Conducting Polymers

The discovery of conducting polymers by MacDiarmid, Shirakawa, and Heeger in 1977, and the ability to dope these polymers to cover the full range from insulator to metal [8, 9] , has created a new field of research at the crossroads of polymer chemistry and condensed matter physics. These newly discovered conjugated polymers with conducting and semiconducting properties provide exciting opportunities for application in opto-electronic devices. The origin of the appealing properties of n-conjugated polymers (Fig. 8.2) that enable their application in devices is related to the extended nature of the electronic wave functions that is created by the alternating single and double bonds of their molecular structure. This provides the basis for charge transport and gives rise to a range of linear and nonlinear optical properties.

Semiconducting polymers offer the promise of achieving a new generation of materials, exhibiting the electrical and optical properties of metals or semiconductors and retaining the attractive mechanical properties and processing advantages of polymers. Many of the envisioned applications of conjugated polymers require transport of charges and therefore high charge carrier mobility. Hole mobilities in highly ordered regioregular polythiophenes [10, 11] and electron mobilities in ladder type polymers [12] as high as 0.1 cm2/Vs have been obtained. In general, the family of n-type conjugated polymers is much less developed than that of p-type


Fig. 8.2 Structures of archetype n-conjugated polymers polyacetylene (PA), poly-p-phenylene (PPP), poly(p-phenylene vinylene) (PPV), polythiophene (PT), and polyfluorene (PF)

conjugated polymers. In fact, the electron mobility of most conjugated polymers is still low compared to the hole mobility in p-type conjugated polymers and the electron mobility in inorganic materials. Increasing attention is therefore given to hybrid conjugated polymer - inorganic materials, that represent a synergistic approach to overcome the limitations of semiconducting polymer devices without losing their beneficial processability properties. New insights show that these materials might strengthen and enhance their intrinsic properties and will eventually lead to a yet newer class of exciting materials with a wide span of applications.

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