LEDs have a mysterious drop in the light produced called “droop” when a higher current is applied. That limits the amount of lumens an LED can make thus holding the technology down as a viable replacement for incandescent bulbs for all-purpose commercial and residential lighting.
The situation could change now that the cause of LED efficiency droop has been explained. Researchers at University of California, Santa Barbara (UCSB) led by 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 collaboration with colleagues at the École Polytechnique in France, have conclusively identified Auger recombination as the mechanism that causes LEDs to be less efficient at high drive currents.
LED Test for Auger Recombination. LED emitting light under forward bias in an ultra high vacuum chamber allowing simultaneous electron emission energy. Image Credit: Ecole Ploytechnique, Ph. Lavialle. Click image for the largest view.
The knowledge gained from the research is expected to result in new ways to design LEDs that will have significantly higher light emission efficiencies. LEDs have enormous potential for providing long-lived high quality efficient sources of lighting for residential and commercial applications.
The significance of a very efficient LED is pointed out by the U.S. Department of Energy’s recent estimate that the widespread replacement of incandescent and fluorescent lights by LEDs in the U.S. could save electricity use equal to the total output of fifty 1 Gigawatt power plants. That would be roughly 10% of U.S. power generation.
It’s a worthwhile pursuit.
Speck who is professor of Materials and the Seoul Optodevice Chair in Solid State Lighting at UCSB said, “Rising to this potential has been contingent upon solving the puzzle of LED efficiency droop. These findings will enable us to design LEDs that minimize the non-radiative recombination and produce higher light output.”
Weisbuch who is distinguished professor of Materials at UCSB is also a faculty member at the École Polytechnique in Paris said, “This was a very complex experiment – one that illustrates the benefits of teamwork through both an international collaboration and a DOE Energy Frontier Research Center.”
Weisbuch enlisted the support of his colleagues Lucio Martinelli and Jacques Peretti in Paris. UCSB graduate student Justin Iveland was a key member of the team working both at UCSB and École Polytechnique.
The research follows work culminating in 2011 of UCSB professor Chris van de Walle and colleagues theorizing 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.
The task has been getting a definitive measurement of Auger recombination in LEDs so supporting the theory. Speck, Weisbuch, and their research team have the measuring technique and the results in the “bank”.
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.
Optimism and theory aside, the work doesn’t propose an engineering solution. But the work does offer that the theory explanation provides a base for engineering LEDs that can take higher current and produce much more luminosity.
The name of the game in lighting is the lumens, or the amount of light emitted for use by our eyes. Today’s LEDs are compositions of many LEDs wired up and arranged to accomplish the task. The research may offer a lot less LEDS in arrangements offering lower costs and better light distribution.
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