Spinless transport

Bipolaron transport offers a reasonable explanation for spinless transport in PA and PPP. The model is not specific to PA, in that it does not require a degenerate ground state. Our model for charge transport via bipolarons in PPP is applicable to the broad range of conducting polymer systems. It should be emphasized, however, that we have made no explicit consideration of the effect of the dopant ion array on the transport process. 

Application of this model obviously assumes the reliability of the observation of spinless transport, for which the evidence is strong(49,80) but not universally accepted. In particular, Epstein et al.(81) suggest that in the I2 doping of PA the magnetic and electrical data can be adequately explained by a model based on variable range hopping. Their model requires a finite density... 

The Porath et al. experiment [14], reviewed in Sect. 2, reports nonlinear transport measurements on 10.4-nm-long poly(dG)-poly(dC) DNA, corresponding to 30 consecutive GC base pairs, suspended between platinum leads (GC-device). DFT calculations indicated that the poly(dG)-poly(dC) DNA molecule has typical electronic features of a periodic chain [58]. Thus, in both models (assuming dephasing or r-stack hybridization) the poly(dG)-poly(dC) DNA molecule is grained into a spinless linear TB chain. A generalization of the dephasing model to spin-transport has been proposed by Zwolak et al. [123]. 

To account for this phenomenon of spinless conductivity, physicists have introduced the concept of transport via structural defects in the polymer chain. In a conventional semiconductor, an electron can be removed from the valence band and placed in the conduction band, and the structure can be assumed to remain rigid. In contrast, an electronic excitation in polymeric materials is accompanied by a distortion or relaxation of the lattice around the excitation, which minimizes the local lattice strain energy. The combined... 

On going to polypyrrole we find that for heavily doped polypyrrole, the resistance is proportional to the logarithm of temperature in the temperature range 1 K to 1 mK. This behavior is appropriate for an amorphous metal [57]. This is supported by the temperature independence of paramagnetic susceptibility that is sometimes seen for heavily doped polypynole. Such a result is also inconsistent with spinless excitations such as bipolarons. Thus one can see various types of temperature behavior for charge transport in electroactive polymers in the dry state. 

The mechanisms by which these polymers conduct electricity have been a source of controversy ever since conducting polymers were hrst discovered. At first, doping was assumed to remove electrons from the top of the valence band, a form of oxidation, or to add electrons to the bottom of the conduction band, a form of reduction. This model associates charge carriers with free spins, unpaired electrons. This results in theoretical calculations of conduction that are much too small (59). To account for spinless conductivity, the concept of transport via structural defects in the polymer chain was introduced. From a chemical viewpoint, defects of this nature include a radical cation for oxidation effects, or radical anion for the case of reduction. This is referred to as a polaron. Further oxidation or reduction results in the formation of a bipo-laron. This can take place by the reaction of two polarons on the same chain to produce the bipolaron, a reaction calculated to be exothermic see Figure 14.17 (55). In the bulk doped polymer, both intrachain and intrachain electronic transport are important. 

One attractive aspect of the soliton theory of charge transport is that the carriers (cations or anions) carry no spin, i,e, the conducting compositions do not contain unpaired electrons, ESR experiments on the doping of PA(49) show that in certain intermediate doping regimes the spin concentration is much lower than expected from the observed conductivity values this phenomenon is referred to as spinless conductivity. If the conduction involved a normal process of defect-induced hole or electron transport, there would be a direct correlation between ESR determined spin concentration and conductivity. The same conclusion of spinless conduction is obtained from ESR experiments on doped PPP(61) however the soliton theory is not applicable to the PPP system(25). In Section VII, we present an alternate transport mechanism based on bipolarons (dications or dianions) which is applicable to all conducting polymer systems(26),... 

We have modeled this process and computed conductivity versus C curves which are very similar to those obtained experimentally (Figure 3). For the purposes of this discussion we will only consider qualitative aspects of the results of this model. At low C, there will be few soliton pairs of sufficiently small x for transport to take place due to the F(x) term i.e., F(x) at the peak in the Qi(x) distribution will be small. As C increases, a++ will increase superlinearly since the peak in Qi(x) will shift to lower x values where F(x) is larger. At still larger C values, a++ will level off and eventually decrease due to the small value of the Q2(x) term, as there will now be few sites available for accepting the bipolaron. Thus, the net result from this model, qualitatively, is an S-shaped curve for the spinless bipolaron contribution to the conductivity. The detailed calculations we have carried out with this model(26) bear out this qualitative discussion and yield results quite similar to the experimental data shown in Figure 3. 

Spectroscopic studies of the equilibrium between oligomeric resonance-delocalized radical cations and spinless dicationic k dimers do not provide direct evidence for the structure of the latter species or for the role of radical cation % dimers in charge transport in solid materials. Evidence for the potential role of %... 

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