Intrachain electronic processes

The studies of the conducting polymers in solution offer a good opportunity to inspeet the eharge transport limited within a single isolated polymer chain. Exclusively the intrachain electronic processes can be studied when the conducting polymers are dispersed dilutely in the solution. The unique technique of FEES has verified the presence of two levels of intrachain transport processes, in which defects or disorder plays a specific role. 

However, one should bear in mind that interaction between the polymer chains and the solution species (counterions, solvent molecules) may influence the electron transport properties of the polymer. In addition, even when assuming intrachain electron transport, this may also influence intrachain electron hopping. In overall charge transport processes during charging/discharging of the polymer film, counterions also participate. Thus these steps may also become rate-determining. 

If the conducting polymer is dilutely dissolved in a solvent and an average separation among the polymer chains is larger than their hydrodynamic diameters [13], the interchain mode is completely inhibited, but the intrachain one still contributes to the electric transport (Fig. 17b). As a result, we can exclusively obtain information on the single-chain phenomena, specifically on the intrachain transport process of the electronic carriers. It is to be noted that the electric polarizability should be measured instead of the conductivity to detect the intrachain mode based on carrier transport restricted within a contour length of a polymer chain. 

At higher polymer concentration, an interchain interaction yields aggregation [42] or the lamellar structure similar to the comblike one in the solid state [182]. If the electronic processes are studied in the semidilute solution, the interchain carrier transport in the solution is expected to be detected and compared with that in the solid state (refer to section 2.3.2). Moreover, the intrachain carrier transport in heavily doped polymers in the dilute solution is also of great interest. The change in the electronic state or in the Coulorabic repulsion between carriers may well affect the intrachain carrier transport as well as the conformation of the polymer chain [28]. 

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. 

A representative example for the information extracted from a TRMC experiment is the work of Prins et al. [141] on the electron and hole dynamics on isolated chains of solution-processable poly(thienylenevinylene) (PTV) derivatives in dilute solution. The mobility of both electrons and holes as well as the kinetics of their bimolecular recombination have been monitored by a 34-GHz microwave field. It was found that at room temperature both electrons and holes have high intrachain mobilities of fi = 0.23 0.04 cm A s and = 0.38 0.02 cm / V s V The electrons become trapped at defects or impurities within 4 ps while no trapping was observed for holes. The essential results are (1) that the trap-free mobilities of electrons and holes are comparable and (2) that the intra-chain hole mobility in PTV is about three orders of magnitude larger than the macroscopic hole mobility measured in PTV devices [142]. This proves that the mobilities inferred from ToF and FET experiments are limited by inter-chain hopping, in addition to possible trapping events. It also confirms the notion that there is no reason why electron and hole mobilities should be principally different. The fact... 

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