Rensselaer researchers boost green LED efficiency

April 26, 2011
Research at Rensselaer Polytechnic Institute leverages a nanoscale pattern to boost the optical efficiency of green LEDs in the lab.

Green LED efficiency has long lagged behind the capabilities of red and blue devices, but researchers at the Rensselaer Polytechnic Institute (RPI) have reported significant improvements in the lab. The team is forming nanoscale patterns on a sapphire substrate that boosts light extraction, internal efficiency, and light output.

More efficient green LEDs would benefit both solid-state lighting (SSL) in general-illumination applications and TV and display technology. Equally efficient red, green, and blue (RGB) LEDs could yield white light for illumination and could improve the color gamut in display or TV applications.

Some early LED backlights for TVs utilized RGB LEDs, but the designs generally had to include two green LEDs for every red and blue one - adding significant expense and power consumption to the design. Likewise, RGB LEDs have been used in lighting but the need for extra green LEDs limits the applicability.

Green LED inefficiency stems from a semiconductor physics phenomenon called the charge separation effect. In green LEDs, electrons and electron holes are separated in the quantum-well region of the device. Light is generated when electrons combine with electron holes, but the separation results in fewer such combinations in green LEDs. Researchers have struggled for years with the problem.

Christian Wetzel, Professor of Physics and the Wellfleet Constellation Professor of Future Chips at RPI, is leading the green LED research team. The team etched a nanoscale pattern between the sapphire substrate and the gallium nitride (GaN) material in which the LED is grown.

The team reports a doubling in the internal quantum efficiency of the LED. Moreover, the design enhances light extraction by a factor of 58%. The result could be brighter green LEDs although its unclear how soon the research could be applied in LED production.

“Every computer monitor and television produces its picture by using red, blue, and green. We already have powerful, inexpensive red and blue LEDs. Once we develop a similar green LED, it should lead to a new generation of high-performance, energy-efficient display and illumination devices. This new research finding is an important step in the right direction,” said Wetzel. “Green LEDs are proving much more challenging to create than academia and the industry ever imagined.”

The research team published the results of its work in a paper entitled “Defect-reduced green GaInN/GaN light-emitting diode on nanopatterned sapphire” that was published in the Virtual Journal of Nanoscale Science & Technology.

Wetzel’s team was supported by the US Department of Energy, National Energy Technology Laboratory (NETL) Solid-State Lighting Contract of Directed Research, and the National Science Foundation (NSF) Smart Lighting Engineering Research Center (ERC), which is led by Rensselaer.