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ELECTRONIC PAPER”: Organic Light Emitting Diodes
It’s
All In The Way We See Things:
If
ever a technology has begged to be disrupted, it is Liquid Crystal
Displays. Invented in 1963 and envisioned as a slimmed-down replacement
for bulky cathode ray tubes or as screens for wall mounted televisions –
a use never realized due to problems scaling up to large surfaces –
liquid crystal displays have instead become the standard for everything
from watches to laptop computers. Despite this, however, remains high
production and commercial expenses that have never come down enough to
successfully mass market these displays, leaving the technology
vulnerable to new innovations.
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With the imaging
appliance revolution underway, the need for more advanced handheld
devices that will combine the attributes of a computer, PDA, and cell
phone is increasing and the flat-panel mobile display industry is
searching for a display technology that will revolutionize the
industry. The need for new lightweight, low-power, wide viewing angled,
handheld portable communication devices have pushed the display industry
to revisit the current flat-panel digital display technology used for
mobile applications. Struggling to meet the needs of demanding
applications such as e-books, smart networked household appliances,
identity management cards, and display-centric handheld mobile imaging
devices, the flat panel industry is now looking at new displays known as
Organic Light Emitting Diodes (OLED).
What
Is Organic Light Emitting Diodes (OLED)?
Organic Light Emitting
Diode technology, pioneered and patented by Kodak/Sanyo, enables full
color, full-motion flat panel displays with a level of brightness and
sharpness not possible with other technologies.
Unlike traditional
LCD’s, OLED’s are self-luminous and do not require backlighting,
diffusers, polarizers, or any of the other baggage that goes with liquid
crystal displays. Essentially, the OLED consists of two charged
electrodes sandwiched on top of some organic light emitting material.
This eliminates the need for bulky and environmentally undesirable
mercury lamps and yields a thinner, more versatile and more compact
display. Their low power consumption provides for maximum efficiency
and helps minimize heat and electric interference in electronic
devices. Armed with this combination of features, OLED displays
communicate more information in a more engaging way while adding less
weight and taking up less space.
There are two forms of OLED displays: Passive-matrix and
Active-matrix.
Passive Displays:
The
passive-matrix OLED display has a simple structure and is well suited
for low-cost and low-information content applications such as
alphanumeric displays. It is formed by providing an array of OLED
pixels connected by intersecting anode and cathode conductors.
Organic materials and cathode metal are deposited into a
“rib” structure (base and pillar), in which the rib structure
automatically produces an OLED display panel with the desired electrical
isolation for the cathode lines. A major advantage of this method is
that all patterning steps are conventional, so the entire panel
fabrication process can easily be adapted to large-area, high-throughput
manufacturing.
To get a passive-matrix OLED to work, electrical current is
passed through selected pixels by applying a voltage to the
corresponding rows and columns from drivers attached to each row and
column. An external controller circuit provides the necessary input
power, video data signal and multiplex switches. Data signal is
generally supplied to the column lines and synchronized to the scanning
of the row lines. When a particular row is selected, the column and row
data lines determine which pixels are lit. A video output is thus
displayed on the panel by scanning through all the rows successively in
a frame time, which is typically 1/60 of a second.
Active Displays:
In contrast to the passive-matrix OLED display,
active-matrix OLED has an integrated electronic back plane as its
substrate and lends itself to high-resolution, high-information content
applications including videos and graphics. This form of display is
made possible by the development of polysilicon technology, which,
because of its high carrier mobility, provides thin-film-transistors (TFT)
with high current carrying capability and high switching speed.
In an active-matrix OLED display, each individual pixel can
be addressed independently via the associated TFT’s and capacitors in
the electronic back plane. That is, each pixel element can be selected
to stay “on” during the entire frame time, or duration of the video.
Since OLED is an emissive device, the display aperture factor is not
critical, unlike LCD displays where light must pass through aperture.
Therefore, there are no intrinsic limitations to the pixel
count, resolution, or size of an active-matrix OLED display, leaving the
possibilities for commercial use open to our imaginations. Also,
because of the TFT’s in the active-matrix design, a defective pixel
produces only a dark effect, which is considered to be much less
objectionable than a bright point defect, like found in LCD’s.
How
It Works:
The basic
OLED cell structure consists of a stack of thin organic layers
sandwiched between a transparent anode and a metallic cathode. The
organic layers comprise a hole-injection layer, a hole-transport layer,
an emissive layer, and an electron-transport layer. When an appropriate
voltage (typically between 2 and 10 volts) is applied to the cell, the
injected positive and negative charges recombine in the emissive layer
to produce light (electro luminescence). The structure of the organic
layers and the choice of anode and cathode are designed to maximize the
recombination process in the emissive layer, thus maximizing the light
output from the OLED device.
Advantages:
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Robust Design
- OLED’s are tough enough to use in portable devices such as cellular
phones, digital video cameras, DVD players, car audio equipment and
PDA’s.
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Viewing Angles
– Can be viewed up to 160 degrees, OLED screens provide a clear and
distinct image, even in bright light.
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High
Resolution –
High information applications including videos and graphics,
active-matrix OLED provides the solution. Each pixel can be turned on
or off independently to create multiple colors in a fluid and smooth
edged display.
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“Electronic
Paper” –
OLED’s are paper-thin. Due to the exclusion of certain hardware goods
that normal LCD’s require, OLED’s are as thin as a dime.
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Production
Advantages –
Up to 20% to 50% cheaper than LCD processes. Plastics will make the
OLED tougher and more rugged. The future quite possibly could consist
of these OLED’s being produced like newspapers, rather than computer
“chips”.
-
Video
Capabilities
– They hold the ability to handle streamlined video, which could
revolutionize the PDA and cellular phone market.
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Hardware
Content –
Lighter and faster than LCD’s. Can be produced out of plastic and is
bendable. Also, OLED’s do not need lamps, polarizers, or diffusers.
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Power Usage
– Takes less power to run (2 to 10 volts).
OLED
LCD
Disadvantages:
-
Engineering
Hurdles –
OLED’s are still in the development phases of production. Although they
have been introduced commercially for alphanumeric devices like cellular
phones and car audio equipment, production still faces many obstacles
before production.
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Color
– The reliability of the OLED is still not up to par. After a month of
use, the screen becomes nonuniform. Reds, and blues die first, leaving
a very green display. 100,000 hours for red, 30,000for green and 1,000
for blue. Good enough for cell phones, but not laptop or desktop
displays.
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Overcoming
LCD’s – LCD’s
have predominately been the preferred form of display for the last few
decades. Tapping into the multi-billion dollar industry will require a
great product and continually innovative research and development.
Furthermore, LCD manufacturers will not likely fold up and roll over to
LCD’s. They will also continue to improve displays and search for new
ways to reduce production costs.
Future Outlook:
The OLED technology faces a bright future in the display
market, as the ever-changing market environment appears to be a global
race to achieve new success. Eventually, the technology could be used
to make screens large enough for laptop and desktop computers. Because
production is more akin to chemical processing than semiconductor
manufacturing, OLED materials could someday be applied to plastic and
other materials to create wall-size video panels, roll-up screens for
laptops, and even head wearable displays.
The OLED
market appears to be expanding at a rapid pace. Sales of passive OLED
displays rose from $2 million to $18 million this year. Projected sales
by 2005 are expected to reach $717 million, with active matrix sales
accounting for half of that.
Summary:
The Organic Light Emitting Diode forms of display still have
many obstacles to overcome before it’s popularity and even more
importantly, its reliability are up to par with standards expected by
consumers. Although the technology presents itself as a major player in
the field of displays, overcoming these obstacles will prove to be a
difficult task. However, the OLED’s advantages over LCD’s and future
outlook have many in the industry goggle-eyed at the realm of
possibilities. For all we know and can hope for…OLED’s could change the
ways in which we see things. |
