Highly Bendable, Transparent Thin-Film Transistors That Use Carbon-Nanotube-Based Conductors and Semiconductors with Elastomeric Dielectrics

We report the use of networks of single-walled carbon nanotubes (SWNTs) with high and moderate coverages (measured as number of tubes per unit area) for all of the conducting (i.e., source, drain, and gate electrodes) and semiconducting layers, respectively, of a type of transparent, mechanically flexible, thin-film transistor (TFT). The devices are fabricated on plastic substrates using layer-by-layer transfer printing of SWNT networks grown using optimized chemical vapor deposition (CVD) procedures. The unique properties of the SWNT networks lead to electrical (e.g., good performance on plastic), optical (e.g., transparent at visible wavelengths), and mechanical (e.g., extremely bendable) characteristics in this “all-tube” TFT that would be difficult, or impossible, to achieve with conventional materials. Invisible circuits based on transparent transistors have broad potential applications in consumer, military, and industrial electronic systems. In backlit display devices, for example, transparent active-matrix circuits can increase the aperture ratio and battery life. Transparent electronic materials that can be printed on low-cost, flexible, plastic substrates are potentially important for new applications, such as bendable heads-up display devices, see-through structural health monitors, sensors, and steerable antennas. More advanced systems, such as electronic artificial skins and canopy window displays, will require materials that can also tolerate the high degrees of mechanical flexing (i.e., high strains) needed for integration with complex curvilinear surfaces. Most examples of transparent TFTs (TTFTs) use thin films of inorganic oxides as the semiconducting and conducting layers. Although the electrical properties of these oxides can be good (mobilities and conductivities as high as 20 cm V s and 4.8 × 10 X cm, respectively), their mechanical characteristics are not optimally suited for use in flexible and mechanically robust devices. For example, the tensile fracture strains for ZnO and indium tin oxide (ITO) thin films are less than 0.03 % and 1 %, respectively. Aligned arrays or random networks of individual SWNTs represent alternative classes of transparent semiconducting and conducting materials. In networks with high coverages of SWNTs, especially when in the form of small bundles, the metallic tubes (normally present with semiconducting tubes in a 1:2 ratio) form a percolating network that behaves like a conducting “film”. At moderate coverages, only the semiconducting tubes form such a percolating network and the film shows semiconducting properties. Unlike the oxides, the SWNT films have excellent mechanical properties due to their high elastic moduli (1.36–1.76 TP nm/tube diameter nm) and fracture stresses (100–150 GPa) of the tubes. SWNT-based semiconductors have been used in flexible TFTs. In one case, solution-deposited SWNT networks also formed the gate electrodes. Although these TFTs can show good electrical properties, especially when CVD tubes are used, the metal (Au, Pd, etc.) source, and drain electrodes limit their optical transparency and C O M M U N IC A TI O N S