North AmEricAN

SPEctrUm.iEEE.orG | North AmEricAN | FEB 2013 | 51
toP: duk-young kim/andrew J. steckl/university of cincinnati; Bottom: Jin-young kim/sungkyungkwan university
Pixels
or developing the right kinds of paper. The best
are those that have special polymer coatings
that help fill in troughs in the surface and seal
the paper to prevent chemical degradation dur-
ing the fabrication process.
Paper would simply combust at the tempera-
tures used to grow and treat the crystalline films
used in traditional semiconductors, so inorganic
paper-based transistors are typically made from
amorphous, noncrystalline films. These can be
formed at a lower temperature using the stan-
dard techniques of depositing a material in a
vacuum, such as evaporation or sputtering. This
fabrication strategy is straightforward, but the
resulting transistors tend to embrace paper’s
inherent textural variation, the result being that
the voltage needed to get current to flow in them
can be a dozen times as much as what’s needed
to move electrons through transistors built on
glass or silicon. John Rogers’s group at the
University of Illinois at Urbana-Champaign, for
example, has been perfecting an alternative ap-
proach in which circuits are built on silicon and
then transferred to paper (or other substrates)
once they’re finished. This method tends to cre-
ate better-performing circuits, although the fab-
rication process is considerably more involved,
and it’s also more costly, because it would start
with a silicon wafer.
When it comes to mass production, however,
the organic semiconductors may be the way to
go. Unlike inorganic materials, organic com-
pounds can be dissolved in fluid and deposited
on paper using roll-to-roll printers, just as with
ordinary ink. But this approach still faces some
obstacles. For one, the transistors tend to be
slower, due to the intrinsic properties of organic
semiconductors. And organic switches are nat-
urally more sensitive to environmental condi-
tions. Oxygen and water vapor, for example,
can degrade or even open up a gap between
an organic material and a metal electrode
through chemical degradation by oxidation or
by partially dissolving the structure. Surface
treatments can help make paper—which nat-
urally sops up moisture from the air that could
affect the device built ontop— relatively imper-
meable. But a fix is still needed to ensure that
organic circuits perform well over long periods
in relatively humid environments.
thin-film circuits built on paper are too slow to be considered for general
purpose computing, but they are an attractive means for controlling and interacting
with “outward-facing” devices such as sensors, displays, and energy-harvesting gear.
Building such devices on paper can be just as challenging as building back-end elec-
tronics. But there has been a lot of progress, particularly in the relatively inexpensive and
low-power realm of reflective displays. One promising approach is the electrochromic
display, which uses pixels made of a conducting polymer. If a sufficient voltage is applied
to such a pixel, electrons will be knocked off and the optical properties of the polymer will
change, turning it from, say, dark blue to transparent. This approach, which was pioneered
by Magnus Berggren’s group at Linköping University, in Sweden, has many advantages.
For one, it requires just a few volts to operate, and it’s structurally fairly simple. But there
are a few drawbacks. The color palette is limited, and the switching speed is quite slow.
It can take anywhere from a fraction of a second to several seconds to complete pixel
transformation, which makes the display unsuitable for full-motion video.
At the University of Cincinnati, my group has been working on adapting an alternative
display approach called electrowetting, which is traditionally used with glass. Electro-
wetting works by confining liquids between two surfaces and then altering their surface
tension, using an applied voltage. Altering the surface tension causes the colored liquid
to either spread out and reflect light or
ball up and allow the light through. Paper
hardly seems a natural fit for this tech-
nique. Electrowetting displays typically
use liquids like water and oil that are
readily absorbed by paper. Pixels also
need to be built on a very smooth, glass-
like surface to ensure reliability and fast
response. With a rough surface, it’s very
hard to guarantee that liquids will move
where they’re supposed to every time.
We first tried the same sort of wax-
coated papers that you might find in your
kitchen cupboard, standard “smooth-
finish” commercial papers, and also
a translucent paper called glassine.
Although the surfaces of all these paper
on dIsplay: Paper can be used
to make displays that either reflect
incoming light or emit their own.
Oneway to make a reflective display
is to alter the surface tension of liquid
pixels[top]. Organic light-emitting
diodes can also be built on paper to
make displays thatglow [bottom].
02.PaperElectronics.NA.indd 51 1/17/13 1:16 PM