Dielectric-semiconductor interface importance

As pointed out before, interfaee stmeture in OFETs is important for both charge injection (metal-semiconductor interface) and charge transport (semi-conductor-dielectric interface). The discussion of the previous section demonstrates that even for the small number of model systems considered here a large variety of sfructures is observed. Because many factors influence the interface and film sfructures, it is difficult to establish general mles. Flowever, a few important observations regarding the stmcture-forming factors can be kept hold of ... 

One frequently examined interface is the solid-liquid interface, where the solid phase may be a dielectric, a semiconductor, or a metal. Species located at these interfaces are of primary importance in electtochemisny and in chemistry of surface-active substances (surfactants). Another common type of interface is the solid-solid interface, specifically dielectric-dielectric, dielectric-semiconductor, dielectric-metal, semiconductor-semiconductor, semiconductor-metal, and metal-metal interfaces. These structures have an extremely important role in such areas as microelectronics and the chemistry of composites. Furthermore, positioning an ultrathin film at the interface of two media, one can substantially increase surface sensitivity of all IR spectroscopic methods. 

The conducting channel where charges are driven from source to drain consists of a very thin layer close to the insulator-semiconductor interface. Accordingly, charge transport there is highly dependent on the quality of this interface. In that respect, the choice of the dielectric is at least as important as that of the semiconductor. More details on the issues linked to the dielectric in organic field-effect transistors can be found in recent reviews [25-27]. 

Several polymer conductors are commercially available, and have been used in the demonstration of printed transistors. These include PEDOT PSS, which is a commercially available polymer conductor, as well as various versions of polyaniline. The latter is typically doped with an acid or salt to increase conductivity. Both of these material systems are water soluble and easily printable. They also typically form good interfaces to organic semiconductors, making them attractive for use in printed transistors. As with polymer dielectrics, however, it is important to note that their usability with inorganic semiconductors is questionable, of course. 

As soon as Ep - Ejjomo becomes smaller than k T, the OFET s conductance increases by several orders of magnitude due to the thermal excitation of the carriers from the localized states into the HOMO band. As a result, a conduction channel is formed at the interface between the semiconductor and the gate dielectric. Overall device operation depends, to a large extent, on the energetics of the semiconductor bands and metal contacts, and therefore studies of the electronic structure of molecular interfaces are important [58],... 

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