A team of researchers at the Rensselaer Polytechnic Institute has identified the reason behind efficiency droop—a mechanism that causes LEDs to lose up to 20 per cent of their efficiency as they are subjected to greater electrical currents. The flaw has been a key obstacle in the development of LED lighting for situations such as household lighting that call for economical sources of versatile and bright light.

In a recent paper, the researchers identified a phenomenon known as "electron leakage" as the culprit. The research offers the first comprehensive model for the mechanism behind efficiency droop, and may lead to new technologies to solve the problem, said E. Fred Schubert, founding director of the university's National Science Foundation-funded Smart Lighting Engineering Research Center, and senior author on the study.

"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," said Schubert. "I think now we have a better understanding of what causes the droop and this opens up specific strategies to address it."

LEDs 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 LED 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."

David Meyaard, first author on the study and a doctoral student in electrical engineering, stated that 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.