The demand for high quality transparent conductive films (TCFs) is very high because of their use in flat panel displays, electrochromic windows, photovoltaic solar cells, hand-held devices, energy technologies, and biosensors [1]. Indium tin oxide (ITO) film-coated transparent conducting materials (TCM) are commonly employed in industrial applications where TCFs are needed. Typically, however, ITO thin films are applied to glass substrates. A glass substrate, of course, is inflexible and easily broken. Additionally, casting an ITO film of uniform thickness onto a curved glass substrate is quite difficult and expensive. ITO-based TCMs are widely used, however, because they afford relatively low sheet resistance (˜20Ω/□) and fairly high transmittance (>80%) in the visible region of the solar spectrum[2]. In other words, TCFs having high conductivity and high transmittance in the visible spectrum are very desirable. However, due to increasing demand and limited availability, the cost of indium is on the rise [2]. ITO thin films can be formed on flexible substrates. However, ITO-based materials become brittle after only a few bending cycles; thus they are unsuitable for high flexibility applications.[3] As a consequence, many other materials, such as single-walled carbon nanotubes (SWNTs) [4-6] and their metal hybrids have been investigated as potential alternatives to ITO [7-9]. See also, for example, U.S. Pat. No. 8,697,180, issued Apr. 15, 2014, to Veerasamy; U.S. Pat. No. 8,048,490, issued Nov. 1, 2011, to Watanabe et al.; and U.S. Pat. No. 7,411,085, issued Aug. 12, 2008, to Hirakata et al.
As-synthesized SWNTs are highly hydrophobic bundles, which must be dispersed in a surfactant solution to take advantage of their many unique properties [10]. Methods to fabricate carbon SWNTs are well known in the art and will not be discussed herein. Carbon SWNTs can also be purchased from a large number of international commercial suppliers, such as Nanocyl s.a. (Sabreville, Belgium) and Nanostructured & Amorphous Materials, Inc. (Houston, Tex.). Dispersions of SWNTs can be formed into TCFs by spin-coating [11], dip-coating [12, 13], spray-coating [14], bar-coating [15] and by vacuum filtration [16, 17]. However, commonly known surfactants tend to stick to the SWNTs firmly, which significantly increases the inter-tube contact resistance. Removing any adhering surfactant by thermal annealing at high temperature is a prerequisite to reducing the sheet resistivity of SWNT films. However, annealing of SWNT films prepared on flexible substrates such as polyethylene terephthalate (PET) is not feasible. Thus, in conventional methods, the SWNT films are first prepared on a solid substrate that can withstand the heat off annealing. The film is then annealed on the solid substrate. After annealing, the film is then transferred from the solid substrate onto a flexible substrate. In short, preparing a flexible SWNT film with low resistivity and high transparency via conventional means is a challenging and laborious task.
There is thus a long-felt and unmet need for new methods or technologies to facilitate using thin flexible films SWNTs in various applications without the aforementioned laborious fabrication techniques.