Materials and Physics

Alan MacDiarmid, Alan Heeger, and Hideki Shirakawa won the 2000 Nobel Prize in Chemistry for the discovery and development of conductive polymers. Most polymers are insulators because their electrons are localized, that is, each of the electrons is attached to its own nucleus. However, the conducting or semiconducting polymers have conjugated double bonds. The double bond consists of a a bond and a k bond. Due to the conjugated structure, the electrons have gained the freedom to move along the entire chain. In metals and conventional semiconductors, charge-transport occurs in delocalized states. Such a model is no longer valid in low conductivity organic semiconductors, where a simple estimate shows that the mean-free path of carriers would become lower than the mean atomic distance. In these materials, carrier-transport occurs through hopping of charges between localized states. The modeling of organic field effect transistors (OFETs) is currently hampered for several reasons. First, the charge-transport in organic semiconductors is still not well understood. Second, there are great differences between the behaviors of OFETs made from different compounds. Furthermore, the fabrication method seems to strongly affect the device performance. Nevertheless, some attempts [354-356] have been made, such as the model based on a simple trip distribution applied on the dihexylquaterthiophene (DH4T).

The charge mobility increases with temperature in most organic materials. A useful model to describe the chargetransport in organic materials is that of the small polaron, developed by Holstein [357]. Localization in conjugated organic materials occurs via the formation of polarons resulting from the deformation of the conjugated chain under the action of the charges. This mechanism of self-trapping is often described through the creation of localized states in the gap between the valence and the conduction bands.

In a multiple trapping and release (MTR) model, a narrow delocalized band is associated with a high concentration of localized levels that act as traps. During their transition through the delocalized levels, the charge carriers interact with the localized levels via trapping and thermal release. The prominent feature of OFETs is that the field-effect mobility is gate bias dependent. A general feature of chargetransport in organic materials is that the mobility becomes field dependent at high electric field (namely, at fields in excess of ~105 V/cm).

Figure 14 shows the structures of the several conducting or semiconducting polymers. The conductivity of the

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