Conduction interchain

The highly conductive class of soHds based on TTF—TCNQ have less than complete charge transfer (- 0.6 electrons/unit for TTF—TCNQ) and display metallic behavior above a certain temperature. However, these soHds undergo a metal-to-insulator transition and behave as organic semiconductors at lower temperatures. The change from a metallic to semiconducting state in these chain-like one-dimensional (ID) systems is a result of a Peieds instabihty. Although for tme one-dimensional systems this transition should take place at 0 Kelvin, interchain interactions lead to effective non-ID behavior and inhibit the onset of the transition (6). 

The temperature of the metal-to-insulator transition in TTF—TCNQ is 53 K. For systems with increased interchain coupling, the transition temperature for the onset of metallic conduction increases roughly as the square of the interaction between the chains. This behavior is tme as long as the coupling between chains remains relatively weak. For compounds with strong interactions between stacks, the material loses its quasi-ID behavior. Thus, the Peieds distortion does not occur even at low temperatures, and the materials remain conductive. 

Later we will describe both oxidation and reduction processes that are in agreement with the electrochemically stimulated conformational relaxation (ESCR) model presented at the end of the chapter. In a neutral state, most of the conducting polymers are an amorphous cross-linked network (Fig. 3). The linear chains between cross-linking points have strong van der Waals intrachain and interchain interactions, giving a compact solid [Fig. 14(a)]. By oxidation of the neutral chains, electrons are extracted from the chains. At the polymer/solution interface, positive radical cations (polarons) accumulate along the polymeric chains. The same density of counter-ions accumulates on the solution side. 

The critical role of the latter process was clearly shown in the extremely elegant work of Wegner and Riihe (1989) who measured the temperature dependence of the DC conductivity of a range of polythiophene and polypyrrole derivatives as a function of the interchain separation. The derivatives... 

The alkyl substitution forced adjacent pyrrole units on the same chain out-of-plane by at least 40°. A consequence of this was that the authors could only evaluate their data at higher temperatures (including room temperature), in terms of the assumption that interchain electronic hopping was the dominant factor in determining the macroscopically measureable electronic conductivity, rather than in/rachain hopping. Under these conditions, the... 

The results of Wegner and Riihe (1989) clearly show that electronic conduction in conducting polymers such as polythiophene and polypyrrole occurs via a hopping mechanism that is dominated by interchain rather than intrachain hopping. 

The temperature dependence of conductivity chain axis [14]. Although oj /charge transport, is nearly identical in both cases (oj and interchain transport plays the limiting role in bulk charge transport properties. 

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