Princeton University researchers have developed a new method to
increase the brightness, efficiency and clarity of LEDs, which are
widely used on smartphones and portable electronics as well as becoming
increasingly common in lighting.
Using a new nanoscale structure, the researchers, led by electrical engineering professor Stephen Chou,
increased the brightness and efficiency of LEDs made of organic
materials (flexible carbon-based sheets) by 57 percent. The researchers
also report their method should yield similar improvements in LEDs made
in inorganic (silicon-based) materials used most commonly today.
The method also improves the picture clarity of LED displays by 400 percent, compared with conventional approaches. In an article published online August
19 in the journal Advanced Functional Materials, the researchers
describe how they accomplished this by inventing a technique that
manipulates light on a scale smaller than a single wavelength.
"New nanotechnology can change the rules of the ways we manipulate
light," said Chou, who has been working in the field for 30 years. "We
can use this to make devices with unprecedented performance."
A LED, or light-emitting diode, is an electronic device that emits
light when electrical current moves through two terminals. LEDs offer
several advantages over incandescent or fluorescent lights: they are far
more efficient, compact and have a longer lifetime, all of which are
important in portable displays.
Current LEDs have design challenges; foremost among them is to reduce
the amount of light that gets trapped inside the LED's structure.
Although they are known for their efficiency, only a very small amount
of light generated inside an LED actually escapes.
"It is exactly the same reason that lighting installed inside a
swimming pool seems dim from outside — because the water traps the
light," said Chou, the Joseph C. Elgin Professor of Engineering. "The
solid structure of a LED traps far more light than the pool's water."
In fact, a rudimentary LED emits only about 2 to 4 percent of the light
it generates. The trapped light not only makes the LEDs dim and energy
inefficient, it also makes them short-lived because the trapped light
heats the LED, which greatly reduces its lifespan.
"A holy grail in today's LED manufacturing is light extraction," Chou said.
Engineers have been working on this problem. By adding metal
reflectors, lenses or other structures, they can increase the light
extraction of LEDs. For conventional high-end, organic LEDs, these
techniques can increase light extraction to about 38 percent. But these
light-extraction techniques cause the display to reflect ambient light,
which reduces contrast and makes the image seem hazy.
To combat the reflection of ambient light, engineers now add
light-absorbing materials to the display. But Chou said such materials
also absorb the light from the LED, reducing its brightness and
efficiency by as much as half.
Princeton researchers have used their expertise in nanotechnology to
develop an economical new system that markedly increases the brightness,
efficiency and clarity of LEDs, which are widely used in smartphones
and other electronics. The illustration demonstrates how a conventional
LED's structure traps most of the light generated inside the device; the
new system, called PlaCSH, guides the light out of the LED. (lllustration courtesy of Stephen Chou et al.)
The solution presented by Chou's team is the invention of a
nanotechnology structure called PlaCSH (plasmonic cavity with
subwavelength hole-array). The researchers reported that PlaCSH
increased the efficiency of light extraction to 60 percent, which is 57
percent higher than conventional high-end organic LEDs. At the same
time, the researchers reported that PlaCSH increased the contrast
(clarity in ambient light) by 400 percent. The higher brightness also
relieves the heating problem caused by the light trapped in standard
LEDs.
Chou said that PlaCSH is able to achieve these results because its
nanometer-scale, metallic structures are able to manipulate light in a
way that bulk material or non-metallic nanostructures cannot.
Chou first used the PlaCSH structure on solar cells, which convert
light to electricity. In a 2012 paper, he described how the application
of PlaCSH resulted in the absorption of as much as 96 percent of the
light striking solar cells' surface and increased the cells' efficiency
by 175 percent. Chou realized that a device that was good at absorbing
light from the outside could also be good at radiating light generated
inside the device — offering an efficient solution for both light
extraction and the reduction of light reflection.
"From a view point of physics, a good light absorber, which we had for
the solar cells, should also be a good light radiator," he 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 physics behind PlaCSH are complex, but the structure is relatively
simple. PlaCSH has a layer of light-emitting material about 100
nanometers thick that is placed inside a cavity with one surface made of
a thin metal film. The other cavity surface is made of a metal mesh
with incredibly small dimensions: it is 15 nanometers thick; and each
wire is about 20 nanometers in width and 200 nanometers apart from
center to center. (A nanometer is one hundred-thousandth the width of a
human hair.)
PlaCSH has a layer of light-emitting material about 100 nanometers
thick that is placed inside a cavity with one surface made of a thin
metal film (shown at left.) The key part of the device is a metal mesh
(center) with incredibly small dimensions: it is 15 nanometers thick;
and each wire is about 20 nanometers in width and 200 nanometers apart
from center to center. An image of the experimental LED is shown at
right. (Images courtesy of Stephen Chou et al.)
Because PlaCSH works by guiding the light out of the LED, it is able to
focus more of the light toward the viewer. 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.
Another benefit for manufacturers is cost. The PlaCSH organic LEDs were
made by nanoimprint, a technology Chou invented in 1995, which creates
nanostructures in a fashion similar to a printing press producing
newspapers.
"It is cheap and extremely simple," Chou said.
Princeton has filed patent applications for both organic and inorganic
LEDs using PlaCSH. Chou and his team are now conducting experiments to
demonstrate PlaCSH in red and blue organic LEDs, in addition to the
green LEDs used in the current experiments. They also are demonstrating
the system in inorganic LEDs.
Besides Chou, the paper's authors are Wei Ding, Yuxuan Wang and Hao
Chen, graduate students in electrical engineering at Princeton. Support
for the research was provided in part by the Defense Advanced Research
Projects Agency and the Office of Naval Research. Chou recently was
awarded a major grant from the U.S. Department of Energy to further
advance the use of PlaCSH as a solution for energy-efficient lighting.