Advances in Materials for Touch-Panel Applications
by Ion Bita
Welcome to 2012, and a Happy New Year to all of you. We start out with a look at advances in the area of materials for touch panels, a display component that has become a must-have for many of today's mobile devices. With the recent explosion in popularity of these devices, many of which incorporate new types of user interaction, it comes as no surprise that the annual touch-module volume has more than tripled in the last 3 years, exceeding 1 billion units in 2011. About half of that volume represents smartphones alone. As a result of this significant growth and business impact, the touch-panel industry is being further shaped by an aggressive pursuit of opportunities for cost and value-added-based differentiation, a pursuit embodied by strategies for sensor-technology development, integration, and manufacturing. While projected-capacitive-based sensing has lately taken center stage, a variety of other technologies are being used as well, including resistive, electromagnetic resonance, surface acoustic, and a host of optical solutions.
Besides the conventional implementation of these technologies in discrete glass- or film-based touch panels, vertical-integration approaches have become more popular in response to factors including cost and display-module thickness. On one hand, display panel makers can pursue integration of touch functionality in the display panel itself, as with in-cell and on-cell approaches in LC-TFT and OLED displays. On the other hand, touch-panel suppliers can take a different approach for vertical integration by adding display lamination capabilities, with some even pursuing integration of the touch sensor into the cover lens to help display-module suppliers meet the current requirements for reduced module thickness and weight.
We are glad to include in this issue two articles showcasing prime examples of the impact of materials developments on two of the trends discussed above – cost-based and value-added-based differentiation in the touch-panel industry.
The first article is from Cambrios Technologies located in California. Michael Spaid (VP Product Development) provides an overview of the material design, processing, and properties of arguably the first high-volume commercially available wet-processed transparent conductor alternative to ITO, ClearOhm. Given the more advantageous resistance – optical property tradeoffs compared to ITO and low-temperature processing compatible with glass and plastic substrates, as well as a reduced cost structure enabled by the use of roll-to-roll coating processes – ClearOhm materials have been received with great excitement by the touch-panel industry. In fact, just in 2011, these films have been used to produce the projected-capacitive touch module for a leading smartphone device, with significant growth expected in 2012. The basis of the Cambrios approach is the use of silver nanowires to create conductive coatings comprising an interconnected mesh of nanowires, with resistivity and optical properties that can be adjusted by tuning the nanowire diameter, length, and substrate surface coverage. The high electrical conductivity of silver is key to achieving sheet-resistance values comparable to ITO while minimizing optical transmission losses due to the low-area coverage of the nanowires. Further, patterning methods are described and show how these films can be used to fabricate touch panels.
The second article is from Peratech, Ltd., located in the UK. David Lussey (co-founder and CTO) introduces an innovative touch-panel technology based on the use of a transparent Quantum Tunneling Composite (QTC) force-sensitive material, QTC Clear. The basis of this approach is a composite employing conductive metal nanoparticles dispersed throughout a clear elastomeric matrix, with a composition tailored to enable significant conductivity changes when the material is compressed. The particle concentration is adjusted so that at rest the composite is insulating, but when pressure is applied the conductivity along the compression direction increases orders of magnitude. The local deformation reduces the interparticle distance and consequently increases conductivity via electron tunneling, or hopping across the nanoparticles. Naturally, due to the sensitivity of the resistance to pressure, this remarkable material should find applications in a new generation of resistive touch panels by replacing the conventional air gap, thus eliminating the optical drawbacks of conventional resistive touch panels. Moreover, the sensitivity of the response to various touch pressures enables a third dimension of sensing besides the (x,y) location, which is another reason for the excitement around the opportunities for user interactions enabled by this new material.
I enjoyed working on these stories about new materials developments, and I hope you learn as much from reading them as much as I did.