An organic field-effect transistor (OFET) is a field-effect transistor using an organic semiconductor in its channel. OFETs can be prepared either by vacuum evaporation of small molecules, by solution-casting of polymers or small molecules, or by mechanical transfer of a peeled single-crystalline organic layer onto a substrate. These devices have been developed to realize low-cost, large-area electronic products and biodegradable electronics. OFETs have been fabricated with various device geometries. The most commonly used device geometry is bottom gate with top drain and source electrodes, because this geometry is similar to the thin-film silicon transistor (TFT) using thermally grown SiO2 as gate dielectric. Organic polymers, such as poly(methyl-methacrylate) (PMMA), can also be used as dielectric. One of the benefits of OFETs, especially compared with inorganic TFTs, is their unprecedented physical flexibility, which leads to biocompatible applications, for instance in the future health care industry of personalized biomedicines and bioelectronics. In May 2007, Sony reported the first full-color, video-rate, flexible, all plastic display, in which both the thin-film transistors and the light-emitting pixels were made of organic materials.
History The concept of a field-effect transistor (FET) was first proposed by Julius Edgar Lilienfeld, who received a patent for his idea in 1930. He proposed that a field-effect transistor behaves as a capacitor with a conducting channel between a source and a drain electrode. Applied voltage on the gate electrode controls the amount of charge carriers flowing through the system. The first insulated-gate field-effect transistor was designed and prepared by Mohamed Atalla and Dawon Kahng at Bell Labs using a metal–oxide–semiconductor: the MOSFET (metal–oxide–semiconductor field-effect transistor). It was invented in 1959, and presented in 1960. Also known as the MOS transistor, the MOSFET is the most widely manufactured device in the world. The concept of a thin-film transistor (TFT) was first proposed by John Wallmark who in 1957 filed a patent for a thin film MOSFET in which germanium monoxide was used as a gate dielectric. Thin-film transistor was developed in 1962 by Paul K. Weimer who implemented Wallmark's ideas. The TFT is a special type of MOSFET. Rising costs of materials and manufacturing, as well as public interest in more environmentally friendly electronics materials, have supported development of organic based electronics in more recent years. In 1986, Mitsubishi Electric researchers H. Koezuka, A. Tsumura and Tsuneya Ando reported the first organic field-effect transistor, based on a polymer of thiophene molecules. The thiophene polymer is a type of conjugated polymer that is able to conduct charge, eliminating the need to use expensive metal oxide semiconductors. Additionally, other conjugated polymers have been shown to have semiconducting properties. OFET design has also improved in the past few decades. Many OFETs are now designed based on the thin-film transistor (TFT) model, which allows the devices to use less conductive materials in their design. Improvement on these models in the past few years have been made to field-effect mobility and on–off current ratios.
Materials One common feature of OFET materials is the inclusion of an aromatic or otherwise conjugated π-electron system, facilitating the delocalization of orbital wavefunctions. Electron withdrawing groups or donating groups can be attached that facilitate hole or electron transport. OFETs employing many aromatic and conjugated materials as the active semiconducting layer have been reported, including small molecules such as rubrene, tetracene, pentacene, diindenoperylene, perylenediimides, tetracyanoquinodimethane (TCNQ), and polymers such as polythiophenes (especially poly(3-hexylthiophene) (P3HT)), polyfluorene, polydiacetylene, poly(2,5-thienylene vinylene), poly(p-phenylene vinylene) (PPV). The field is very active, with newly synthesized and tested compounds reported weekly in prominent research journals. Many review articles exist documenting the development of these materials. Rubrene-based OFETs show the highest carrier mobility 20–40 cm2/(V·s). Another popular OFET material is pentacene, which has been used since the 1980s, but with mobilities 10 to 100 times lower (depending on the substrate) than rubrene. The major problem with pentacene, as well as many other organic conductors, is its rapid oxidation in air to form pentacene-quinone. However if the pentacene is preoxidized, and the thus formed pentacene-quinone is used as the gate insulator, then the mobility can approach the rubrene values. This pentacene oxidation technique is akin to the silicon oxidation used in the silicon electronics. Polycrystalline tetrathiafulvalene and its analogues result in mobilities in the range 0.1–1.4 cm2/(V·s). However, the mobility exceeds 10 cm2/(V·s) in solution-grown or vapor-transport-grown single crystalline hexamethylene-tetrathiafulvalene (HMTTF). The ON/OFF voltage is different for devices grown by those two techniques, presumably due to the higher processing temperatures using in the vapor transport grows. All the above-mentioned devices are based on p-type conductivity. N-type OFETs are yet poorly developed. They are usually based on perylenediimides or fullerenes or their derivatives, and show electron mobilities below 2 cm2/(V·s).
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