University of Florida researchers may help resolve the public
debate over America’s future
light source of choice: Edison’s incandescent
bulb or the more energy efficient compact fluorescent lamp.
It could be neither.
Instead, America’s
future lighting needs may be supplied by a new breed of light-emitting diode,
or LED, that conjures light from the invisible world of quantum dots. According
to an article in an online issue of Nature
Photonics, moving a QD LED from the laboratory to market is a step closer
to reality thanks to a new manufacturing process pioneered by two research
teams in UF’s department of materials science and engineering.
“Our
work paves the way to manufacture efficient and stable quantum dot-based LEDs
with really low cost, which is very important if we want to see wide-spread
commercial use of these LEDs in large-area, full-color flat-panel displays or
as solid-state lighting sources to replace the existing incandescent and
fluorescent lights,” says Jiangeng Xue, the research leader and an associate
professor of materials science and engineering “Manufacturing costs will be
significantly reduced for these solution-processed devices, compared to the
conventional way of making semiconductor LED devices.”
A
significant part of the research carried out by Xue’s team focused on improving
existing organic LEDs. These semiconductors are multilayered structures made up
of paper thin organic materials, such as polymer plastics, used to light up
display systems in computer monitors, television screens, as well as smaller
devices such as MP3 players, mobile phones, watches, and other handheld
electronic devices. OLEDs are also becoming more popular with manufacturers
because they use less power and generate crisper, brighter images than those
produced by conventional LCDs (liquid crystal displays). Ultra-thin OLED panels
are also used as replacements for traditional light bulbs and may be the next
big thing in 3D imaging.
Complementing
Xue’s team is another headed by Paul Holloway, distinguished professor of
materials science and engineering at UF, which delved into quantum dots, or
QDs. These nanoparticles are tiny crystals just a few nanometers wide,
comprised of a combination of sulfur, zinc, selenium, and cadmium atoms. When
excited by electricity, QDs emit an array of colored light. The individual
colors vary depending on the size of the dots. Tuning, or “adjusting,” the
colors is achieved by controlling the size of the QDs during the synthetic
process.
By
integrating the work of both teams, researchers created a high-performance
hybrid LED, comprised of both organic and QD-based layers. Until recently, however,
engineers at UF and elsewhere have been vexed by a manufacturing problem that
hindered commercial development. An industrial process known as vacuum
deposition is the common way to put the necessary organic molecules in place to
carry electricity into the QDs. However, a different manufacturing process
called spin-coating, is used to create a very thin layer of QDs. Having to use
two separate processes slows down production and drives up manufacturing costs.
According
to the Nature Photonics article, UF
researchers overcame this obstacle with a patented device structure that allows
for depositing all the particles and molecules needed onto the LED entirely
with spin-coating. Such a device structure also yields significantly improved
device efficiency and lifetime compared to previously reported QD-based LED
devices.
Spin-coating
may not be the final manufacturing solution, however.
“In
terms of actual product manufacturing, there are many other high through-put,
continuous ‘roll-to-roll’ printing or coating processes that we could use to
fabricate large area displays or lighting devices,” Xue says. “That will remain
as a future research and development topic for the university and a start-up
company, NanoPhotonica, that has licensed the technology and is in the midst of
a technology development program to capitalize on the manufacturing
breakthrough.”
