Liquid Crystals for Organic Field-Effect Transistors
Columnar, smectic and lamellar polymeric liquid crystals are widely recognized as very promising charge-transporting organic semiconductors due to their ability to spontaneously self-assemble into highly ordered domains in uniform thin films over large ar
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Liquid Crystals for Organic Field-Effect Transistors Mary O’Neill and Stephen M. Kelly
9.1 Introduction Organic semiconductors are becoming a viable alternative to amorphous silicon for a range of thin film transistor devices. Indeed, plastic and organic electronics are considered disruptive technologies enabling new applications including intelligent or interactive packaging, RFID tags, e-readers, flexible power sources and lighting panels. The future success of the industry depends on the availability of high performance, solution-processable, materials for low cost manufacturing as well as low voltage device operation. The organic field effect transistor (OFET) is the fundamental building block of plastic electronics and is used to amplify and switch electronic signals. The key figure of merit for OFETs is the charge carrier mobility, (), the hole/electron velocity per unit field. Carrier transport occurs by hopping via - interactions between sites, which may be traps, single molecules, several repeat units of a polymer chain or even a number of delocalised chain segments in high-mobility conjugated mainchain polymers. There has been excellent progress in the development of solution processed organic semiconductors for OFETs [1–3]. The state-of-the-art performance is now equivalent to that of amorphous silicon.
M. O’Neill () Department of Physics and Mathematics, University of Hull, Hull, HU6 7RX UK e-mail: [email protected] S.M. Kelly Department of Chemistry, University of Hull, Hull, HU6 7RX UK e-mail: [email protected] R.J. Bushby et al. (eds.), Liquid Crystalline Semiconductors, Springer Series in Materials Science 169, DOI 10.1007/978-90-481-2873-0 9, © Springer ScienceCBusiness Media Dordrecht 2013
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M.O’Neill and S.M. Kelly
9.2 Principle of Organic Field-Effect Transistors Transistors are organic semiconductor devices used to amplify and switch electronic signals. In an organic field-effect transistor (OFET) a voltage applied to the gate electrode controls the current between the source and drain electrodes [4, 5]. The construction of a typical OFET is shown in Fig. 9.1. An insulator is placed between the gate and the organic semiconducting thin film. The source and drain electrode are separated by a semiconducting channel of length L and width W . Ideally, when no gate voltage, VG , is applied, the conductance of the semiconductor film is extremely low, because there are no mobile charge carriers, i.e., the device is “off”. The application of VG gives an electric field across the dielectric, which induces mobile charges in the semiconductor film. These move in response to the voltage, VD , applied between the source and drain, i.e., the transistor is “on”. Figure 9.1 is an example of a bottom-gate, top-contact device configuration. The source and drain electrodes may be deposited between the dielectric and semiconducting thin film giving a bottom-gate, bottom-contact device. Alternatively the layers can be applied sequentially in the order: source and drain electrodes, semiconductin
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