Boffins explain LED inefficiencies

Electrons misbehave at higher energies

One of the problems with using LED-based lamps to replace incandescent or fluorescent lamps is that they're expensive: not only do they need more electronics than the alternatives, LED efficiency is capped by a fall in light output at higher current.

To answer the so-what question: getting rid of the “droop effect” allows LEDs to be made more efficient – and that would make them cheaper. But to eliminate the droop, you first need to find out why it happens.

Enter a group of scientists from the US Department of Energy-sponsored Centre for Energy-Efficient Materials at the University of California, Santa Barbara. Working with colleagues from France's École Polytechnique, they have watched what's going on at an atomic level and concluded that the “droop” is down to what's known as an “Auger recombination” process.

The process has been known for some time, and is well-understood in semiconductors, but its status as a candidate for LED droop has been controversial since it was first proposed in 2011. The Auger effect describes how an electron falling from a high energy level to an inner-shell state can release energy as a photon, or transfer its energy to another electron that leaves the atom.

As Wikipedia explains, in Auger recombination, an electron and a hole recombine and energy is transferred to an electron in the conduction band. The result is that some of the electrons energy arriving in the LED's drive current is given off as heat instead of light.

Observing the Auger recombination taking place should pave the way for more efficient LED designs, the researchers explained to Phys.org.

One of the researchers, professor James Speck said the findings “will enable us to design LEDs that minimise the non-radiative recombination and produce higher light output.”

The researchers observed the phenomenon using electron emission spectroscopy to see what was taking place in a gallium nitride (GaN) LED. The surface of the LED was prepared so as to allow the researchers to directly measure the energy spectrum of electrons emitted by the device.

“The signature of Auger electrons is observed through high energy peaks which appear in the electron energy distribution curves (EDCs) at high injected current densities. The Auger electron current is found to correlate with the simultaneously observed droop in emission efficiency”, they write in their paper, available at Arxiv.

The paper is also to be published in Physical Review Letters. ®

Bootnote: In the original version of this story I wrote "electrons" where I meant to write "energy". Thanks for pointing out the error. ®