Optical image of flexible and stretchable thin film transistor array covering a baseball shows the mechanical robustness of this backplane material for future plastic electronic devices. |
Imprinting
electronic circuitry on backplanes that are both flexible and
stretchable promises to revolutionize a number of industries and make
“smart devices” nearly ubiquitous. Among the applications that have been
envisioned are electronic pads that could be folded away like paper,
coatings that could monitor surfaces for cracks and other structural
failures, medical bandages that could treat infections and food
packaging that could detect spoilage. From solar cells to pacemakers to
clothing, the list of smart applications for so-called “plastic
electronics” is both flexible and stretchable. First, however, suitable
backplanes must be mass-produced in a cost-effective way.
Researchers
with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National
Laboratory (Berkeley Lab) have developed a promising new inexpensive
technique for fabricating large-scale flexible and stretchable
backplanes using semiconductor-enriched carbon nanotube solutions that
yield networks of thin film transistors with superb electrical
properties, including a charge carrier mobility that is dramatically
higher than that of organic counterparts. To demonstrate the utility of
their carbon nanotube backplanes, the researchers constructed an
artificial electronic skin (e-skin) capable of detecting and responding
to touch.
“With
our solution-based processing technology, we have produced mechanically
flexible and stretchable active-matrix backplanes, based on fully
passivated and highly uniform arrays of thin film transistors made from
single walled carbon nanotubes that evenly cover areas of approximately
56 square centimeters,” says Ali Javey, a faculty scientist in Berkeley
Lab’s Materials Sciences Division and a professor of electrical
engineering and computer science at the University of California (UC)
Berkeley. “This technology, in combination with inkjet printing of metal
contacts, should provide lithography-free fabrication of low-cost
flexible and stretchable electronics in the future.”
(From left) Kuniharu Takei, Toshitake Takahashi and Ali Javey at the microscope electric probe station used to characterize flexible and stretchable backplanes for e-skin and other electronic devices. (Photo by Roy Kaltschmidt, Berkeley Lab) |
Javey
is the corresponding author of a paper in the journal NanoLetters that
describes this work titled “Carbon Nanotube Active-Matrix Backplanes for
Conformal Electronics and Sensors.” Co-authoring this paper were
Toshitake Takahashi, Kuniharu Takei, Andrew Gillies and Ronald Fearing.
With
the demand for plastic electronics so high, research and development in
this area has been intense over the past decade. Single walled carbon
nanotubes (SWNTs) have emerged as one of the top contending
semiconductor materials for plastic electronics, primarily because they
feature high mobility for electrons – a measure of how fast a
semiconductor conducts electricity. However, SWNTs can take the form of
either a semiconductor or a metal and a typical SWNT solution consists
of two-thirds semiconducting and one-third metallic tubes. This mix
yields nanotube networks that exhibit low on/off current ratios, which
poses a major problem for electronic applications as lead author of the
NanoLetters paper Takahashi explains.
“An
on/off current ratio as high as possible is essential for reducing the
interruption from pixels in an off-state,” he says. “For example, with
our e-skin device, when we are pressure mapping, we want to get the
signal only from the on-state pixel on which pressure is applied. In
other words, we want to minimize the current as small as possible from
the other pixels which are supposed to be turned off. For this we need a
high on/off current ratio.”
To
make their backplanes, Javey, Takahashi and their co-authors used a
SWNT solution enriched to be 99% semiconductor tubes. This highly
purified solution provided the researchers with a high on/off ratio
(approximately 100) for their backplanes. Working with a thin substrate
of polymide, a high-strength polymer with superior flexibility, they
laser-cut a honeycomb pattern of hexagonal holes that made the substrate
stretchable as well. The holes were cut with a fixed pitch of 3.3 mm
and a varied hole-side length that ranged from 1.0 to 1.85 mm.
(Left) Optical image of e-skin with an L-shaped object placed on top. (Right) Two-dimensional pressure mapping obtained from the L-shaped object. |
“The
degree to which the substrate could be stretched increased from 0 to
60% as the side length of the hexagonal holes increased to 1.85 mm,”
Takahashi says. “In the future, the degrees of stretchability and
directionality should be tunable by either changing the hole size or
optimizing the mesh design.”
Backplanes
were completed with the deposition on the substrates of layers of
silicon and aluminum oxides followed by the semiconductor-enriched
SWNTs. The resulting SWNT thin film transistor backplanes were used to
create e-skin for spatial pressure mapping. The e-skin consisted of an
array of 96 sensor pixels, measuring 24 square centimeters in area, with
each pixel being actively controlled by a single thin film transistor.
To demonstrate pressure mapping, an L-shaped weight was placed on top of
the e-skin sensor array with the normal pressure of approximately 15
kilo Pascals (313 pounds per square foot).
“In
the linear operation regime, the measured sensor sensitivity reflected a
threefold improvement compared with previous nanowire-based e-skin
sensors reported last year by our group,” Takahashi says. “This improved
sensitivity was a result of the improved device performance of the SWNT
backplanes. In the future we should be able to expand our backplane
technology by adding various sensor and/or other active device
components to enable multifunctional artificial skins. In addition, the
SWNT backplane could be used for flexible displays.”
This research was supported in part by the DOE Office of Science and in part by the National Science Foundation.
Carbon Nanotube Active-Matrix Backplanes for Conformal Electronics and Sensors