Page 44

EETE JUN 2015

DISPLAYS & interfaces Rethinking 2.5 and 3D displays with new materials WBy Alan Tsai ith the need for information to be constantly at our fingertips, the challenge becomes how to integrate touch screen functionality into spaces where design and aesthetics are also important – such as in an automotive interior or wearable technology – or where extremely large format touch screens are required, such as digital signage or electronic white boards. Other challenges include transparency and, of course, cost containment wherever possible. Because of consumer demands for devices that integrate form and function, there is currently a great deal of interest in highly sensitive touch screen displays and the materials that can enable them, such as indium tin oxide (ITO)-based solutions, silver nanowires, metal mesh, carbon nanotube/graphene and conductive PET. While each of these has their merits, a new material – transparent Various views of the silver nanoparticle technology, which is wet coated onto polycarbonate film to achieve a highly transparent, flexible conductive film capable of achieving 2.5 and 3D forms. conductive polycarbonate film – is emerging as a less costly technology that can provide extremely high transparency, impressive sheet resistance or conductivity and, most importantly, formability into complex 2.5 and 3D shapes. Let’s compare each one of these in turn, beginning with the most often-used incumbent material, ITO. The material is used today to make a variety of displays, such as LCDs, flat panels and touch screens. Using the example of touch screens for consumer electronics and similar devices, we find that the touch screens today are typically made using ITO on glass or an ITO film laminated to a cover glass. The drawbacks of glass for touch screens are that it can shatter and glass can be heavy, especially in large size displays, which can provide complications for mounting and adds to the expense of shipping. The use of ITO is also challenging when considered for flexible (multi-bending) touch screens due to its brittle nature. When flexed, ITO will crack, thus preventing the operation of the touch screen. ITO also has a challenge when moving to larger format touch screens due to the conductivity requirements. Large format touchscreens (> 40”display sizes) require conductivity of 50 ohms/m2 or less in order to maintain the fast 10-point multi-touch response time the market requires. And lastly, ITO-based solutions are considered to be relatively high cost, due to the relative scarcity of ITO and the expense of processing. Metal mesh, while providing low resistance and high optical performance, has the potential to exhibit moiré or other visible patterns, has very limited flexibility and can have low production yields. Because of these attributes, it has a low ability to achieve 2.5 or 3D shapes. Silver nanowire is flexible and also has relatively low resistance when used in small formats (under 10”), but exhibits resistance levels of 100 Ω/m2 in large formats (> 46”). Because it is created using a lower-cost coating process, silver nanowire can require less investment. Potential downsides include the formation of a milky haze and silver migration, which can impact performance, and only average formability. Carbon nanotube (CNT) or graphene solutions have not yet been commercialized, but have the potential for high optical quality and flexibility. They may exhibit lower conductivity than other solutions, may have average formability, and may be higher in cost. Similar technology exists using a polyethylene terephthalate (PET) substrate instead of polycarbonate. This alternative can match other materials’ transmittance performance, is flexible and relatively low in cost. However, its resistance is relatively high (>150 Ω/m2) and the material exhibits lower conductivity, and can develop a blue haze. It, too, exhibits average formability. As a semi-crystalline material, PET is not capable of withstanding the higher temperatures needed to thermoform the material into complex designs, a definite advantage of polycarbonate, which is an amorphous material. Transparent, conductive polycarbonate film Recently introduced transparent, conductive polycarbonate (PC) film represents a completely new class of display materials with outstanding transmittance and resistance, especially in large formats (>46”) and exceptional formability. Prior to this development, achieving a transparent conductive polycarbonate film Alan Tsai is Director of Technology & Innovation for Consumer Electronics Innovative Plastics at SABIC – www.sabic.com 36 Electronic Engineering Times Europe June 2015 www.electronics-eetimes.com


EETE JUN 2015
To see the actual publication please follow the link above