The rising demand for better touchscreens means consumers will increasingly expect more performance at a lower cost. Today’s touchscreens should be thin, light, visible in varying ambient light conditions, highly responsive and, most importantly affordable. Quick response, transparent touchscreens are now critical to a great user experience and can only be achieved through transparent conductors invisible to the naked eye. A key component behind these innovative technologies is silver nanowires
Pro-Cap: Popular for Now
The most popular touchscreen technology is projected capacitance or pro-cap. At the core of this technology is a transparent conductor, a layer of material that needs to conduct electricity. Yet it must remain transparent and allow light from the underlying display to shine through the screen. Today’s touchscreens must be highly responsive, bright and visible in varying ambient light conditions. This calls for highly conductive transparent conductors with high transmission. But pro-cap technology won’t recognize touch from a thick-gloved hand and it won’t work with all types of stylus or a prosthetic hand.
Inherent Versatility of Transparent Conductors
Transparent conductors can also 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, and no side effects like moiré or pattern visibility. Add to this the ability to flex a hundred thousand times to support flexible touch screens and one begins to see the inherent versatility of transparent conductors. The industry, of course, wants all of this at a cost lower than today’s incumbent traditional technology. So what are the distinctive benefits of silver nanowires for various types of touchscreen applications?
Silver Nanowires: The New ‘Gold’ Standard
Silver nanowires are expected to become the new gold standard in touchscreen displays, delivering a bounty of benefits at reduced cost. The wires are usually developed and suspended in a fluid. The resultant ink is used to coat roll-to-roll plastic film substrates to create transparent conductive films 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 (usually PET), these high-aspect-ratio (1,000:1) silver nanowires, overlap to create a highly conductive, yet transparent network. See Figure 1. This relatively sparse network of high aspect ratio silver nanowires allows light to pass through with high transmission rates. State of the art conductive films using silver nanowires now have less than 0.2% delta haze, which is as good or better than ITO.
Meeting Touchscreen Requirements
Sheet resistance requirements for transparent conductors vary by application and touchscreen size. The conductivity requirements for a 30-inch monitor are significantly higher than that of a touchscreen used in a four-inch mobile phone. Touchscreen applications require highly transmissive materials for clear visibility; excellent conductivity to ensure fast touch response: and thin, light materials for sleek, aesthetically pleasing end products, all at a low cost. These requirements are constantly evolving.
Today, device makers are looking for conductivity below 70W/sq to make their touchscreens more responsive and further improve the user experience. For large area touchscreens in devices, such as 30-inch monitors or 42” electronic writing boards, 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 will need the ability to be bent or rolled. Most importantly, transparent conductor prices must be low enough to encourage mass production of touch-enabled consumer electronic devices.
Transmission vs. Conductivity
High transmission (greater than 90 percent) along with low resistance (20 to 70W/sq) allows for 10-finger touch—the key to a great user experience. This is particularly true for laptops, All-In-One (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 130W/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.
In contrast, silver nanowire ink can be coated at around 100° C—much lower than the softening temperatures of plastic films. Mass production throughput is also 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).
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 due to the random distribution of the nanowires.
Weight And Thickness
Nobody wants today’s consumer electronic devices to look like yesterday’s clunky machines. Tablet and laptop devices are becoming increasingly thinner 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 is usually deposited on glass, resulting in a fragile, heavy glass touch sensor that is about 0.7 – 1.5 mm thick. In comparison, a silver nanowire film-based touch sensor is only 0.2 – 0.4mm thick. Silver nanowire sensors on film are roughly 45 percent lighter and 50 percent thinner than its ITO counterpart, giving film-based silver nanowire sensors a big advantage.
Flexible, Wearable Displays And Touchscreens
Necessity being the mother of invention, it didn’t take long for display manufacturers to realize that too many mobile phone users were sitting on and cracking their touchscreens. Result: touchscreens are becoming increasingly flexible. In fact, touchscreens have become so flexible that you may soon be able to fold or roll up your smartphone or tablet when not in use.
Flexibility is clearly the next big trend in touchscreens and displays, allowing for 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.
Touchscreens Ideal for Large Area Displays
Often seen in corporate meeting rooms, executive or customer briefing areas, huddle spaces, and college/universities, large touch screen based devices have become increasingly popular. They are popular as Ideation platforms, they make for great electronic writing, sharing, real-time collaboration and offer better security than the traditional white board where someone takes a picture before it is erased. In these types of devices, there is a need for highly transparent and highly conductive touch sensors and silver nanowires play an enabling role in such applications.
Touchscreens Help Drive Automotive Market
Automotive manufacturers are integrating touchscreens into center stack displays, navigation systems, human-machine interfaces (HMIs), keyless entry, and rear-seat entertainment systems. According to a recent study, shipments of touchscreen displays for automotive applications rose sharply in 2018. This trend is expected to increase over the next four years, with projected shipments exceeding 65 million units in 2021.
For emerging touchscreen applications, including large area touchscreens, as well as flexible display applications, silver nanowires offer a significant advantage, both in cost and performance. Currently used in several consumer products, silver nanowire material offers lower manufacturing and per-unit costs and makes scaling much easier. Roll-to-roll processed silver nanowire transparent conductors are the clear choice for 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.
Sri Peruvemba is a Board Member and Chair of Marketing of The Society for Information Display (SID), a professional organization focused on the display industry.