Intricate experiment reveals cause of LED droop

Ashley N. Rice, ashley.rice@photonics.com

The future looks bright for alternative lighting, now that scientists have solved the mystery that causes LEDs to “droop” in efficiency at high currents.

The phenomenon known as LED droop has intrigued researchers for years. In 2011, University of California, Santa Barbara, professor Chris Van de Walle and colleagues theorized that a complex nonradiative 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.

Now, a definitive measurement of this complex process has been accomplished by James Speck and Claude Weisbuch of UCSB, in collaboration with colleagues at École Polytechnique in Paris. The achievement, which could help optimize cost per lumen, could put the technology back into the spotlight as a viable replacement for incandescent bulbs for residential and commercial lighting.

The experiment used an LED with a specially prepared surface that permitted the investigators 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.

“This was a very complex experiment – one that illustrates the benefits of teamwork through both an international collaboration and a DOE (Department of Energy) Energy Frontier Research Center,” said Weisbuch, distinguished professor of materials at UCSB’s Center for Energy Efficient Materials – an Energy Frontier Research Center sponsored by the DOE – and a faculty member at École Polytechnique.

Knowledge from this study could result in new ways to design LEDs that will have significantly higher light emission efficiencies. Replacing incandescent and fluorescent lights with LEDs in the US could save electricity equal to the total output of 50 1-GW power plants, according to the DOE.

“Rising to this potential has been contingent upon solving the puzzle of LED efficiency droop,” said Speck, professor of materials and the Seoul Optodevice Chair in Solid-State Lighting at UCSB. “These findings will enable us to design LEDs that minimize the nonradiative recombination and produce higher light output.”

The study appeared in Physical Review Letters (doi: 10.1103/physrevlett.110.177406).

For additional information, see “LED Efficiency Puzzle Reportedly Solved”.