Researchers from the University of Nebraska-Lincoln (UNL) and Stanford University reported in Nature Communications on January 8, 2014 an organic thin-film transistor with the highest speed rating ever measured. For liquid crystal displays, faster transistor switching speed allows for smaller transistors and smaller pixels (higher resolution), which translates into better performance. See also: Liquid crystals; Transistor

Today, the semiconductor material for thin-film transistors in most active-matrix liquid crystal displays is amorphous silicon. That material is favored because of its low production costs, even though the speed or mobility of charge carriers (electrons or holes) in amorphous silicon is far slower than that of polycrystalline silicon (their respective ratings are 1 cm²/Vs and 100 cm²/Vs). Amorphous silicon’s mobility is too low for applications demanding larger currents, such as in the pixels of organic light-emitting diodes (OLEDS). Polycrystalline silicon has its own disadvantages; for example, its processing temperature is too high for use in the flexible polyester substrates of a new generation of products such as color electronic paper (e-paper). Interest in thin-film transistors made of organic materials that might offer both flexibility and high charge mobility has therefore been strong. See also: Amorphous solid; Band theory of solids; Color electronic paper; Conduction (electricity); Electronic display; Flat-panel display device; Polyester resins; Semiconductor; Silicon

Unlike amorphous materials, organic molecules need to be well aligned to function as thin-film transistors. To achieve good molecular alignment, the UNL and Stanford researchers used a novel technique called off-center spin-coating: they dispensed a solution containing the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (also called C8-BTBT) and a polystyrene binder onto a glass substrate positioned relatively near the edge of a turntable and spun it at a much higher speed than normal. This technique is impractical for the large-scale manufacture of thin-film transistors but it produced an average carrier mobility of 25 cm²/Vs as well as the record value of 43 cm²/Vs and demonstrated the potential of materials such as C8-BTBT. See also: Organic-inorganic interfaces; Polystyrene resin