Theory confirmed on why LEDs are less efficient at high drive currents

Researchers have long sought to discover the mechanism responsible for causing light-emitting diodes (LEDs) to be less efficient at high drive currents. 

 
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The phenomenon is referred to as the “LED droop” and researchers at the University of California – Santa Barbara (UCSB) and École Polytechnique believe they have developed a method that confirms Auger recombination to be its root cause. This is a noteworthy discovery because  until now, only theories existed as to why a sudden drop in light would occur when a higher current was applied to the LED.

“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,” commented Claude Weisbuch, distinguished professor of Materials at UCSB.

“Rising to this potential has been contingent upon solving the puzzle of LED efficiency droop,” commented James 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 non-radiative recombination and produce higher light output.”

The study actually dates back to 2011. That’s when UCSB professor Chris van de Walle and colleagues first theorized that a complex non-radiative process called “Augber recombination” was responsible for nitride semiconductor LED droop; specifically, that injected electrons colliding with other electrons lost energy to heat rather than emit light.

Using this theory as the groundwork for their study, Speck, Weisburch, and their research team developed a definitive measurement for Auger recombination. They then employed this technique in an experiment using an LED that had its surface specially prepared to allow the researchers to directly measure the energy spectrum of electrons emitted from the LED.

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

The results of the experiment were groundbreaking, as they showed a very distinct, very clear signature line of electrons being produced on the surface of the LED by the Auger process. A paper was produced by the group that details the full study and all of its results. It will be published in the journal Physical Review Letters.

If you don’t want to wait for the paper to come out, you can download a similar copy of their manuscript via the Cornell University Library.

Story via: engineering.ucsb.edu