Proved: Why LED efficiency drops at high current
Researchers at the US Rensselaer Polytechnic have proven a link between LED efficiency drop at high current and carrier mobility. Better lighting LEDs could result.
“Efficiency droop, first reported in 1999, has been a key obstacle in the development of LED lighting for situations, like household lighting, that call for economical sources of versatile and bright light,” said the university.
Published in Applied Physics Letters, the work shows that at high current an electric field develops within the p-type region of the diode allowing electrons to escape the active region where they would otherwise re-combine with holes to emit photons – a mechanism previously proposed, but not proven, said Rensselaer.
“We measure excellent correlation between the onset of field build-up and the onset of droop. This is clear evidence that the mechanism is electron leakage, and we can describe it quantitatively,” said Rensselaer researcher David Meyaard. “For example, in one result reported in the paper, we show the onset of high injection and the onset of droop and you can see that they are very nicely correlated. And that was just not possible in the past because there was really no theoretical model that described how electron leakage really works.”
“If the holes and the electrons had similar properties, there is a symmetry; both would meet in the middle, where the quantum well is, and there they recombine,” said Professor Fred Schubert. “What we have instead is a material system where the electrons are much more mobile than the holes. And because they are very mobile, they diffuse more easily, they also react more easily to an electric field. Because of that asymmetry, or disparity, we have a propensity of the electrons to shoot over and to be extracted from the quantum well. And so they don’t meet the hole in the active region and so they don’t emit light.”
Based on the theoretical model, Meyaard and Schubert will look for LED structures that reduce the issue.Tags: asymmetry, disparity, electrons, household lighting