The top figure showcases "LED droop", which decreases LED lighting efficiency and brightness. The Rensselaer Polytechnic Institute and Samsung have come up with a new LED, whose output profile is shown below, which greatly reduces droop.  (Source: Rensselaer Polytechnic Institute/Samsung)
New technology may help LED lighting inch closer to market

In the U.S. and elsewhere, lighting accounts for as much as a third of electricity usage.  Thus, when it comes to fossil fuels conservation, there is a large impetus to adopt more efficient lighting solutions.  Right now the most efficient lighting solution is LED lighting, which if widely adopted could save 10 percent of the power used in the U.S.  Unfortunately, LED lighting is also by far the most expensive form of lighting.  To spur research in to lower LED lighting costs, the U.S. Department of Energy (DOE) has offered a $20M USD "L Prize" for the first team to meet a rigorous set of standards.

A new breakthrough by researchers with Rensselaer Polytechnic Institute's National Science Foundation-funded Smart Lighting Engineering Research Center and Samsung Electro-Mechanics may bring LED lighting closer to affordability and help the researchers creep closer to the DOE-funded jackpot.

The new type of LED is said to be "polarization matched", but what's most important is its improved metrics.  The light offers 18 percent increase in light output and a 22 percent increase in wall-plug efficiency, which essentially measures the amount of electricity the LED converts into light.

The new kind of LED greatly diminishes a common problem with LEDs called "efficiency droop".  This phenomenon involves LEDs being most efficient when operating on low current densities, and seeing their efficiency greatly droop at higher current densities.  While all the factors have yet to be determined, electron leakage is one source of droop.  The end result of droop is that LEDs are forced to operate at lower current densities, which feature much lower brightness and efficiency in achieving light output.

Project leader E. Fred Schubert, Wellfleet Senior Constellation Professor of Future Chips at Rensselaer states, "This droop is under the spotlight since today’s high-brightness LEDs are operated at current densities far beyond where efficiency peaks.  This challenge has been a stumbling block, because reducing the current densities to values where LEDs are more efficient is unacceptable. Our new LED, however, which has a radically re-designed active region, namely a polarization-matched active region, tackles this issue and brings LEDs closer to being able to operate efficiently at high current densities." 

His team discovered that a cause of electron leakage was mismatched polarization.  The used a quantum-barrier design to help to greatly reduce the mismatch.  The conventional Gallium Indium Nitride/Gallium Nitride (GaInN/GaN) layer of the LED active region was replaced with Gallium Indium Nitride/ Gallium Indium Nitride (GaInN/GaInN).  The decrease in polarization mismatch, in turn decreased electron leakage, in turn lessening droop and delivering superior brightness and efficiency.

The gains present in lab testing were similar to theoretical models, created in computer simulations.  Professor Schubert says he expects discoveries like this one to propel solid state lighting into the mainstream, which he says will amount to vast environmental, energy, and cost benefits as well as innovations in healthcare, transportation systems, digital displays, and computer networking.

LED lighting has already found its way into many vehicles.  The list of models sporting LED headlights continues to grow, as the technology prepares to exit the high-end market and move into the mainstream auto market.

The new research is reported in this week's Applied Physics Letters.

Other researchers on the project include Rensselaer physics, Future Chips, and electrical engineering graduate students Jiuru Xu, Martin F. Schubert, and Ahmed N. Noemaun; Rensselaer Future Chips research assistant Di Zhu; Jong Kyu Kim, research assistant professor of electrical, computer, and systems engineering at Rensselaer; along with Samsung Electro-Mechanics researchers Min Ho Kim, Hun Jae Chung, Sukho Yoon, Cheolsoo Sone, and Yongjo Park.

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