Published on August 6th, 2013 | by James Ayre2
LED “Efficiency Droop” Problem Solved
August 6th, 2013 by James Ayre
One of the key obstacles to the wider adoption of LED lighting may have just been solved, nearly 15 years after its discovery — the mechanism behind the flaw known as “efficiency droop” has finally been identified, potentially leading the way to new techniques/technologies to address it.
Efficiency droop is a flaw in LED lightbulbs that causes efficiencies to drop by as much as 20% when the LEDs are subjected to greater electrical currents — the flaw, first discovered back in 1999, has been a major obstacle to the “development of LED lighting for situations, like household lighting, that call for economical sources of versatile and bright light.”
The newly identified culprit is “electron leakage,” as the researchers from the Rensselaer Polytechnic Institute researchers have termed it. The new work has resulted in the first comprehensive model of this phenomena.
“In the past, researchers and LED manufacturers have made progress in reducing efficiency droop, but some of the progress was made without understanding what causes the droop,” stated E Fred Schubert, the Wellfleet Senior Constellation Professor of Future Chips at Rensselaer, the founding director of the university’s National Science Foundation-funded Smart Lighting Engineering Research Center, and primary author of the new study. “I think now we have a better understanding of what causes the droop and this opens up specific strategies to address it.”
The Rensselaer Polytechnic Institute provides background and explanation:
Light-emitting diodes take advantage of the fact that high-energy electrons emit photons, i.e. particles of light, as they move from a higher to a lower energy level. The light-emitting diode is constructed of three sections: an “n-type” section of crystal that is loaded with negatively charged electrons; a p-type section of crystal that contains many positively charged “holes;” and a section in between the two called the “quantum well” or “active region.”
Electrons are injected into the active region from the n-type material as holes are injected into the active region from the p-type material. The electrons and holes move in opposite directions and, if they meet in the active region, they recombine, at which point the electron moves to a lower state of energy and emits a photon of light. Unfortunately, researchers have noticed that as more current is applied, LEDs lose efficiency, producing proportionally less light as the current is increased.
What the new research has shown is that, under the “high current regime,” an electric field develops inside the p-type region of the diode, which allows electrons to escape from the active region. As a result, these escaped electrons are unable to recombine with holes and emit photons of light — hence the decreased efficiency. Such a phenomenon was first put forward as a potential explanation over 5 years ago, but this new research has provided the first “incontrovertible evidence that it is the cause behind efficiency droop.” The researchers identified the electric field as it was built up, and observed that, after a strong enough field was present, the electrons were able to “escape” from the active region.
“We measure excellent correlation between the onset of field-buildup and the onset of droop,” stated David Meyaard, first author on the study and a doctoral student in electrical engineering. “This is clear evidence that the mechanism is electron leakage, and we can describe it quantitatively. For example, in one key 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.”
“The work shows that because electrons have a greater ‘mobility’ than holes, the diode is made from disparate types of carriers. 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,” stated 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.”
The researchers say that they will now focus their efforts on the development of a new structure for LEDs.