From: Michael M. Butler (mmb@spies.com)
Date: Thu Jan 30 2003 - 11:59:26 MST
The most remarkable thing in this piece is that the editors of the NYTimes
let the horrible "effect-for-affect" gaffe slip through. Th barbarians are
truly at the gates. Sigh.
===MMB
Red Light, Green Light: A 2-Tone L.E.D. to Simplify Screens
By IAN AUSTEN
http://www.nytimes.com/2003/01/30/technology/circuits/30next.html
T 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.
________________________________________
Daniel J. Lynch
Sp3d, Inc.
http://www.sp3d.com
415-864-3302
888-9-CARROT
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