TECH | FOCUS
Best-in-Class Touchscreens with
Silver Nanowires
U
sed in a broad range of applications such as tablets, mobile phones, laptops, monitors and Global Positioning
Systems (GPS), touchscreens are one of
the most intuitive consumer electronics
interfaces. According to DisplaySearch,
worldwide shipments of touchscreen
panels are set to double from 2012 to
2018, shipping an estimated 2.5 billion
panels in 2018.
As the demand for better touchscreens
increases, expectations for low-cost,
high-performance touchscreens increase
as well. In order to meet today’s advanced standards, touchscreens should
be thin, light, visible in various ambient light conditions, highly responsive
and most importantly low-cost. Fast responding transparent touchscreens are
now critical to a great user
experience and can only be
achieved through transparent conductors that are not
visible to the naked eye. A
key component behind these
innovative technologies is
silver nanowires.
Traditional materials, such
as indium tin oxide (ITO),
are neither very conductive
nor very transparent and are
too brittle for flexible display
and touch applications, one
of the most discussed and
highly anticipated products
today. The quest to replace
ITO with a better performing
Figure 1: Transparent conductors in touchscreens
material resulted in the development of silver nanowPlan View
ire-based transparent conductors. A lot of companies
are creating new materials
to compete with and replace
ITO; each with its own set
of benefits and advantages,
as well as challenges. This
70 Degree Tilt
article will discover the distinctive benefits of silver
Figure 2: Silver nanowires coated on film, viewed
nanowires for various types
through an electron microscope
of touchscreen applications.
The most popular touchscreen technology is projected capacitance, or procap. At the core of this technology is a
transparent conductor, a layer of material that needs to conduct electricity, yet
remain transparent and allow the light
from the underlying display to shine
through the screen.
In order to meet today’s standards,
touchscreens need to be very responsive, and the display needs to be bright
and visible in all types of ambient light
conditions, requiring highly conductive
transparent conductors with high transmission.
Transparent conductors also can be
used as electrodes for LCD, OLED, thin
film photovoltaic cells, shutters for 3D
TVs and a whole host of applications.
In general, the requirements are the
same – higher conductivity, better light
transmission, no side effects like moiré
or pattern visibility, coupled with the
ability to flex a hundred thousand times
to support flexible touch screens. The
industry, of course, wants all of this at
a cost lower than that of the incumbent
traditional technology.
Silver Nanowires
Silver nanowires are usually developed and suspended in a fluid and the
resultant ink is used to coat roll-to-roll
plastic film substrates to create transparent conductors of varying sheet resistances. Silver nanowires can also be
coated on glass or other substrates but
roll-to-roll film is most popular. The
nanowires are a few tens of nanometers in diameter and a few tens of micrometers in length. When coated on a
plastic substrate, these high aspect ratio
(1,000:1) silver nanowires (typically
PET), overlap to create a highly conductive, yet transparent network, as shown
in Figure 2. This relatively sparse network of high aspect ratio silver nanowires allows light to pass through with
high transmission rates.
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TECH | FOCUS
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Transmission vs. Conductivity
High transmission (greater than 90
percent) along with low resistance (50
to 80Ω/sq) enables 10-finger touch – a
key component of a great user experience, particularly for laptops, All-InOne (AIO) computers and other large
area capacitive touchscreens. Higher
transmission also improves battery life
per charge in mobile devices and creates
brighter displays since the touch sensor
does not impede light as much.
For sheet resistances of less than
130Ω/sq, traditional transparent conductors like ITO are only available on glass;
as their annealing temperature is too
high and will damage plastic substrates.
Higher conductivity with traditional
methods is obtained by depositing a
thicker layer of transparent conductor on
a glass substrate, which takes more time
to deposit, thus reducing throughput.
Figure 3 compares light transmission of
silver nanowires films with both non30
AEI August 2014
Copyright©2014 Dempa Publications, Inc.
99
98
97
% Light Transmission
Touchscreen Requirements
Sheet resistance requirements for
transparent conductors vary by application and touchscreen size. The conductivity requirements for a 27-inch monitor are significantly higher than that of a
touch screen used in a four-inch mobile
phone. Touchscreen applications require
highly transmissive materials for clear
visibility; excellent conductivity to enable a fast response to touch, as well
as thin, light materials for sleek, aesthetically pleasing end products, at a low
cost.
These requirements are constantly
evolving. Today, device makers are
looking for conductivity below 100/
sq to make their touchscreens more responsive and further improve user experience. For large area touchscreens
in devices, such as 20-inch monitors,
higher conductivity is essential for a fast
response time with the ability to detect
10-finger touch. For mobile devices like
laptops and smartphones, film-based
transparent conductors are in demand
to create thinner, lighter and stronger
touchscreens. With flexible displays on
the horizon, transparent conductors that
can be bent or rolled become necessary.
Most importantly, transparent conductor prices must be low enough to enable
mass adoption of touch-enabled consumer electronic devices.
96
95
94
93
92
91
90
Cambrios ClearOhm
89
Index matched ITO
Non-index matched ITO
88
10
100
Sheet Resistance
Figure 3: Light transmission vs. resistance comparison for silver nanowires vs. ITO on film
Source: Cambrios
index matched and index matched ITO
layers that increase transmission of light.
In contrast, silver nanowire ink can be
coated at around 100°C – much lower
than the softening temperatures of plastic films. Mass production throughput
also is consistently high, regardless of
sheet resistance requirements. For lower
sheet resistance, product designers need
only apply a thicker coating of silver
nanowire ink at the same coating speed
(hence same throughput). As seen in
Figure 3, silver nanowire material offers
higher transmission than ITO. It also offers more than 95 percent light transmission, even at sheet resistances significantly lower than those achievable with
film-based ITO.
True Single Layer Sensor
For tablets and mobile phones, single
layer touch sensors are in demand, and
they offer very low cost because they
use fewer layers of adhesives and conductors in the touchscreen stack. They
do have high performance requirements,
which make silver nanowires an ideal
fit. A seven-inch true single layer design
using silver nanowires has been demonstrated recently, which is twice as big as
what is possible with ITO and the narrow line/space requirements rule out
competing metal mesh technologies as
well. This seven-inch true single layer
touch sensor offers multi-touch capability, very high transmission (over 90 percent) and is ideally suited for most price
sensitive mobile consumer electronics
products. This single layer touchscreen
can be matched with either glass or plastic cover lens offering original equipment manufacturers (OEMs) increased
flexibility with their design.
Pattern Visibility And Moiré
Moiré effect occurs when the eye sees
a set of lines or dots over another set of
lines or dots. This visual image can considerably degrade the quality and resolution of images, particularly on a touchscreen. Silver nanowires have no moiré
issues and almost no pattern visibility
because of the random distribution of
the nanowires.
An emerging competing technology
based on metal mesh has transmission
and conductivity advantages over ITO,
however, suffers from pattern visibility
and moiré, or pattern interference created when the metal mesh pattern conflicts
with the display’s pixel pattern. While
there are ways to minimize the moiré effect, these tactics cause additional logistical work for OEMs and original device
manufacturers (ODMs), including additional design time for different transparent conductor patterns and pixel structures. This type of customization limits
the OEMs’ ability to use the same sensor
design or firmware for various display
resolutions or even displays with the
same resolution from different suppliers.
Weight, Thickness
Nobody wants today’s consumer electronic devices to look like yesterday’s
clunky machines. Tablet and laptop devices are becoming increasingly thin-
ner and stationary devices like monitors
and kiosks are becoming sleek and aesthetically pleasing, driving demand for
thinner, lighter components. Electronic
components with reduced mass tend to
be more rugged and durable.
ITO usually is deposited on glass,
resulting in a fragile, heavy glass touch
sensor that is about 0.7 to 1.5mm thick.
In comparison, silver nanowire film
based touch sensor is only 0.2 to 0.4mm
thick. Silver nanowires sensors on film
is roughly 40 percent lighter and 40 percent thinner than its ITO counterpart,
making this a strong area of advantage
for film-based silver nanowire sensors.
Flexible, Wearable Displays And
Touchscreens
Flexibility, the next big trend in
touchscreens and displays, will enable
enhanced portability, durability and
unique designs. Imagine unbreakable
phone screens that would flex instead
of shattering when dropped, the ability
to fold your seven-inch tablet so that it
fits in your pocket, or displays that wrap
around your arm, a pillar or building.
Products like these are slowly becoming
a reality and are driving demand for flexible, bendable and rollable touchscreens.
In customer tests, silver nanowire
coated films withstood greater than
100,000 turns around a 3mm radius of
bend demonstrating great fit in flexible
and rollable electronic devices.
Total Cost of Ownership
Silver is the best conductor of electricity on the planet and is roughly 100
times more conductive than ITO. From
a material standpoint, much less silver is
needed versus ITO for a given transpar-
ent conductor design.
Silver nanowires material also trumps ITO
in terms of coating infrastructure and equipment costs. Coating ITO
requires vacuum deposition equipment – a multimillion dollar investment.
In contrast, silver nanowire materials are solution
coated, requiring a significantly lower upfront
equipment
investment.
ITO throughput also is
influenced by an appli- Figure 5: Roll-to-roll manufacture of silver nanowire transcation’s conductivity re- parent conductors
quirements; for example,
a 50Ω/sq layer of ITO means four times
pattern quality is very high with excelless throughput than a 200Ω/sq layer in
lent optical performance. Laser patterncontrast, silver nanowires throughput
ing is not a viable option for ITO on film.
does not vary based on conductivity;
ITO requires more power or longer duroll-to-roll coating process is more efration to pattern, which could result in
ficient, allows for rapid capacity expandamaged film and low throughput.
sion and does not produce the extensive
Overall, silver nanowire-based touchwaste associated with the ITO deposiscreens range from slightly less to sigtion process.
nificantly less expensive than equivalent
For photo patterning or wet-etch patITO film-based solutions.
terning methods, silver nanowires and
ITO’s costs are similar. Silver nanowConclusion
ires, however, are less expensive to
For newer factories and emerging
pattern using a room temperature laser
touchscreen applications, including
process, which offers high throughput
large area touchscreens, as well as flexand quality similar to that achieved with
ible display applications, silver nanowhigh-end photo processing. Laser patires offer a significant advantage, both
terning roughly is one-fourth the cost of
in cost and performance. The silver
photo patterning as equipment costs are
nanowire material is currently used in
lower and there are no consumables like
several consumer products, offers lower
photo resist, etchants or strippers. Furmanufacturing and per unit costs, and
thermore, since the process does not use
makes scaling much easier. Roll-to-roll
chemicals, there are no waste disposal
processed silver nanowire transparissues. Minimal laser power is required
ent conductors are the clear choice for
to pattern silver nanowires film and the
new production facilities that need high
throughput and easy processing, as well
as for device manufacturers that need a
thin, light, flexible material to deliver
high performance for innovative devices. For design engineers that need higher
performance than conventional touchscreens, this article is a call to action to
ask their suppliers about true single layer
touchscreens, about higher conductivity
silver nanowire-based solutions that are
ready for wearable and flexible devices.
Figure 4: Flexible silver nanowire-based touchscreens
About This Article:
The author, Sri Peruvemba, is the Vice
President for Corporate Marketing at
Cambrios Technologies.
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Copyright©2014 Dempa Publications, Inc.
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