Electron-Transporting Thiophene-Based Semiconductors Exhibiting Very High Field Effect Mobilities
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ELECTRON-TRANSPORTING THIOPHENE-BASED SEMICONDUCTORS EXHIBITING VERY HIGH FIELD EFFECT MOBILITIES Antonio Facchetti, Myung-Han Yoon, and Tobin J. Marks* Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, USA. ABSTRACT Organic semiconductors exhibiting complementary n-type carrier mobility are the key components for the development of the field of “plastic electronics”. We present here a novel series of oligothiophenes designed to improve performance and stability under electrontransporting conditions. Furthermore, the key structural features of these compounds allows additional modifications of the n-type conducting core to achieve material solubility and processability. Thin film transistor (TFT) devices were fabricated employing both vacuumand solution-deposited semiconducting layers. Field-effect transistor measurements indicate that all the members of this new series are n-type semiconductors with mobilities and Ion:Ioff ratios approaching 1 cm2/(Vs) and 107, respectively. This family represents a key milestone in the design, understanding, and development of the next generation of highly efficient n-type OTFT components. INTRODUCTION The field effect transistor (FET) based on inorganic electrodes, insulators, and semiconductors is the most important component for the development of (micro)electronics during the last one-half of the century.1,2 Instead of competing with inorganic materials, organic FETs based on molecular and polymeric materials (OFET) may find applications in low-performance memory elements3 as well as integrated optoelectronic devices, such as pixel drive and switching elements in active-matrix organic light-emitting diode (LED) displays.4 These systems have been widely pursued since they offer numerous advantages for easy evaporation/solution processing and good compatibility with a variety of substrates including flexible plastics,5 and great opportunities for structural modifications.6 The simplest and most common OFET device configuration is that of a thin-film transistor (TFT), in which a thin film of the organic semiconductor is deposited on top of a dielectric with an underlying gate (G) electrode. Charge-injecting drain–source (D-S) electrodes providing the contacts are defined either on top of the organic film (top-configuration) or on the surface of the FET substrate prior to the deposition of the semiconductor (bottomconfiguration). The current between S and D electrodes is low when no voltage is applied between G and D electrodes, and the device is in the so called ‘off’ state. When a voltage is applied to the gate, charges can be induced into the semiconducor at the interface with the dielectric layer. As a result, the D–S current increases due to the increased number of charge carriers, and this is called the ‘on’ state of a transistor. The key parameters in characterizing a FET are the field-effect mobility (µ) which quantifies the average charge carrier drift velocity per unit electric
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