Nanocavities Increase LED Display Output, Contrast

Nanocavities can help light escape from LEDs to produce brighter, clearer and more efficient displays.

Researchers at Princeton University reported increasing the efficiency of green organic LEDs (OLEDs) to 60 percent — 57 percent higher than conventional high-end OLEDs — using a new system called a plasmonic cavity with subwavelength hole-array. The results should be applicable to other colors of OLEDs, as well as to silicon-based LEDs.

The approach is based on earlier work by professor Dr. Stephen Chou in solar cells which resulted in absorption of 96 percent of sunlight and a 175 percent increase in cell efficiency.

The illustration demonstrates how a conventional LED's structure traps most of the light generated inside the device; the new system guides the light out of the LED. Courtesy of Dr. Stephen Chou/Princeton University.

“From a viewpoint of physics, a good light absorber, which we had for the solar cells, should also be a good light radiator,” Chou said. “We wanted to experimentally demonstrate this is true in visible light range, and then use it to solve the key challenges in LEDs and displays.”

The new system involves a layer of light-emitting material about 100 nm thick inside a cavity with one surface made of a metal thin film. The other cavity surface is made of a metal mesh of 20-nm-diameter antennas, each about 200 nm apart. The OLEDs were made by nanoimprint over an area of about 1000 cm².

The system also replaces the conventional brittle, transparent electrode, making it far more flexible than most current displays. “It is so flexible and ductile that it can be weaved into a cloth,” Chou said.

Rudimentary LEDs emits only about 2 to 4 percent of the light they generate. The trapped light not only makes the LEDs dim and energy inefficient, the trapped light produces heat and reduces the LED’s lifespan.

Metal reflectors, lenses and other optical structures can increase light extraction to about 38 percent, but cause the display to reflect ambient light, which reduces contrast and makes the image seem hazy. Efforts to counteract glare often result in light from the LED being absorbed along with ambient light.

OLEDs incorporating the new plasmonic cavities demonstrated a contrast increase of 400 percent, the researchers said.

Princeton has filed patent applications based on the research, which was funded by DARPA and the Office of Naval Research.

The research was published in Advanced Functional Materials (doi: 10.1002/adfm.201400964).

For more information, visit www.princeton.edu.