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Carbon-nanotube film on plastic as transparent electrode for resistive
touch screens
David S. Hecht
David Thomas
Liangbing Hu
Corinne Ladous
Tom Lam
Youngbae Park
Glen Irvin
Paul Drzaic (SID Fellow)
Abstract
— Carbon-nanotube (CNT) films on plastic are incorporated as the touch electrode in a
four-wire resistive touch panel. Single-point actuation tests show superior mech anical performance
to ITO touch electrodes, with no loss of device functionality up to 3 million actuations. Sliding-
stylus-pen tests reveal no loss of device linearity after 1 million stylus cycles. A CNT refractive index
of ~1.55 leads to CNT touch panels with low reflection (<9% over the visible range) without costly
anti-reflective coatings. CNT films on PET currently have 86% total transmission (including the PET)
over the visible and 600 Ω/䊐, with lab scale tests giving 88% at 500 Ω/䊐. CNT films are neutrally
colored (
a
*~0,
b
* ~ 1.5), low haze (<1%), uniform, and both chemically and environmentally stable.
Unidym’s solution-based coatings can be printed directly onto both flexible and rigid polycarbonate
using solution coating processes. Unidym films can be patterned using subtractive methods such as
laser ablation with resolution down to 10 µm, or additive methods such as patterned gravure. CNTs
are grown, purified, formulated into inks, and coated using scalable processes, allowing films to be
attractive from a cost perspective as well.
Keywords — Carbon nanotubes, resistive touch screens, flexible, transparent electrodes.
DOI # 10.1889/JSID17.11.941
1 Introduction
Transparent electrodes are a necessary component in many
modern devices such as touch screens, LCDs, OLEDs, and
solar cells, devices which are growing in demand. Currently,
these electrodes predominantly consist of vacuum-sput-
tered oxides, the most common of which is indium tin oxide
(ITO). ITO, however, suffers from several problems, such as
poor mechanical robustness, yield loss during manufactur-
ing (often at the device level), high reflectivity, a yellowish
tint, high cost, limited production capacity, and a dwindling
supply of indium.1Unidym solves these issues by replacing
ITO with a thin film of carbon nanotubes (CNTs) printed on
plastic using scalable printing techniques. Currently, films are
coated in rolls up to 5 ft. wide, at speeds up to 300 ft./minute
and with a coating thickness that can be precisely tuned to
allow for varying sheet resistance (Rs), from 100 to
100,000 Ω/䊐(OPS). Our first product is a CNT film on
polyester terephthalate (PET) that is 600 OPS and greater
than 86% transmission (including the plastic substrate). Our
films offer several benefits over conventional ITO electrodes
including low reflectivity (<9% over the visible range), neu-
tral color (a*~0;b*~ 1.5), and mechanical robustness
(R/R0< 1.1 after three million touch actuations). We report
here, for the first time, prototype four-wire touch panels
usingaCNTfilmasthetop(touch) electrode. We find
excellent performance in a number of mechanical and opti-
cal characteristics.
2 Film manufacturing
2.1 Inks and coatings
There are three major steps in manufacturing Unidym’s
CNT film product: CNT growth/purification, CNT ink for-
mulation, and thin-film coating. Unidym has chosen scal-
able processes for each of these steps, enabling for the first
time a commercial supply of CNT-coated film with excellent
performance. Tubes are grown using thermal CVD with a
proprietary catalyst/reactor. Purified CNTs are processed
into aqueous-based inks using a system that can produce
enough ink per day to coat ~50,000 ft.2of product. Inks are
formulated to the proper rheological profile for high-
throughput coating using standard printing techniques such
as spray coating, slot coating, ink-jet printing, and gravure
[Fig. 1(a)]. Unidym has demonstrated production scale
coatings of CNT films on plastic substrates up to 5 ft. wide
at speeds up to 300 ft./minute [Fig. 1(b)]. For comparison,
vacuum-deposited oxides such as ITO typically coat at 1–10
ft./minute, and use costlier vacuum deposition. The thick-
ness of the CNT films can easily be varied by controlling
both the concentration and wet laydown thickness of the
CNT ink. Variations in CNT film thickness will affect the
final film Rsand transmission.2By using a standard slot
coater, final film resistance can be quickly and simply tuned
from 100 up to 10,000 OPS (78–93% total film transmit-
tance, including plastic substrate) using the same CNT ink
by modulating the flow rate of the ink through the coating
die. This is in contrast to sputtered oxide films, which are
more difficult to continuously tune tothe desired resistance,
The authors are with Unidym, 1244 Reamwood Ave., Sunnyvale, CA 94089; telephone/fax 650/462-1935, e-mail: dhecht@unidym.com.
©Copyright 2009 Society for Information Display 1071-0922/09/1711-0941$1.00
Journal of the SID 17/11,2009 941
and cannot be easily made to have resistance greater than
1000 OPS since very thin ITO films tend to be extremely
brittle. Unidym’s first product is 600 OPS and 86% total
transmission in the visible region (film thickness ~10–20 nm).
The final CNT film product also incorporates a polymer top-
coat, whose primary function is to improve the adhesion of
the CNT film to the plastic substrate. Unidym has demon-
strated coating on PET substrates with clear, anti-glare,
and/or anti-Newton ring hardcoats; the data in this publica-
tion is for PET with a clear hardcoat. Note that Unidym’s
CNT ink can be coated on a variety of surfaces, including
PET, polyethylene naphthalate, polycarbonate (PC), and
glass, as well as on rough or curved surfaces, such as those
found on anti-Newton ring hardcoats, without significant
loss of performance. This may open up new applications,
especially for flexible and curved displays where ITO has
mechanical robustness issues.
2.2 Polycarbonate
Unidym’s wet-coating process enables coating of substrates
inaccessible to ITO, such as rigid or flexible PC. This is
important as companies seek to move away from fragile
glass components, and towards durable and lighter PC sub-
strates that give additional robustness to the product with-
out sacrificing performance. ITO coating on PC has proven
elusive, as the thermal coefficient of PC causes elongation
and contraction that leads to cracking of the ITO layer.Typi-
cally ITO on PC has an order-of-magnitude higher resis-
tance at a given transmission than ITO on PET.3Currently,
companies who seek to use PC are forced to laminate ITO
on PET to a rigid PC backer using an optically clear adhe-
sive. Direct coatings on PC are preferred, as it removes a
PET layer, costly optically clear adhesive, and a manufactur-
ing step. Unidym has demonstrated direct coatings of CNT
on PC using standard wet-coating processes. As shown in
Fig. 2, CNT on PC offers a much simpler device stack than
the ITO/PET/PC laminate structure. It is also possible to
coat both sides of a PC substrate with CNTs, which could
have future applications in capacitive touch screens.
Though the data in this publication focuses on CNT films on
PET, the electrical, optical, and mechanical performance of
the films on PC are largely similar.
2.3 Patterning
For both resistive and capacitive touch screens, the trans-
parent conductor material needs to be patterned for proper
device functionality. In four-wire and five-wire resistive
touch screens, edge-deletion patterning is required, while
in capacitive, more complicated patterns throughout the
film are necessary. ITO films are typically patterned by
screen printing of a wet acid etchant. However, this method
generates hazardous chemical waste in the process, which is
expensive to treat and not environmentally friendly. Also,
acid etching of ITO films actually decreases its mechanical
integrity, making it more prone to cracking near the etch
points. Unidym’s CNT films, due to their high chemical sta-
bility, cannot be etched by the same acid-based methods.
However, Unidym has demonstrated other methods for
both subtractive and additive patterning, including oxygen
plasma etching, laser ablation, and ink-jet printing. Laser
ablation has many advantages over acid etch including no
wet chemicals, no lithography, low operating cost, effective-
ness, and ability to cheaply change the pattern. ITO can also
be patterned by laser ablation; however, a higher fluence is
needed which can damage the underlying substrate, intro-
ducing unwanted haze or other defects. Figure 3 shows
CNT films that have been patterned by laser ablation.
Laser-ablated patterns on CNT films have been demon-
strateddownto10µm with both electrical isolation (>10-
MΩresistance between conducting regions) and no visible
FIGURE 1 — (a) (Main) CNT film and ink (inset) SEM image of CNT film
(image, 3 µm2). (b) Larg est coated roll that Unidym employees can lift.
Rolls twice as wide (but twice as heavy) have been made. FIGURE 2 — (Left) ITO polymer laminate stack, which necessitates
laminating ITO/PET to PC using OCA. (Right) simpler and less expensive
stack enabled by CNT wet coating directly onto PC.
FIGURE 3 — Examples of patterned CNT films using laser ablation. High
absorbance of CNTs allow patterning at lower fluence than ITO, without
damage to underlying substrate.
942
Hechtetal./
Carbon-nanotube film on plastic as transparent electrode for resistive touch screens
substrate damage. Laser patterning is cost effective and per-
formed at high throughputs (up to 1-m/sec roll-to-roll) and
has been demonstrated using both UV and IR lasers on PET
andonPC.Theresolutionprovidedbylaserpatterningis
sufficient for both resistive and capacitive touch screens.
For resistive touch screens, where edge deletion is the only
patterning required, an additive patterning step is feasible
with wet-deposition techniques such as gravure. Additive
patterning is largely preferred as it eliminates the need for
expensive additional patterning steps.
3 Film properties
Unidym’s CNT films on PET have been extensively character-
ized with regards to their electrical, optical, and mechanical
performance. Two primary properties of the film are the Rs
and the visible transmittance. When integrated into a four-
wire resistive touch panel, current controllers can accept a
certain range of input impedances. Typically, ITO with an Rs
between 250 and 500 OPS is used, though much of this is
due to the ITO legacy, as there is no fundamental reason
why films with Rsgreaterthan500OPScannotbeused
(though when the Rsgets too high it might introduce unwanted
time constant delays). With the advent of new touch-screen
controllers and minimal touch-screen redesign (for exam-
ple, switching the xand yelectrodes), CNT films with Rsup
to 800–1000 OPS could be used as the touch electrode with
no degradation in touch-screen response. The transmit-
tance is also important so that the integrated display will
appear undistorted by the touch panel and is typically
between 85 and 90% (the higher the better).
Figure 4 shows the four-point Rsof Unidym’s CNT
film vs. total transmission through the CNT layer and PET
(measured using a Jasco V670 spectrophotometer with inte-
grating sphere, weighted over the visible wavelength range).
For a given CNT material and processing method, there is
an inevitable tradeoff between Rsand transmittance. Light
adsorption (not reflection) limits transmission through CNT
films; therefore, thicker films are darker, but have lower Rs.
The red line shows current production-scale film perform-
ance on both PET and PC substrates of 86% transmission
(L*= 94.2) and 600 OPS, which is already suitable for inte-
gration into many touch-screen products. Recent lab scale
coatings on a PET substrate with a low-reflection coating
opposite the CNT coating side, and using a higher purity,
lower yield CNT, have shown results of 500 OPS and 88%
or 300 OPS at 86%. The latter performance does not repre-
sent in any way the limit of what CNTs can achieve. Moder-
ate improvements in tube purity, length, diameter, metal/
semiconducting ratio, doping, formulation, and coating
couldconservativelyleadtoafilmproductwith500OPSat
91% transmission (L*= 96) or 250 OPS at 88% transmis-
sion.4As these fundamental optoelectronic parameters con-
tinue to improve, it will open up new opportunities for our
films in various touch-screen applications beyond resistive
andpossiblyforothermarketsinLCD,OLEDs,orsolar.
Additional parameters beyond Rsand optical trans-
mission of these films need to be considered, including film
stability under accelerated aging and touch actuation, uni-
formity, color, reflection, and mechanical properties. These
are addressed below.
3.1 Reduced reflection
Unidym’s CNT films offer substantially lower reflection
than a comparable ITO film without the need for any anti-
reflective coating (which tends to distort the film color), due
to its superior index of refraction match to most plastics
such as PET (nforCNT~1.5–1.6;PET~1.5;ITO~2).
Figure 5(a) demonstrates the low reflection of a CNT film
on PET, with most of the reflection due to the PET film
itself, in contrast to the higher reflection from most ITO
films on PET (without the addition of costly anti-reflective
coatings). The carbon-nanotube coating contributes less
FIGURE 4 — Sheet-resistance/transmission tradeoff for Unidym CNT on
PET/PC products (red line). Lab results using low-reflectance PET
substrate and improved CNT material also shown (black line).
Commercially available ITO on PET plotted for comparison (red stars).
FIGURE 5 — (a) Ref lection spectrum of Un idym CNT/PET product (black)
and ITO/PET (red). Low reflection from Unidym film is due to CNTs index
of refraction matching with PET. (b) LCD in bright daylight when viewed
through a CNT/PET (top) and ITO/PET (bottom) film. Notice the
additional glare from the reflection off of the ITO/PET film decreases the
contrast.
Journal of the SID 17/11,2009 943
than 1% additional reflection to that of uncoated PET. The
low reflection from a CNT film improves the daylight read-
ability by reducing unwanted specular reflection, or glare.5
Figure 5(b) demonstrates this principal, showing a photo-
graph of an LCD monitor viewed outdoors in bright condi-
tions through both a CNT/PET (top) and ITO/PET
(bottom) film. Reflection from the ITO/PET film effectively
reduces the contrast, making the display more difficult to
read. Using PET with a low-reflection hardcoat leads to
films with a total reflection less than 5% across the visible
spectrum.
3.2 Color neutrality
An additional benefit of CNT films is that the CNTs absorb
relatively equally across the visible spectrum. The benefit of
this is that CNT films have a “neutral” color and will not
significantly affect the colors from the display behind it. Fig-
ure 6 shows the a*and b*color coordinates for a CNT film,
as well as various commercial ITO and conducting polymer
films. The CNT film comes closest to (0,0), while the ITO
films have a yellow tinge. A neutral color is beneficial in
display applications so that the touch panel will not distort
the image color. Recent coatings on a low b*PET (b*=–0.2)
can lead to CNT films with a b*<0.5.
3.3 Haze
Haze is the term for light that is scattered from the film
surface as it is transmitted. Large haze values are typically
not desired for touch-screen applications where optical clar-
ity is important. Some transparent conductor technologies,
such as silver nanowires, suffer from unwanted haze result-
ing from light scattered from the relatively large diameter
(50–100 nm) wires. For CNT films, however, haze is not an
issue, since the surface roughness of the films is on the order
of 5–10 nm. Typically, the addition of CNT films contributes
approximately 0.1–0.3% haze; therefore, the haze level is
controlled by the choice of an appropriate substrate.
3.4 Uniformity
The electrical and optical uniformity of CNT films are impor-
tant parameters for proper functionality of a touch-screen
device. A uniform Rsis especially critical for four-wire resis-
tive touch screens, as it impacts the linearity of the touch
response. The electrical uniformity of our CNT films is con-
trolled by the film-thickness uniformity, which is set by the
properties of the CNT ink and coating instrument. Typically,
our films show a film Rsstandard deviation of 5–10% over
an A4 sheet (measured in a 2-D array with 1-in. spacing),
which is within the range desired by touch-screen manufac-
turers. This will be further addressed in Sec. 4. The optical
uniformity is typically a 0.2% standard deviation over an A4
sheet.
3.5 Environmental/chemical stability
It is critical that a touch-screen device retain normal func-
tionality after 2–3 years of use in typical daily conditions.
These conditions may include high relative humidity (RH)
and high/low temperatures. We performed accelerated
aging tests on out CNT films at 60°C/90% RH. After 1000
hours at this condition, we find less than a 15% change in Rs
and less than a 0.1% change in transmission (film is allowed
to equalize for 24 hours in ambient before measurement).
The CNT film also must be able to withstand touch-screen
manufacturing process steps, which sometimes includes a
150°C bake for 1 hour. Unidym’s product shows less than a
10% change in Rsand less than a 0.5% change in haze after
this aggressive processing step. Last, due to the relative
chemical inertness of carbon nanotubes, Unidym’s films
show less than a 10% change in resistance after a 10-minute
soak in toluene, acetone, or isopropanol. CNT’s chemical
inertness makes them insensitive to trace acids that may be
found in various processing steps.
3.6 Mechanical properties
Unidym’s CNT films consist of a network of flexible nanotubes
[see insert of Fig. 1(a)] on a plastic substrate. The mechani-
cal properties of such a network of micron-long high-aspect-
ratio wires is in stark contrast to a ceramic material such as
ITO, which tends to be brittle and can crack under relatively
low strains (~2%) leading to cataclysmic device failure,
while the CNTs can maintain film integrity (and more
importantly Rs) up to strains of 20%. Unidym has previously
shown that CNT films on plastic can be bent around a man-
drel of 2-mm diameter with <20% change in Rs,whilean
ITO films shows a 1200% increase in resistance at the same
FIGURE 6 — Color coordinates for ITO, conducting polym er, and CNT
films on PET. The CNT is the most neutrally colored film.
944
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Carbon-nanotube film on plastic as transparent electrode for resistive touch screens
bending diameter.6ITO films can be made more durable by
laminating the ITO/PET film to a second piece of PET
using an OCA, which can help reduce film stress. However,
this adds substantial additional cost to the product, which
CNT coated PET can avoid. The mechanical durability of
nanotube films opens up new opportunities for touch pan-
els, such as devices that are on a curved surface. Such a
curved surface could introduce excess stress that would lead
to quick failure in an ITO device, while the CNTs could
withstand such stress. In fact, CNT devices could thrive in
any market where rugged touch screens are needed.
4 Touch-screen integration
Unidym’s CNT films were integrated into a fully functional
10.4-in. four-wire resistive touch screen with the architec-
ture shown in the bottom portion of Fig. 7(a). A CNT coated
PET film with a clear hardcoat was used as the top electrode
andanITO-coatedglasswithspacerdotsasthebottom
electrode. The full stack was assembled into a touch panel
in a standard fabrication line. This demonstrates the com-
patibility of CNT-coated films with current manufacturing
processes and techniques, including application and curing
of silver bus bars. The final device is shown in Fig. 7(b), and
a slightly smaller touch panel integrated with a full-color
LCD is shown in Fig. 7(c).
The mechanical robustness of the touch panel shown
in Fig. 7(b) was measured using a single-point actuation tap
test using a stylus with a radius of 0.8 mm and 250 g of force.
A similar test was also performed on a commercial ITO
touch panel. The curvature induced by the stylus creates
high tensile and compressive stress which, in the case of
ITO, leads to cracking of the conductive film7,8 and an
asymptotic rise in the switch resistance. The end result is a
panel that is inaccurate, unresponsive, and has poor resolu-
tion. The flexible, web-like topology of a CNT film provides
natural stress relief, making it almost impossible to crack or
break by such mechanical tapping. Figure 8 shows that a
panel with a CNT touch electrode continues to operate with
negligible change in switch resistance, long after an ITO
panel has failed. Unidym has subsequently tested a device
out to 3 million touch actuations without touch-panel fail-
ure. It should be noted that the actuation test was per-
formed with the device under constant electrical bias of 5 V.
The mechanical robustness demonstrated by CNT touch
panels promises to increase the lifetime and durability of
current touch screens, while opening future applications in
flexible and curved touch screens, where the ITO would
crack quickly under the increased strains. A particularly
weak point for ITO-based devices is near the edge bezel,
where strain is maximized. CNT films show no such suscep-
tibility to tapping at the edge bezel region. Thus, use of
CNT films would decrease the needed bezel size, thereby
increasing viewable area. Another overlooked, but impor-
tant, benefit of CNT film robustness is the improvement in
yield loss that occurs during film handling and device manu-
facturing in ITO-based devices. For mechanically demand-
ing processing steps, such as ITO film lamination to a backer
substrate, yield losses can exceed 30%; this would be a non-
existent problem for CNT films.
An important touch-panel device parameter is response
linearity, which should not change over time as the device is
FIGURE 7 — (a) Schematic of ITO/ITO (top) and CNT/ITO (bottom)
four-wire touch panels. (b) CNT/ITO 10.4-in.-diagonal functional touch
panel. (c) Unidym touch panel integrated with a full-color LCD.
FIGURE 8 — Switch resistance between CNT (black) or ITO (red) top
electrode bus bar and ITO bottom electrode bus bar as a function of
touch actuation number. Notic e that the ITO/ITO device fails after
50,000 touch actuations while the CNT device shows no change in
functionality after 1 million actuations. CNT device shows such marked
robustnes s even when test is performed near device bezel.
Journal of the SID 17/11,2009 945
used. To test this, we subjected the device to repeated cycles
of a sliding stylus (Palm 3181WW) under a 250-g load. The
stylus reciprocated against the device at 4 Hz, and the device
was held under a constant 5-V bias. The device linearity was
periodically measured every 1000strokes(upto1million
strokes) by measuring the device voltage at approximately
4-mm intervals. Figure 9 shows that the device is linear over
the area tested, and that the linearity does not change as a
function of stroke number. This shows that the film Rs
remains constant over the 1 million sliding stylus cycles,
indicating no damage to the film from the sliding stylus and
no film delamination. Essentially, the device response does
not change up to 1 million cycles using a sliding stylus,
another indication of the robustness of Unidym’s films.
Section 3.1 discusses the reduced reflection from
CNT films compared with ITO. This translates directly to
reduced reflection in touch panels incorporating the corre-
sponding films. Figure 10 shows the reflection from a
CNT/ITO touch panel, as well as from an ITO/ITO panel.
The reduced reflection from the fully integrated CNT/ITO
panel agrees qualitatively with the reduced reflection from
the CNT film shown in Fig. 5. It is anticipated that using a
CNT/CNT panel could further reduce the reflection.
5 Conclusions
The explosive growth in the demand for touch panels in dis-
plays (driven in large part by the iphone) has increased the
urgency of finding alternatives to expensive and brittle ITO-
based devices. Unidym has demonstrated a scalable set of
materials and processes that meets the needs of analog-resis-
tive touch panels and with further improvements will meet
the demands of other touch-panel applications as well. The
ability to form a transparent conductive layer on PC is par-
ticularly important, as this is difficult to achieve with ITO.
The ability to function under curved and stressed conditions
without failure will assuredly motivate CNT-based films to
be an integral part of future flexible displays. The replace-
ment of expensive vacuum-based sputtering with aqueous-
based roll-to-roll coating, coupled with the falling price of
CNTs and rising price of indium, make CNT-based devices
increasingly attractive from a cost perspective as well.9
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FIGURE 9 — Device response as a function of sliding-stylus cycle
number. The constant voltage at each position as a function of stroke
number indicates that the device linearity is not degrading with use.
FIGURE 10 — Reflection from CNT/ITO (black) and ITO/ITO (red) touch
panels. Decreased reflection from CNT/ITO device is due to superior
index matching.
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Carbon-nanotube film on plastic as transparent electrode for resistive touch screens
