A new “spintronic” organic light-emitting diode (OLED) glows orangish (center) when the device, chilled well below freezing, is exposed to a magnetic field from the two poles of an electromagnet located on either side of the device. Image: Tho Nguyen, University of Utah |
University of Utah physicists invented a new “spintronic” organic
light-emitting diode (OLED) that promises to be brighter, cheaper, and more
environmentally friendly than the kinds of LEDs now used in television and
computer displays, lighting, traffic lights, and numerous electronic devices.
“It’s a completely different technology,” says Z. Valy Vardeny, University
of Utah distinguished professor of physics and senior author of a study of the
new OLEDs in Science. “These new organic LEDs can be brighter than
regular organic LEDs.”
The Utah physicists made a prototype of the new kind of LED—known
technically as a spin-polarized organic LED or spin OLED—that produces an
orange color. But Vardeny expects it will be possible within two years to use
the new technology to produce red and blue as well, and he eventually expects
to make white spin OLEDs.
However, it could be five years before the new LEDs hit the market because
right now, they operate at temperatures no warmer than about -28 F, and must be
improved so they can run at room temperature, Vardeny adds.
Vardeny developed the new kind of LED with Tho D. Nguyen, a research
assistant professor of physics and first author of the study, and Eitan
Ehrenfreund, a physicist at the Technion-Israel Institute of Technology in
Haifa.
The study was funded by the U.S. National Science Foundation, the U.S.
Department of Energy, the Israel Science Foundation, and U.S.-Israel Binational
Science Foundation. The research was part of the University of Utah’s new
Materials Research Science and Engineering Center, funded by the National
Science Foundation and the Utah Science Technology and Research initiative.
The evolution of LEDs and OLEDs
The original kind of LEDs, introduced in the early 1960s, used a conventional
semiconductor to generate colored light. Newer organic LEDs or OLEDs—with an
organic polymer or “plastic” semiconductor to generate light—have become
increasingly common in the last decade, particularly for displays in MP3 music
players, cellular phones, and digital cameras. OLEDs also are expected to be
used increasingly for room lighting. Big-screen televisions with existing OLEDs
will hit the market later this year.
The new kind of OLED invented by the Utah physicists also uses an organic
semiconductor, but isn’t simply an electronic device that stores information
based on the electrical charges of electrons. Instead, it is a “spintronic”
device—meaning information also is stored using the “spins” of the electrons.
Invention of the new spin OLED was made possible by another device—an “organic
spin valve”—the invention of which Vardeny and colleagues reported in Nature in 2004. The original
spin-valve device could only regulate electrical current flow, but the
researchers expected they eventually could modify it to also emit light, making
the new organic spin valve a spin OLED.
“It took us eight years to accomplish this feat,” Vardeny says.
Spin valves are electrical switches used in computers, televisions, cell
phones, and many other electrical devices. They are so named because they use a
property of electrons called “spin” to transmit information. Spin is
defined as the intrinsic angular momentum of a particle. Electron spins
can have one of two possible directions, up or down. Up and down can correlate
to the zeroes and ones in binary code.
Organic spin valves are comprised of three layers: an organic layer that
acts as a semiconductor and is sandwiched between two metal electrodes that are
ferromagnets. In the new spin OLED, one of the ferromagnet metal electrodes is
made of cobalt and the other one is made of a compound called lanthanum
strontium manganese oxide. The organic layer in the new OLED is a polymer known
as deuterated-DOO-PPV, which is a semiconductor that emits orange-colored
light.
The whole device is 300 um wide and long and 40 nm thick.
A low voltage is used to inject negatively charged electrons and positively
charged “electron holes” through the organic semiconductor. When a magnetic
field is applied to the electrodes, the spins of the electrons and electron
holes in the organic semiconductor can be manipulated to align either parallel
or antiparallel.
Two advances make new kind of organic LEDs possible
In the new study, the physicists report two crucial advances in the materials
used to create “bipolar” organic spin valves that allow the new spin OLED to
generate light, rather than just regulate electrical current. Previous organic
spin valves could only adjust the flow of electrical current through the
valves.
The first big advance was the use deuterium instead of normal hydrogen in
the organic layer of the spin valve. Deuterium is “heavy hydrogen” or a
hydrogen atom with a neutron added to regular hydrogen’s proton and electron.
Vardeny says the use of deuterium made the production of light by the new spin
OLED more efficient.
The second advance was the use of an extremely thin layer of lithium
fluoride deposited on the cobalt electrode. This layer allows negatively
charged electrons to be injected through one side of the spin valve at the same
time as positively charged electron holes are injected through the opposite
side. That makes the spin valve “bipolar,” unlike older spin valves, into which
only holes could be injected.
It is the ability to inject electrons and holes at the same time that allows
light to be generated. When an electron combines with a hole, the two cancel
each other out and energy is released in the form of light.
“When they meet each other, they form ‘excitons,’ and these excitons give
you light,” Vardeny says.
By injecting electrons and holes into the device, it supports more current
and has the ability to emit light, he says, adding that the intensity of the
new spintronic OLEDs can be a controlled with a magnetic field, while older
kinds require more electrical current to boost light intensity.
Existing OLEDs each produce a particular color of light—such as red, green,
and blue—based on the semiconductor used. Vardeny says the beauty of the new
spin OLEDs is that, in the future, a single device may produce different colors
when controlled by changes in magnetic field.
He also says devices using organic semiconductors are generally less
expensive and are manufactured with less toxic waste than conventional silicon
semiconductors.
Source: University of Utah
