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steveo at syslang.net
Date: Fri, 31 Jan 2003 00:12:35 -0500 (EST)
From: [EMAIL PROTECTED]
Subject: NYTimes.com Article: Red Light,
Green Light: A 2-Tone L.E.D. to Simplify Screens
This article from NYTimes.com
has been sent to you by [EMAIL PROTECTED]
The next big thing...
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Red Light, Green Light: A 2-Tone L.E.D. to Simplify Screens
January 30, 2003
By IAN AUSTEN
IT started in the Netherlands with work on organic
light-emitting diodes, or O.L.E.D.'s, the glowing
electronic devices made with plastics rather than silicon
and other traditional semiconductors.
J. W. Hofstraat, the head of the polymers and organic
chemistry section at Philips Research, was trying to make
O.L.E.D.'s that would be brighter by generating nearly all
of their light within the visible spectrum. Steve Welter, a
graduate student at the University of Amsterdam, where Dr.
Hofstraat teaches, was given the job of testing various
experimental O.L.E.D.'s created by the Philips group. That
is when the unexpected happened.
During a routine test, Mr. Welter found that the sample
O.L.E.D.'s could glow either green or red. The notion that
one diode could produce two colors was so alien that Dr.
Hofstraat and his colleagues did not at first accept the
student's report. "We had to see it ourselves, let me put
it that way," Dr. Hofstraat said. "We were rather
surprised."
Organic light-emitting diodes are relatively new and have
so far been used in relatively simple displays. Philips,
for example, uses an O.L.E.D. display on an electric razor.
But organic diodes have several potential advantages over
the liquid crystal displays now used in computers and
mobile telephones. To start with, O.L.E.D.'s can be printed
on plastic, even flexible sheets. O.L.E.D. displays also
use less power and produce brighter images. Several
companies, including Toshiba, Sony, Eastman Kodak and
Sanyo, have built prototype full-color displays with
organic diodes.
"With O.L.E.D.'s you can roll up your display and put it in
your pocket and it weighs an ounce, not a pound," said
Stephen R. Forrest a professor of electrical engineering at
Princeton and an O.L.E.D. researcher.
Philips's discovery could further enhance the charms of
O.L.E.D.'s for display makers. Among other things, it may
permit full-color screens that use only a repeating pattern
of elements in pairs. (Current displays, including liquid
crystal screens, use elements in groups of three, that is,
red, green and blue). Researchers believe that the
discovery could lead to three-color organic diodes that
switch between red, green and blue, further simplifying
screen construction and increasing brightness.
Other research groups have built O.L.E.D.'s with complex
structures that can produce two or more colors. But George
G. Malliaras, an assistant professor of materials science
and engineering who heads an organic semiconductor group at
Cornell University, said that the simplicity of the Philips
discovery sets it apart. "This is very exciting," Dr.
Malliaras said. "These are the best discoveries: a student
does something and discovers a new physical phenomena."
While Dr. Hofstraat is still studying the fundamental
nature of his two-tone O.L.E.D.'s, making them change color
could not be easier. The color changes from red to green
(or, in subsequent models, between any two primary colors)
when the direction of the electrical current running
through it is reversed.
Making the O.L.E.D.'s is also simple, at least in
comparison with the manufacture of silicon semiconductors.
Like many organic semiconductors, the dual color Philips
models are created by spraying their plastic solution out
of specially designed ink jet printers.
The magic ingredient giving the Philips O.L.E.D.'s their
color-switching ability is a solution of a metal known as
dinuclear ruthenium. Originally, the researchers mixed it
into the O.L.E.D. plastic to boost the amount of visible,
rather than infrared, light that the diode creates while
glowing red.
When Dr. Hofstraat studied Mr. Welter's finding, he
observed that when the current headed in one direction
through the diode, it excited the metal additive and
created a red glow. When the direction was reversed, it was
the plastic's turn to be excited by the voltage and the
device glowed green. (A paper about the group's discovery
appeared in the Jan. 2 issue of the journal Nature.)
So far, Dr. Hofstraat said, tests have shown that the added
metal does not effect the life span of the O.L.E.D.'s. He
anticipates that it will take three to five years to
produce displays with the color-switching diodes.
Two-color O.L.E.D.'s offer increased brightness in a couple
of ways. Because they are created with printers, the
O.L.E.D.'s can be very large. As a result, a simple display
like a warning sign that must change from green to red
could theoretically be made with one giant color-changing
diode.
On a sophisticated computer display, a grid made with a
repeating pattern of paired elements, whether diodes or the
elements in an L.C.D., allows more of the screen to be
lighted more of the time than a display using groups of
three elements. When an L.C.D. on a laptop is displaying
the color red, for example, just a third of its elements
are letting light through. All of the blue and green
elements are dark. But a screen based on two-color diodes
could have all of them lit up when displaying a single
primary color.
The ideal, of course, would be a diode that emits all three
primary colors. And Dr. Malliaras at Cornell said that the
Philips discovery offered, at least in theory, some new
ways to do that. He suggested that it might be possible to
trigger a third color from a variation of the diode that
would use a second, higher voltage in addition to the
reversing current.
Dr. Forrest at Princeton has already developed an
O.L.E.D.-based pixel that can display all three primary
colors. Rather than use an inkjet printer, he makes his
O.L.E.D.'s with thin films created using vacuum deposition,
a process in which the coating material is heated and
evaporated in a high vacuum and condenses on the chilled
surface of the material to be coated. His full-color diode
is a carefully constructed stack of three thin film diodes,
one for each of the primary colors. The technology is
licensed to the Universal Display Corporation of Ewing,
N.J.
Dr. Malliaras said that potential problems that might
derail either system. Creating the sandwich of diodes in
the Princeton three-color system, he said, makes
manufacturing difficult. To make sophisticated displays
with the Philips two-color O.L.E.D., he added, extensive
work will be needed to create electronics that switch
pixels on and off. Fully assessing the Philips two-color
diode is difficult because its basic properties are not
completely grasped, he added.
But Dr. Malliaras said the questions raised by the Philips
finding were helpful. "It simply shows that although there
has been a lot of research in the field of organic
light-emitting devices, there are still issues that are not
fully understood," he said.
http://www.nytimes.com/2003/01/30/technology/circuits/30next.html?ex=1044989955&ei=1&en=3192e61e409674db
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