After two decades of intensive study and testing,
flexible displays are just about ready to take off.
Researchers have strived to find the right combination
of flexible glass, polymer, and metal-foil substrates
along with thin-film-transistor (TFT) backplates—a combination that will turn the flexible display into a commercial reality. Ultimately, they're looking to produce a
thin, flexible, clear substrate with the barrier properties of glass.
Anticipated mass-market applications include newspapers, books, and magazines as alternatives to
paper; point-of-sale (POS) terminals; outdoor and
indoor signage; smart cards; and labeling for retail
shelves. The technology's potential in the automotive
market looms particularly large, with windshields and
dashboards as well as bumper stickers, upholstery,
GPS, and other infotainment functions.
Market research company iSuppli Corp. expects the
flexible-display market to ramp up from nearly nothing
today to about $338 million by 2013. Market analysis
firm NanoMarkets has forecast a $668 million "paperlike" displays market by 2008, though most electronic-paper displays are rigid, not flexible. Often called electronic ink (e-ink), electronic paper involves the deposition
of electronic functions embedded in ink films that are
deposited on a flexible substrate material.
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Manufacturers of
flexible displays are looking at existing processes used to make rigid displays,
like LCDs, to create
low-cost flexible displays. They're also
investigating flexible
substrate materials
like plastic, flexible
glass, metal foils, and
polymers, as well as
display materials like
electronic ink (electrophoretics), LCDs,
organic LEDs (OLEDs),
and even LEDs.
Yet they're finding that moving from traditional rigid substrates used in the manufacture of ICs and display materials to a flexible
substrate isn't easy. Many flexible materials can't handle the high processing temperatures encountered
when making rigid displays.
No single material can satisfy the requirements of
both substrate and deposited electronics during manufacturing. Such bendable materials can't reliably
operate at high temperatures without being affected
by stresses. There's also a need for laminate adhesives that can perform reliably at high temperatures
without being affected by stresses.
The technology often is confused with electrochromic displays, another form of electronic ink.
According to Dave Jackson, director of marketing and
planning at E Ink Inc., electrophoretic displays function
by moving clusters of charged, colored particles in an
electric field. In contrast, electrochromic displays contain chemical compounds that change color when an
electric current is applied.
Both are reflective and can hold their image with no
power. Yet electrochromic displays typically require a
large amount of power to drive the color-changing reaction, making them less energy-efficient than electrophoretic displays.
So far, electrophoretic inks printed on rigid plastic
substrates have had the most commercial success in
devices that are generally known as electronic-paper
displays (EPDs). E Ink has pioneered and holds many
patents for e-ink technology. EPD substrates comprise
tiny pockets containing charged particles suspended
in an opaque liquid ink.
An electrophoretic display is an information display
that forms visible images by rearranging charged pigment particles via an applied electric field ().
Electrophoretic displays are considered prime examples of the electronic-paper category because of their
paper-like appearance and
low power consumption.
In the simplest implementation of an electrophoretic display, titanium-dioxide particles
approximately 1 mm in diameter are dispersed in a hydrocarbon oil. A dark-colored dye is
also added to the oil, along
with surfactants and charging
agents that cause the particles to take on an electric
charge. This mixture is placed
between two parallel, conductive plates separated by a gap
of 10 to 100 mm.
When a voltage is applied
across the two plates, the
particles migrate electrophoretically to the plate
bearing the opposite charge
from that on the particles.
When the particles are located at the front (viewing) side
of the display, the display
appears white, because light
is scattered back to the viewer by the high-index titania
(titanium-dioxide) particles.
When the particles are
located at the rear of the display, it appears dark because
the colored dye absorbs the
incident light. If the rear electrode is divided into a number of small picture elements
(pixels), an image can be
formed by applying the
appropriate voltage to each region of the display to create a pattern of reflecting
and absorbing regions.
Examples of commercial electrophoretic displays
include the high-resolution active-matrix displays used
in the Sony Librie and Sony Reader, as well as the iRex
iLiad e-readers. These displays are constructed from
an electrophoretic imaging film manufactured by E Ink.
Also, the Motorola Motofone uses the technology to
achieve its remarkable slimness.