With its two chief properties of excellent electrical conductivity and optical transparency, indium tin oxide (ITO) can be found in transparent conductive coatings for displays found in all kinds of products, such as TVs, mobile phones and laptops, and is also used as a transparent electrode in thin-film solar cells. Unfortunately indium is a rare metal and available supplies could run out in as little as ten years. This has prompted researchers to search for alternatives with some success already reported using carbon nanotubes and copper nanowires. The latest ITO replacement material also uses carbon nanotubes, as well as other commonly available materials, and is environmentally friendly.
The replacement material developed by researchers at Eindhoven University of Technology is a transparent, conducting film that is produced in water using carbon nanotubes and plastic nanoparticles. The researchers start by dissolving standard, widely available carbon nanotubes in water. They then add conducting latex, which is a solution of polymer beads in water, and a binder in the form of polystyrene beads. Upon heating, the polystyrene beads fuse together to form the film, which contains a conducting network of nanotubes and beads from the conducting latex. The water is then removed by freeze-drying leaving the transparent, conductive film.
Because high concentrations of carbon nanotubes would make the film black and opaque, the researchers have kept the concentration as low as possible, with the nanotubes and the conducting latex together accounting for less than one percent of the weight of the film.
While the conductivity of the film is still a factor 100 lower than that of ITO, the researchers say it is already good enough to be used as an antistatic layer for displays, or for EMI shielding to protect against electromagnetic radiation. The researchers also expect the gap in electrical conductivity between their film and ITO to be quickly closed.
"We used standard carbon nanotubes, a mixture of metallic conducting and semiconducting tubes", says Cor Koning. "But as soon as you start to use 100 percent metallic tubes, the conductivity increases greatly. The production technology for 100 percent metallic tubes has just been developed, and we expect the price to fall rapidly."
Another advantage the new film has over ITO is that it is environmentally friendly. All the materials used to produce it are water based and, unlike ITO, no heavy metals are used. The film is also more suited to flexible displays than ITO layers, which are fragile and lack flexibility.