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‘Auger recombination theory’ is responsible for LED ‘droop’ phenomenon


April 25, 2013
By Anthony Capkun

April 25, 2013 – Researchers at University of California, Santa Barbara (UCSB), in collaboration with colleagues at the Ecole Polytechnique in France, say they have conclusively identified Auger recombination as the mechanism that causes light emitting diodes (LEDs) to be less efficient at high drive currents.

LED ‘droop’ is a drop in the light produced when a higher current is applied. The cost per lumen of LEDs has held the technology back as a viable replacement for incandescent bulbs, say the researchers, for all-purpose commercial and residential lighting. But this could change now that the cause of LED efficiency droop has been explained, say researchers James Speck and Claude Weisbuch of the Center for Energy Efficient Materials at UCSB (an Energy Frontier Research Center sponsored by the U.S. Department of Energy).

In 2011, UCSB professor Chris van de Walle and colleagues theorized that a complex non-radiative process known as Auger recombination was behind nitride semiconductor LED droop, whereby injected electrons lose energy to heat by collisions with other electrons rather than emitting light. A definitive measurement of Auger recombination in LEDs has now been accomplished by Speck, Weisbuch, and their research team.

The experiment used an LED with a specially prepared surface that permitted the researchers to directly measure the energy spectrum of electrons emitted from the LED. The results unambiguously showed a signature of energetic electrons produced by the Auger process.

Knowledge gained from this study is expected to result in new ways to design LEDs that will have significantly higher light emission efficiencies. Researchers add that the U.S. Department of Energy estimates that the widespread replacement of incandescent and fluorescent lights by LEDs in the States could save electricity equal to the total output of 50 1-GW power plants.

“These findings will enable us to design LEDs that minimize the non-radiative recombination and produce higher light output,” said Speck, professor of Materials and the Seoul Optodevice Chair in Solid State Lighting at UCSB.

PHOTO 1: LED emitting light under forward bias in an ultra-high vacuum chamber allowing simultaneous electron emission energy.

PHOTO 2: (From left) Jacques Peretti, Claude Weisbuch and Lucio Martinelli.

PHOTO 3: (From left) UC Santa Barbara researchers Justin Iveland and Professor James Speck.