Combination of materials improves light emission
By Steven Schultz
Princeton scientists have created light-emitting
materials that could accelerate the development of
flat-panel computer screens and other compact video
displays.
The discovery, a feat of engineering materials at the
level of quantum mechanics, also may yield insights into the
basic properties of light-emitting substances.
The researchers, led by James S. McDonnell Distinguished
University Professor of Electrical Engineering Stephen
Forrest, found that they could combine the phenomena of
fluorescence and phosphorescence in a way that allows
extremely efficient production of light. This could allow
developers of display devices to choose from a much wider
range of materials than previously available, adding
flexibility to their products and reducing costs of
production.
Forrest and graduate student Marc Baldo collaborated on
the work with University of Southern California chemistry
professor Mark Thompson. Their results appeared in the
February 17 issue of Nature.
OLEDs
The subject of the research is a type of device called an
organic light-emitting diode (OLED). These are thin films of
molecules that can be induced to emit light. They have
advantages over liquid crystal displays, which are used in
laptop computer screens (for example), because they are
brighter, use less electricity, offer potentially truer
colors and allow smaller pixel size.
OLEDs can be made from two types of molecules,
fluorescent and phosphorescent. Until now, the choice
between the two has been a tradeoff. Fluorescence offers
variety, because scientists have identified many more
fluorescent than phosphorescent molecules with suitable
properties, such as good color quality and operational
lifetime. Thanks to a discovery published by the same
researchers in Nature in 1998, phosphorescence is
much more efficient in terms of energy consumption.
The new finding gives developers of OLED devices the best
of both materials: Adding small quantities of
energy-efficient phosphorescent molecules to fluorescent
materials has resulted in final products that emitted
fluorescent light in a highly efficient manner.
Forrest said electronics manufacturers could use the new
technique within six months in certain applications such as
car stereo displays. Eventually the technique could lead to
the ubiquitous use of OLEDs in products such as palm pilots,
cell phones and laptop computers.
"It offers manufacturers exactly what they want," said
Forrest. "You want a laptop that doesn't run down the
battery in three hours; you want the battery to last 10
hours."
Four excited states
The efficiency of light-emitting devices depends on a
detail of quantum mechanics: How well do molecules take
advantage of two "excited" states that they enter when they
receive an electric charge? The two states are called
singlets and triplets; they always occur with three triplets
for every singlet. The material emits light when singlets or
triplets release their energy and return to a "ground
state." Fluorescent materials are inefficient, because only
singlets produce light and the three triplets are
wasted.
In their 1998 paper, Forrest's group showed that they
could engineer materials to use the singlets and the
triplets and produce light through phosphorescence. Because
they use all four excited states, these phosphorescent
materials are four times more efficient than fluorescent
materials.
The researchers then sought to bring the same efficiency
to fluorescent materials, which exist in great variety. They
found they could use one of their high-efficiency phosphors
to "collect" all the triplet states, convert them into
usable singlets and transfer them into a fluorescent
material.
"The fluoresor that would miss all those triplets now
gets them from the phosphor. The phosphor essentially
sensitizes it," said Forrest. The process takes advantage of
the fact that phosphorescence is a slow phenomenon compared
to fluorescence; the fluorescent material grabs the
converted triplets and turns them into light before the
phosphorescence has a chance to occur.
The University has applied for a patent on Forrest's work
and has licensed rights to the discovery to Universal
Display Corp., which is dedicated to developing the display
technologies from Forrest's lab and which partially funded
the research. Additional funding came from the Department of
Defense, Air Force and National Science Foundation.
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