The US Rensselaer Polytechnic Institute has designed a power LED that retains efficiency at high current.
“The device achieves a notable reduction in efficiency droop, a well-known phenomenon that provokes LEDs to be most efficient when receiving low-density currents, but then to lose efficiency as higher currents are fed into the device,” said the university.
“The cause of this droop is not yet fully understood, but studies have shown that electron leakage is likely a large part of the problem.”
The polarisation-matched LED, developed in collaboration with Samsung Electro-Mechanics, exhibits less leakage and a lower forward voltage leading to 18% increase in light output and a 22% increase in wall-plug efficiency, claims Rensselaer.
“Droop is under the spotlight since today’s high-brightness LEDs are operated at current densities far beyond where efficiency peaks,” said project leader Professor Fred Schubert.
“Our LED has a radically re-designed active region, namely a polarization-matched active region, and brings LEDs closer to being able to operate efficiently at high current densities.”
In Applied Physics Letters (PDF), the Rensselaer team said: “Previous studies have indicated that electron leakage from the active region is enhanced by sheet charges at heterointerfaces that result from polarisation mismatch between layers in a conventional LED active region,” and that leakage can be strongly reduced by changing the quantum-barrier design – for example by replacing the conventional GaInN/GaN heterojunction with GaInN/GaInN.
“This substitution allows the layers of the active region to have a better matched polarisation, and in turn reduce both electron leakage and efficiency droop,” claimed Rensselaer.
“The benefits seen by testing the GaInN/GaInN LED were consistent with theoretical simulation s showing polarisation matching reducing electron leakage and efficiency droop.”
However, low current density efficiency – below 50A/cm2 – was worse than the conventional structure.