New clues to LEDs' efficiency droop

In the move to use LEDs to illuminate buildings and neighborhoods, gallium nitride–based semiconductors have taken center stage. Of the few semiconductors that can emit at green, blue, and shorter wavelengths, they are by far the most efficient and, as such, are key ingredients in LED-generated white light. But GaN-based LEDs can more efficiently light a smartphone display than a parking lot: Above a modest current threshold—around 10 A/cm²—they convert progressively less of their incoming electricity into light, a phenomenon known as "droop." Now a collaboration led by Claude Weisbuch (University of California, Santa Barbara, and École Polytechnique, Palaiseau, France) may know why. By manipulating the surface electric field of a GaN LED, the team coaxed the device to emit some of its electrons into an ultrahigh vacuum, where they could be analyzed with an electron spectrometer. Whenever the injection current was sufficiently high to produce efficiency droop, the LED spat out some electrons with energies roughly 1.5 eV higher than the rest. Weisbuch and his colleagues concluded that the high-energy electrons could only have been generated by the Auger effect, in which recombining electron–hole pairs, instead of releasing energy as light, transfer their energy to nearby charge carriers. If that's indeed the case, it would come as vindication for researchers from Philips Lumileds Lighting Co who seven years ago proposed just such an explanation. (J. Iveland et al., Phys. Rev. Lett. 110, 177406, 2013.)—Ashley G. Smart