OFET
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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, or by solution-casting of polymers or small molecules. These devices have been developed to realize low-cost, large-area electronic products. 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 Si/SiO₂ oxide as gate dielectric. Organic polymers, such as poly(methyl-methacrylate) (PMMA), can be used as dielectric, too.

In May 2007, Sony Corp. reported the first full-color, video-rate, flexible, all plastic display [1], in which both the thin film transistors and the light emitting pixels were made of organic materials.

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, 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 such as Advanced Materials, Journal of the American Chemical Society, Chemistry of Materials, Angewandte Chemie, and many others. Many review articles exist documenting the development of these materials ¹².

Carrier transport

The carrier transport in OFET is specific for two-dimensional (2D) carrier propagation through the device. Various experimental techniques were used for this study, such as Haynes-Shockley experiment on the transit times of injected carriers, time-of-flight (TOF) experiment for the determination of carrier mobility, pressure-wave propagation experiment for probing electric-field distribution in insulators, organic monolayer experiment for probing orientational dipolar changes, the optical time-resolved second harmonic generation (TRM-SHG) and etc. Recently it was found that carriers propagate through the OFET has a diffusionlike behaviour ³.

Light emitting OFETs

Because an electric current flows through such a transistor, it can be used as a light-emitting device, thus integrating current modulation and light emission. In 2003, a German group reported the first organic light-emitting field-effect transistor (OLET)⁴. The device structure comprises interdigitated gold source- and drain electrodes and a polycrystalline tetracene thin film. Both, positive charges (holes) as well as negative charges (electrons) are injected from the gold contacts into this layer leading to electroluminescence from the tetracene.

References

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