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Connecting Theory and Practice in Optoelectronics
I conducted a little poll among the attendees of the LED conference at Photonics West in San Francisco last week. They still turn out to be divided over the microscopic mechanism behind the high-power efficiency loss in GaN-based LEDs. This droop phenomenon threatens the success of solid-state lighting applications since the energy efficiency of LED light bulbs remains close to that of traditional fluorescent lamps. I found a similar division in research publications on this topic, details are given in this review. Most agree that the efficiency loss is caused by non-radiative electron-hole recombination. However, the debate is over which specific mechanism dominates and what to do about it. The two leading explanations are Auger recombination and electron leakage, respectively, which are mainly based on modeling. Very few direct measurements of either mechanism are published thus far, none of which established a dominating magnitude. Auger recombination is a fundamental process that cannot be eliminated and the LED industry seems to have settled for this pragmatic explanation. Electron leakage, on the other hand, should be avoidable in a cleverly designed LED, and this optimistic approach is still favored by many university researchers. A third, defect-related mechanism is suspected by other scientists, based on the non-ideal nano-structure of the light-emitting layers. After 10 years of intense research worldwide and more than 1000 papers written on this topic, what can we possibly do to find more clarity?
Update 2015-09-01: Latest results are now published in Applied Physics Letters.