AIST, and others have developed a technology that dramatically improves the long-term stability of the conductivity of transparent conductive films that use carbon nanotubes (CNTs).
As nanoparticles of metal halides such as copper iodide are grown in a film, the newly developed CNT transparent conductive film has a hybrid structure in which the nanoparticles connect CNTs to each other. The film retains 85 % of the transmittance of the base material, while sheet resistance (surface resistance rate) is 60 Ω/square. The film has sufficiently high transparency and conductivity for practical CNT transparent conductive films. The film also achieved long-term stability of sheet resistance when kept in the open air, which has been an issue with the conventional doping technology used to improve conductivity.
The developed film is expected to have applications in touch panels, sensors, flexible solar cells, and more as a flexible conductive material using the flexible characteristics of CNTs. It is also expected to have applications in wearable electronics that require elasticity.
Indium tin oxide (ITO) film is currently used mainly as a transparent electrode material for mobile information devices and touch panel PCs. ITO film is brittle and does not stand bending which makes it difficult to use for developing flexible next-generation electronic devices therefore, an alternative transparent conductive material is needed to resolve these issues.
CNTs are a material having a tubular structure made of curled graphene sheet(s). They are known to be flexible like polymers and very highly conductive. However, the conductivity of CNT films is much lower than that of individual CNTs. This is thought to be because the contact resistance between CNTs has a significant effect on conductivity. Doping, which involves adding a small amount of oxidizing agent such as nitric acid, is a method often used to increase the conductivity of CNT films. Nitric acid doping involves making volatile nitric acid adsorb to the CNTs, so if such films are kept in the open air for long periods, volatile molecules are gradually released from the doped film, raising sheet resistance and creating a problem of durability.
In this study, a film of metal halides such as copper iodide on the top or bottom of the CNT film by vacuum deposition instead of doping the CNTs with nitric acid (Fig. 1). This was then irradiated with light of a few hundred milliseconds pulse width to rapidly raise and lower the film temperature. The process caused the metal halide to transfer to the interior of the film, producing a transparent conductive film. Using single-wall CNTs produced by the eDIPS method yielded sheet resistance of 60 Ω/square with 85 % transmittance (relative value at wavelength 550 nm when the transmittance of the base material is defined as 100 %). These are the highest levels of transparency and conductivity recorded for a CNT transparent conductive film.
Source and top image: AIST
Top image shows: (a) CNT film without nanoparticles; (b) CNT film produced by the developed technology; (c) Enlarged image of (b)