Semiconductor/semiconductivity with polymers

In comparison with traditional inorganic semiconductors, semiconducting polymers cannot be considered materials with ultra-high purity. As a result, although many device concepts have been demonstrated using semiconducting polymers as the active materials, there has been considerable skepticism that these novel semiconductors could be used in commercial applications. 

As already outlined, partial oxidation or reduction at suitable potentials converts the semiconducting neutral polymer to much more conductive catioific or anionic forms, respectively (see Fig. 2.4). As already outlined, according to a physically improper analogy with inorganic semiconductors, oxidation is said to lead to the p-doped polymer, while reduction to the n-doped form. 

The discovery of semiconducting, conjugated polymers and the ability to dope these polymers over the full range from insulator to metal has resulted in the creation of a class of new materials that combines the electronic and optical properties of semiconductors and metals with the attractive mechanical properties and processing advantaged of polymers [15—17]. Moreover,... 

There is a class of polymers that are inherently semiconductive, namely, conjugated polymers. This was initially discovered by Heeger, McDiarmid, and Shirakawa in seminal studies for which they received the Nobel Prize in Chemistry in 2000. 27-129 With various doping procedures, these polymers can be transformed from semiconductors into conductive polymers. Some examples of conjugated polymers are shown in Scheme 1. 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

The most unusual and interesting feature of these polymers is their capacity to switch between insulating and conducting (or semiconducting) states. All other materials, with the only additional exception of some intercalation compounds, are normally found only as conductors or semiconductors or insulators, without the facility to switch between these states. 

So far we have outlined the conceptual framework in which we discuss charge transfer in organic semiconductors. It is based on a molecular picture where the molecular unit is considered central, with interactions between molecular units added afterwards. For amorphous molecular solids and for molecular crystals this approach is undisputed. In the case of semiconducting polymers, a conceptually different view has been proposed that starts from a one-dimensional (ID) semiconductor band picture, and that is generally known as the Su-Schrieffer-Heeger (SSH) model [21-24]. 

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