Charge carrier properties below and above the metal–insulator transition in conjugated polymers – recent results

Abstract

The strong doping dependence of the dc conductivity in conjugated polymers below the metal-insulator transition (MIT), illustrated for PPV, is argued to be due to an increase in the density of states and the growth in size of the regions, in which the charge is delocalized. Obviously in the metallic states these regions have to overlap. Phase sensitive dielectric spectroscopy data show that when this overlap is achieved less than 1% of the donated charges (at least in polypyrrole) participates in metallic transport. The data up to the MIT allow a tentative picture of the density of states. 1 Introduction Chemical doping of conjugated polymers, like polyacetylene, polypyrrole (PPy) and poly(p-phenylene vinylene) (PPV) makes them conducting and even let them pass through a metalinsulator transition (MIT) to become metallic. Often the conduction processes below the metal-insulator transition are described by (variable range) hopping and above the MIT as Drude-like. Here we show that if one has to explain the strong doping-concentration dependence of the dc conductance below the MIT, the variable-range hopping-mechanism between point-like sites has to be modified to allow for the growing size of the localized regions and indicate how such a modification can be justified theoretically [1]. For a characterization of the metallic state close to the MIT we use dielectric data on PPy from 10