs with larger emission power at lower energy use, as well as faster switching for communication and sensors. Pictured standing (l-r) are NRL researchers Dr. Alex Efros and Dr. Noam Bernstein; sitting (l-r) are Dr. John Michopoulos and Dr. John Lyons. (Source: US Naval Research Laboratory/James Marshall)

The nanocrystals are made out of cesium lead halide perovskites. The nanocrystals make a great material for more efficient solid-state lasers and LEDs because they are the first material to have a bright ground exciton state.

To create these crystals, the research team closely watched the three compositions of halide perovskites: chlorine, bromine and iodine. They were watching these compositions to see how much light is emitted and how much fast light emission is retained from them at wavelengths all over the spectrum. This is the key to better-performing LED lights.

Perovskite nanocrystals have the lowest energy exciton while still being bright. Their radiative lifetime -- the time it takes an electron and hole to recombine and emit light -- is 20 times faster than conventional materials at room temperature. The exciton becomes 1000 times faster at cryogenic temperatures.

"The discovery of such material, and understanding of the nature of the existence of the ground bright exciton, opens the way for the discovery of other semiconductor structures with bright ground excitons," said Dr. Alexander Efros, research physicist at NRL. "An optically active bright exciton in this material emits light much faster than in conventional light emitting materials and enables larger power, lower energy use, and faster switching for communication and sensors."

Researchers already know that QLEDs suffer from “droop” (reduced efficiency) with a high pumping intensity. Droop is caused by excitons dissipating before they have emitted light. This means that if the perovskites are used to create LEDs, they may use all of their energy to create the light before it has had a chance to disperse.

"The increased rate of light emission of these materials holds great promise for various technological applications that rely on LEDs and lasers," Efros said. "In principle, the 20 times shorter lifetime could, therefore, lead to 20 times more intense LEDs and lasers."

Laser power is dependent on the gain of the material from which it is made. The gain is proportional to the radiative emission rate.

It is possible to spread information in beams over long distances without copper or fiber optic cables. This would be a huge benefit from increased light emission rates.

"The maximum bandwidth of the communication system is limited by the rate at which the LEDs can turn on and off, and the shorter radiative lifetime translates directly into faster switching and therefore a higher data transmission rate," says Efros.

A paper on this research was published in Nature.

To contact the author of this article, email Siobhan.Treacy@ieeeglobalspec.com


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