Carbon nanotube transparent conducting films

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oduction Over the last decade, the market for transparent conducting films has experienced exponential growth due to the proliferation of LCD displays, touch screens, and thin-film solar cells. Today, the transparent conducting film is ubiquitous. Popular devices such as LCD TVs, computer screens, solar cells, GPS units, cell phones, tablet computers, and e-book readers are all dependent on it. To make a smartphone, tablet computer, or GPS function requires five layers of transparent conducting films: one layer for the back-plane transistors, two layers for switching liquid crystal orientation, and two layers for the touch screen. For a single material, this represents more layers than any other material used for these devices. To date, LCDs are the largest application for transparent conducting films, and they are forecast to continue to grow for the foreseeable future. In 2010, about 362 million touch screen panels were shipped. An annual growth rate of 20% is expected through 2013 for the touch screen market. With the popularity of e-readers such as the Kindle increasing, demand for transparent conducting films for electrophoretic displays (EPDs) is forecast to grow 30-fold from 2008 to 2014.1 Most significantly, the market demand for transparent conducting films in green energy applications, including electrochromatic windows, electroluminescent light (EL) panels, organic lightemitting devices (OLEDs), and thin-film solar cells, is growing at a rapid pace.

Tin-doped indium oxide (ITO) as a transparent conducting film dominates all market segments, except thin-film solar cells, wherein fluorine-doped tin oxide (SnO2:F) and aluminumdoped zinc oxide (AZO) have made great inroads. According to the Information Network, “ITO commanded a transparent conducting film market share of 97.6% in 2010, the revenue for the target alone was $6.99 billion.” With market demand increasing and new applications emerging, ITO increasingly becomes problematic in several aspects. An ITO film contains ∼90% indium oxide (In2O3). The indium is produced mainly as a by-product of zinc mining. Its supply is limited. The most viable method for ITO deposition is vacuum sputtering. The process is expensive and problematic for substrates, such as polycarbonate (PC), which has a tendency to de-gas under vacuum. ITO film is brittle—easy to crack when it is subject to deformation—a serious issue for both manufacturing yield and lifetime for flexible devices. Additionally, the ITO surface is active when in contact with a corrosive or organic material, resulting in interface instability. Therefore, there is a great need for alternatives to ITO (e.g., conducting polymers). Several new technologies are emerging, including silver nanoparticles and nanowires, graphene, and carbon nanotubes (CNTs). This article focuses on CNT technology. For a broad perspective of the field, readers can refer to several recent review articles.1–4 The first publication on CNT transparent conducting films can be traced back to a 1998 U.S. patent5 issued to Hyperion

Chunming Niu, Unidym, Inc.,

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