Over the past decade, touchscreens have risen to dominate mobile phone and other mobile consumer electronic device interfaces – and their popularity shows no sign of waning. Capacitive touchscreens, the type most commonly used in consumer electronics, usually use a conductor made of indium tin oxide (ITO). This material is well suited to this purpose due to its excellent conductivity and its transparency in thin layers. Unfortunately there are few deposits of indium in the world, which has prompted a search for alternatives. One such new alternative are touchscreens containing carbon nanotubes, which researchers claim offer comparable performance to ITO, but are much cheaper.
The main components of the new electrode material developed by researchers at Fraunhofer are carbon nanotubes and low-cost polymers. The electrode foil is composed of two layers. One is a foil made of inexpensive polyethylenterephthalate (PET) like that used to make plastic bottles. The second is thin film that is formed when a mixture of carbon nanotubes and electrically conducting polymers that is applied to the PET as a solution dries.
These combinations of plastics have not been particularly durable in comparison to ITO because humidity, pressure or UV light put a strain on the polymers causing the layers to become brittle and break down. The carbon nanotubes solve this problem by hardening on the PET to create a network that firmly anchors the electrically conducting polymers and provide much improved durability.
Ivica Kolaric, project manager from Fraunhofer Institute for Manufacturing Engineering and Automation IPA, concedes that "the electrical resistance of our layer is somewhat greater than that of the ITO, but it’s easily enough for an application in electrical systems,” and its advantages over ITO far outweigh its drawbacks.
In addition to indium’s limited supply, ITO layers are also fragile, lack flexibility, and the process to deposit them onto a surface requires a vacuum and is costly. In contrast, the researchers say carbon is renewable, low-cost and available all over the world from organic matter such as wood and the thin foil is flexible, making it suitable for applications other than touchscreens.
“We could even make photovoltaic foils out of it to line corrugated roofs or other uneven structures,” says Kolaric, who has already set up a pilot production where the foil can be enhanced for a wide range of applications.