Organic field-effect transistors (OFETs) were first described in 1987. Their characteristics have undergone spectacular improvements during the last two or three years. At the same time, several models have been developed to rationalize their operating mode. In this review, we examine the performance of OFETs as revealed by recently published data, mainly in terms of field-effect mobility and on±off current ratio. We compare the various compounds that have been used as the active component, and describe the most prominent fabrication techniques. Finally, we analyze the charge transport mechanisms in organic solids, and the resulting models of OFETs.

The organic thin-film transistor (OTFT) is now a mature device that has developed tremendously during the last twenty years. The aim of this paper is to update previous reviews on that matter that have been published in the past. The operating mode of OTFTs is analyzed in view of recent model development. This mainly concerns the distribution of charges in the conducting channel and problems connected with contact resistance. We also delineate what differentiates n- and p-type semiconductors, and show how this concept differs from what it covers in conventional semiconductors. In the chapter devoted to fabrication techniques, emphasis is placed on solution-based techniques and particularly printing processes. Similarly, soluble materials are given a prominent place in the section dedicated to the performance of devices. Finally, special attention is given to devices at the nanoscale level, which demonstrate a new route toward molecular electronics.

With the advent of devices based on single crystals, the performance of organic field‐effect transistors has experienced a significant leap, with mobility now in excess of 10 cm2 V−1 s−1. The purpose of this review is to give an overview of the state‐of‐the‐art of these high‐performance organic transistors. The paper focuses on the problem of parameter extraction, limitations of the performance by the interfaces, which include the dielectric–semiconductor interface, and the injection and retrieval of charge carriers at the source and drain electrodes. High‐performance devices also constitute tools of choice for investigating charge transport phenomena in organic materials. It is shown how the combination of field‐effect measurements with other electrical characterizations helps in elucidating this still unresolved issue.

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In order to analyze the correlation between charge transport and structural properties in conjugated oligomers, sexithiophene, 6T, was substituted by hexyl groups, both on the terminal a positions (a,cvDH6T) and as pendant groups in the ß position (ß,ß' 6 ). Structural characterizations by X-ray diffraction show that vacuum-evaporated thin films of 6T and , consist of layered structures in a monoclinic arrangement, with all-trans planar molecules standing on the substrate. When compared to 6T, , 6 is mainly characterized by a very large increase of molecular organization at the mesoscopic level, evidenced by a much longer range ordering. Electrical characterizations indicate that the conductivity of , 6 is largely anisotropic, with a ratio of 120 in favor of the conductivity parallel to the substrate plane, i.e. along the stacking axis. The charge carrier mobility, determined on field-effect transistors fabricated from these conjugated oligomers, also shows an increase by a factor of 25 when passing from 6T to , 6 , reaching a value of 5 X 10-2 cm* 12 3V-1 s-1. In contrast, ß,ß' 6 presents very low conductivity and mobility, the latter being below detection limit. These results are attributed to the self-assembly properties brought by alkyl groups in the a,w position.

We have performed current–voltage measurement on polycrystalline sexithiophene (6 T) thin film transistors at temperatures ranging from 10 to 300 K. A method is developed to extract the carrier mobility from an analysis of the transfer characteristics. In particular, data are corrected for contact resistance. The carrier mobility is found to increase quasilinearly with gate voltage at room temperature. The dependence becomes superlinear at low temperatures. The temperature dependence shows three domains. For 100 K<T<300 K, the mobility is thermally activated with an activation energy of around 0.1 eV. The activation energy reduces to 5 meV for 25 K<T<100 K. Finally, the mobility is practically temperature independent for temperatures lower than 25 K. The data are explained by a model where charge transport is limited by a high concentration of traps at grain boundaries. At high temperatures, charge transfer at boundaries occurs via thermionic emission, while tunnel effect takes place at low temperatures. The energy distribution of traps is determined, and various features predicted by the model are outlined.

Organic field-effect transistors (OFETs) promise for printed intelligence embedded into, and coated onto many different items that previously have been considered impossible to make electronically active. It is crucial that this technology is driven at low voltage and power. Also, we need to obtain solid understanding of the charge transport in organic semiconductors. Different materials [1] and architectures have been utilized as the probe to reveal the nature of charge transport along the transistor channel and to achieve low-voltage switching. In transistors operating according to field-effect or electrochemical principles, respectively, vacuum, air, [2] oxides, [3] high-permittivity dielectrics, [4] organic mono-layers, [5] and electrolytes [6] have successfully served as the medium to electronically separate the gate from the transistor channel, of which the latter three allow low-voltage operation. In particular electrolytes have attracted much attention lately since they generate very high electric fields at the organic transistor channel/electrolyte interface already at very low voltages, i.e., below 1 V. One issue with those devices is that electrochemical switching and field-effect modulation of the organic channel often coexist, [7] which result in transistors that are typically slow and that exhibit a great degree of hysteresis. Here, we report OFETs gated via pure water that operates entirely in the field-effect mode of operation. Our findings shed new light on low-voltage operating OFETs, their charge transport characteristics under exposure to water [8] and opens for sensor applications using water-gated OFETs as transducers in aqueous media.[9] Because of the simplicity and readiness of its production, it could also reveal a very helpful tool for rapid testing of new organic semiconductor compounds.Electrolyte (insulator)/semiconductor interfaces have attracted much attention during the last decades, in part driven by an interest to achieve high-performing sensors operating in water, to reach low-voltage operation for OFETs and to study the fundamentals of charge transport in semiconducting solids. In ion-sensitive field-effect transistors (ISFETs), the electrical potential at the electrolyte/insulator interface is translated into a modulation of the transistor output characteristics.[10] Actually, the modulation originates from that the threshold voltage (V T ) is sensitive to the ion concentration. The high electric field that is possible to establish at electrolyte/solid interfaces becomes a powerful tool for probing various features of the transport and accumulation of charges inside solids. Electrostatic field-operated transistors and switches, including for instance silicon, carbon nanotubes, [11] rubrene, [12] or manganites [13] as the active material, have been extensively studied in the past. In all those cases pure field-effect operation, without any parasitic electrochemical reactions of the bulk of the solid, is achieved simply because the materials included in the devices are known ...

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