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OLED TV - An Emerging Technology
George Graves, November 11, 2005
For decades we were told that
flat TV screens, those darlings of science fiction writers past and
present were just around the corner. When they finally arrived, the
technologies quickly multiplied. First there was Liquid Crystal Display
technology (LCD) then there was plasma, then Surface-conduction
Electron-emitter (SED). Now these three are about to be joined by a fourth
flat screen technology, one that has the potential of being the roll-up,
wallpaper screen of fiction. It's called OLED or Organic Light Emitting
Diode technology and it promises to revolutionize almost everything that
uses displays from cell phones, PDAs and keyboards, to computer monitors
and HDTVs.
The first OLEDs were developed by Kodak in the late 1980s and since
then, companies such as Samsung and Sony have been working to perfect both
the technology itself and its manufacturability. This work is now
beginning to bear fruit and products using this technology are right
around the corner.
How OLED Works
The earliest solid-state light-producing device to hit the market was the
Light-Emitting Diode, or LED. This semiconductor device was found to emit
a bright light when current was passed through it. Being solid-state, it
lacked a filament to burn-out and so it was seen to be a very reliable,
long lasting light source. Soon all manner of displays appeared using this
technology, most of which were alpha-numeric. They showed up on our
wrist-watches, our radio dials, and even our automobile dashboards. In the
middle 1970s, Sony started using LEDs in very large screen TV monitors for
stadiums, auditoriums and concert halls, but they were never able to make
an LED-based video display that was practical for home use because of the
size and power requirements of the then current LED technology. It took a
passive system, the LCD, to make the breakthrough to home and office
electronics. LCD is a very low-power technology and the individual pixel
size is a function of manufacturing processes so it lends itself to a
myriad of tasks and screen resolutions. Unfortunately, LCD is
non-emissive, that is to say that it doesn't produce any light in and of
itself, but merely controls the amount of light that actually reaches our
eyes from a supplied light source located behind the screen. The need for
a flat, uniformly bright back-light over the entire screen area has made
the low-power characteristics of the LCD somewhat moot. The backlight
accounts for most of the power budget in these displays. That's why your
digital camera batteries go flat so quickly when you use the LCD
viewfinder.
What is needed is an emissive technology that has low power consumption
and that can be made with manufacturing techniques akin to those producing
LCD screens. In other words, instead of individual LED devices, this
technology needs to be producible as a continuous film containing all
three primary subtractive colors (Red, Green, Blue) in a matrix containing
as many pixels as are needed for the application in question. OLED meets
these requirements by placing a series of organic thin films between two
transparent electrodes. An electric current causes these films to produce
a bright light. By using semiconductor technology, each pixel can be
addressed individually thus controlling the patterns of light and color
which combine to form a picture.
The organic process used in OLED is called electrophosphorescence and
is a biological phenomenon that has been noted and wondered at for eons.
Fireflies, plankton, and many sea creatures all possess this
characteristic naturally. But it's only in the last few years that
researchers have been able to synthesize it non-biologically.
Even though these OLED panels are made up of several layers of doped
fluorocarbon polymers, the result is a system which is very thin, usually
less than 0.5 thousandths of a millimeter thick. These OLEDs produce
self-luminous displays that do not require backlighting and can operate at
very low current with only 2-10 volts. These thin displays can be made
flexible, and have a wide viewing angle of up to 170 degrees.
Two Ways to Address OLED
Just like liquid crystal and plasma, OLED can be made as either a passive
or an active matrix. A passive matrix would be one where rows and columns
of pixels are selected, i.e. energizing row 6 and column 5 would mean that
only the pixel at that intersection, 6 rows down and 5 columns over would
light. This particular operating mode is ideal for small, cheap displays
such as smart-cards and watches. The second method, the active matrix mode
is one in which each LED element is brought out separately to the edge of
the display and is driven by its own transistor using TFT
(Thin-Film-Transistor) technology. In this case each LED can be turned on
or off without regard to what's happening on the rest of the display. This
mode produces faster response times and allows for more control over
brightness levels and contrast.
Advantages of OLED
OLEDs possess several advantages that make them ideal for HDTV. They are
brighter than any back-lit passive technology and they react faster to
changes in signal than do either plasma or LCD TVs making them much better
suited for full-motion video. They also are much lighter in weight, and
are more durable. Using a flexible substrate, these devices would be
almost impervious to shock or torsional forces. They also enjoy a much
greater operating temperature range, making them ideal for hostile
environments. No backlight means lower power consumption and one less
component to fail or wear out.
We have already seen that OLED offers displays which are thinner than
current LCD or plasma HDTVs, and because the polymer films which make up
the OLED are very flexible as well as thin, they don't need a rigid
substrate as do these other technologies. This means that OLED screens
could be rolled-up like a newspaper. OLEDs are also brighter than LCD
because they are emissive devices not merely transmissive. LCD has to pass
light produced by a backlight and in so doing, it naturally absorbs and
attenuates some of it. Also because OLEDs do not function by blocking
light but rather by producing it, they don't have the problems with
off-axis viewing that LCD possesses.
Another advantage to OLED is that once production ramps up, displays
will be much easier to produce and can be made in much larger sizes.
Because OLEDs are essentially plastics, they can be made into large thin
sheets much more easily than one can grow and lay down liquid crystal
elements or arrange thousands of tri-colored plasma cells onto a glass
substrate.
Disadvantages of OLED
While OLED seems to be, on the surface, the perfect display technology,
there are some problems. First of all, even though the red and green LEDs
have long lifetimes of 10,000 to 40,000 hours, the blue component suffers
high failure rates after about 3,000 – 5,000 hours. It is believed at this
time that with refinements in manufacturing technology, this limitation
can be overcome fairly quickly, but as it stands now, the entire screen's
life is limited by the blue LEDs' life. Also, as of this writing,
manufacturing processes are expensive. But like everything else, cost is a
direct result of yield. As experience is gained manufacturing these
devices, the cost will come down just as it did with LCD. Finally, the
OLED matrix is easily damaged by water. Without some way to seal the
display in a waterproof outer covering, a spilled soft drink could spell
curtains for the display.
Conclusion
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Samsung 40
inch OLED TV
This unit is not yet for sale.
(Picture Courtesy of Samsung Electronics of America)
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Large screen HDTVs as thin as wallpaper are a very appealing prospect
and may well be the first large-scale use of OLEDs. Although Samsung has
announced the first 40" OLED TV based on this technology, it is still
unclear when OLED technology will be in mass production and when it will
become price competitive with existing technologies. If it can fulfill its
promise, a wide variety of applications will benefit.
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