25 September 2003
25 September 2003
Philips demonstrates video-speed electronic-paper technology
based on electrowetting
In the September 25 issue of Nature, scientists of Philips present a novel technology
for
reflective, paper-like displays. The technology is fast enough to display video content and
can
be used to build a reflective full-color display that is at least two times brighter than
what is
possible with any other technology. The display is based on electrowetting - a technology
that
allows rapid control and manipulation of fluid motion on a micrometer scale.
The technology is based on controlling the shape of a confined water/oil interface by an
applied voltage, shown schematically for one pixel in figure 1. With no voltage applied,
the (colored) oil forms a flat film between the water and a hydrophobic (water-repellent),
insulating coating of an electrode, resulting in a colored pixel. When a voltage is applied
between the electrode and the water, the interfacial tension between the water and the coating
changes. As a result the stacked state is no longer stable, causing the water to move the
oil
aside. This results in a partly transparent pixel, or, in case a reflective white surface
is used
under the switchable element, a white pixel. Because of the small size of the pixel, the
user
only experiences the average reflection, which means that a high-brightness, high-contrast
switchable element is obtained, which forms the basis of the reflective display.
Displays based on electrowetting have several attractive features. The switching between
white and colored reflection is fast enough to display video content. Furthermore, it is
a lowpower
and low-voltage technology, and displays based on the effect can be made flat and
thin. The reflectivity and contrast are better or equal to those of other reflective display
types
and are approaching those of paper.
In addition, the technology offers a unique path towards high-brightness full-color displays,
leading to displays that are four times brighter than reflective LCDs and twice as bright
as
other emerging technologies. Instead of using red, green and blue (RGB) filters or alternating
segments of the three primary colors, which effectively result in only one third of the display
reflecting light in the desired color, electrowetting allows for a system in which one sub-pixel
is able to switch two different colors independently. This results in the availability of
two
thirds of the display area to reflect light in any desired color. This is achieved by building
up
a pixel with a stack of two independently controllable colored oil films plus a color filter.
The
colors used are cyan, magenta and yellow, which is a so-called subtractive system,
comparable to the principle used in inkjet printing for example. Compared to LCD another
factor two in brightness is gained because no polarizers are required.
The technology is particularly appealing for electronic-paper like applications, for which
high-brightness and contrast-rich reflective displays are needed, and opens up new
application areas that make use of the full color and video speed capability.
Electrowetting display principle. If no voltage is applied (situation a), the flat oil film
results
in a colored pixel. If a voltage is applied (b), the oil film contracts, resulting in reflection
of
incident light from the white substrate.
