https://phys.org/news/2019-02-firefly-inspired-surfaces-efficiency-lightbulbs.html
[links and images in online article]
Firefly-inspired surfaces improve efficiency of LED lightbulbs
February 19, 2019 by A'ndrea Elyse Messer, Pennsylvania State University
A new type of light-emitting diode lightbulb could one day light homes
and reduce power bills, according to Penn State researchers who suggest
that LEDs made with firefly-mimicking structures could improve efficiency.
"LED lightbulbs play a key role in clean energy," said Stuart (Shizhuo)
Yin, professor of electrical engineering. "Overall commercial LED
efficiency is currently only about 50 percent. One of the major concerns
is how to improve the so-called light extraction efficiency of the LEDs.
Our research focuses on how to get light out of the LED."
Fireflies and LEDs face similar challenges in releasing the light that
they produce because the light can reflect backwards and is lost. One
solution for LEDs is to texture the surface with
microstructures—microscopic projections—that allow more light to escape.
In most LEDs these projections are symmetrical, with identical slopes on
each side.
Fireflies' lanterns also have these microstructures, but the researchers
noticed that the microstructures on firefly lanterns were asymmetric—the
sides slanted at different angles, giving a lopsided appearance.
"Later I noticed not only do fireflies have these asymmetric
microstructures on their lanterns, but a kind of glowing cockroach was
also reported to have similar structures on their glowing spots," said
Chang-Jiang Chen, doctoral student in electrical engineering and lead
author in the study. "This is where I tried to go a little deeper into
the study of light extraction efficiency using asymmetric structures."
Using asymmetrical pyramids to create microstructured surfaces, the team
found that they could improve light extraction efficiency to around 90
percent. The findings were recently published online in Optik and will
appear in the April print edition.
According to Yin, asymmetrical microstructures increase light extraction
in two ways. First, the greater surface area of the asymmetric pyramids
allows greater interaction of light with the surface, so that less light
is trapped. Second, when light hits the two different slopes of the
asymmetric pyramids there is a greater randomization effect of the
reflections and light is given a second chance to escape.
After the researchers used computer-based simulations to show that the
asymmetric surface could theoretically improve light extraction, they
next demonstrated this experimentally. Using nanoscale 3-D printing, the
team created symmetric and asymmetric surfaces and measured the amount
of light emitted. As expected, the asymmetric surface allowed more light
to be released.
The LED-based lighting market is growing rapidly as the demand for clean
energy increases, and is estimated to reach $85 billion by 2024.
"Ten years ago, you go to Walmart or Lowes, LEDs are only a small
portion (of their lighting stock)," said Yin. "Now, when people buy
lightbulbs, most people buy LEDs."
LEDs are more environmentally friendly than traditional incandescent or
fluorescent lightbulbs because they are longer-lasting and more energy
efficient.
Two processes contribute to the overall efficiency of LEDs. The first is
the production of light—the quantum efficiency—which is measured by how
many electrons are converted to light when energy passes through the LED
material. This part has already been optimized in commercial LEDs. The
second process is getting the light out of the LED—called the light
extraction efficiency.
"The remaining things we can improve in quantum efficiency are limited,"
said Yin. "But there is a lot of space to further improve the light
extraction efficiency."
In commercial LEDs, the textured surfaces are made on sapphire wafers.
First, UV light is used to create a masked pattern on the sapphire
surface that provides protection against chemicals. Then when chemicals
are applied, they dissolve the sapphire around the pattern, creating the
pyramid array.
"You can think about it this way, if I protect a circular area and at
the same time attack the entire substrate, I should get a volcano-like
structure," explained Chen.
In conventional LEDs, the production process usually produces
symmetrical pyramids because of the orientation of the sapphire
crystals. According to Chen, the team discovered that if they cut the
block of sapphire at a tilted angle, the same process would create the
lopsided pyramids. The researchers altered just one part of the
production process, suggesting their approach could easily be applied to
commercial manufacture of LEDs.
The researchers have filed for a patent on this research.
"Once we obtain the patent, we are considering collaborating with
manufacturers in the field to commercialize this technology," said Yin.
=====================================
To subscribe, unsubscribe, turn vacation mode on or off,
or carry out other user-actions for this list, visit
https://www.freelists.org/list/keiths-list