1.5 Thin Film Organic Field-Effect Transistors (OTFT)

A field-effect transistor is a three-terminal device configured like a parallel plate capacitor, where one conduction electrode, the gate electrode, is electrically insulated from the organic semiconductor layer (see Fig 1.4) [28,29]. Two electrodes, the source and the drain, are connected to the organic semiconductor layer. By controlling the voltage on the gate, a charge can be induced. These charges are injected from the source electrode and cross the conducting channel towards the drain by applying voltage between the two electrodes. Silicon has been the most widely used semiconductor material in field-effect transistors, because these devices exhibit fast switching speeds and are therefore suitable for use in modern processors.

Figure 1.4: Left: A schematic view of a bottom contact OFET. The source electrode is grounded, while the drain and the gate are biased negatively. In this mode, holes are injected from the source and collected at the drain. Right: a top contact OFET with the electrodes patterned on top of the organic semiconductor.

However, there are many applications for field-effect devices where fast switching speed is not a requirement, such as, for example, large-area coverage, mechanically flexible and low cost integrated circuits. With the successful synthesis of the first organic transistors in 1986 [30], the prospect of replacing costly and labor-intensive inorganic devices with cheaper and more flexible organic electronic materials entered a new era.

Despite considerable improvement in the fabrication and characterization of thin-film organic field-effect transistors, the physics of charge injection and transport in these devices is not well understood.