Zinc Oxide microwires improve performance light-emitting diodes

Zinc Oxide microwires improve performance of light-emitting diodes

1:31 AM, 8th November 2011
Zinc Oxide microwires improve performance of light-emitting diodes
Georgia Tech Regents Professor Zhong Lin Wang (right) and Graduate Research Assistant Ying Liu study light-emitting diodes whose performance has been enhanced through the piezo-phototronic effect.

 

ATLANTA, GEORGIA: Researchers have used zinc oxide microwires to significantly improve the efficiency at which gallium nitride light-emitting diodes (LED) convert electricity to ultraviolet light. The devices are believed to be the first LEDs whose performance has been enhanced by the creation of an electrical charge in a piezoelectric material using the piezo-phototronic effect.

By applying mechanical strain to the microwires, researchers at the Georgia Institute of Technology created a piezoelectric potential in the wires and that potential was used to tune the charge transport and enhance carrier injection in the LEDs. This control represents another example of how materials that have both piezoelectric and semiconducting properties can be controlled mechanically.

“By utilizing this effect, we can enhance the external efficiency of these devices by a factor of more than four times, up to eight per cent,” said Zhong Lin Wang, Regents Professor in the Georgia Tech School of Materials Science and Engineering. “From a practical standpoint, this new effect could have many impacts for electro-optical processes - including improvements in the energy efficiency of lighting devices.”

Details of the research were reported in the Sept 14 issue of the journal Nano Letters. The research was sponsored by the Defense Advanced Research Projects Agency (DARPA) and the US Department of Energy (DOE). In addition to Wang, the research team mainly included Qing Yang, a visiting scientist at Georgia Tech from the Department of Optical Engineering at Zhejiang University in China.

Because of the polarization of ions in the crystals of piezoelectric materials such as zinc oxide, mechanically compressing or otherwise straining structures made from the materials creates a piezoelectric potential -- an electrical charge.

The LEDs fabricated by the research team produced emissions at ultraviolet wavelengths (about 390 nanometres), but Wang believes the wavelengths can be extended into the visible light range for a variety of optoelectronic devices. “These devices are important for today’s focus on green and renewable energy technology,” he said.

In the experimental devices, the researchers studied the change in light emission produced by varying the amount of strain in 20 different devices. Half of the devices showed enhanced efficiency, while the others - fabricated with opposite orientation of the microwires - showed a decrease.

High-efficiency ultraviolet emitters are needed for applications in chemical, biological, aerospace, military and medical technologies. Although the internal quantum efficiencies of these LEDs can be as high as 80 per cent, the external efficiency for a conventional single p-n junction thin-film LED is currently only about three percent.

Beyond LEDs, Wang believes the approach pioneered in this study can be applied to other optical devices that are controlled by electrical fields.

“This opens up a new field of using the piezoelectric effect to tune opto-electronic devices,” said Wang. “Improving the efficiency of LED lighting could ultimately be important, bringing about significant energy savings because so much of the world’s energy is used for lighting.”

(C) Georgia Institute of Technology News

 

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