Polaron Interactions

At higher doping levels the polaron states interact the authors calculated that a bipolaron level is 0.49 eV more stable than two polaron levels so unfavourable polaron interactions are avoided via the formation of a more stable bipolaron, in agreement with the epr data of Scott and colleagues (1983). 

As we will see in Section 5, it is not a straightforward matter to obtain conjugated polymers with a high content of pendent radicals either chemical introduction of radicals into polymers is inefficient or radical centres cannot be kept intact during polymerization. A possible way out of this problem has been proposed by Fukutome, namely, doping of closed-shell cross-conjugated polymers to generate the open-shell centres. The polaronic interaction in these systems has been discussed (Fukutome et al., 1987). 

We now set up a Hamiltonian for the polaron interacting with the screening charge due to the water [64] ... 

Electronic Interactions.—As mentioned above, elastic strain is not the only contender for long-range interactions in these materials, and electronic interactions are also likely candidates. The problem here is that it is difficult to know precisely the valence states of the atoms in these structures. In addition, the electrical properties of the materials are not always well known and conductivity data for many of these phases has not yet been obtained. In the literature only electrostatic interactions and polaron interactions have been considered. 

Because of the small size of polarons in molecular crystals, the conduction channel in organic transistors extends in the transverse direction for only a few molecular layers [20-22]. For the same reason, polarons interact strongly with chemical impurities and structural defects. As a result, polaronic transport in organic OFETs is very sensitive to the morphology of the semiconductor surface and to the presence... 

Increasing the purity of organic crystals and reducing the contact resistance in OFETs is another challenging direction of future experimental work, which will help to extend the temperature range where the intrinsic polaronic transport can be studied. The development of more advanced techniques for purification of molecular materials will enable the expansion of the intrinsic transport regime to much lower temperatures, where the effects of quantum statistics and polaron-polaron interactions should become experimentally accessible. 

In many cases, it has been found that the conductivity of the system is spinless, which suggests that charge carriers other than polarons would be appropriate in these cases [32, 33]. Therefore, it was proposed that polaron interaction would produce a new charge carrier with no spin and 2e charge corresponding to a positive bipolaron (Figure 1.5). 

While we have been calling the broadening effect as phonon broadening, it must be pointed out that the PAM likewise predicts a broadening of PES features with temperature, but not associated with phonons. Experimentally there is no way to distinguish between phonons and the magnetic polaron interactions inherent in the PAM. Suffice it to say that the temperature effects observed in PES measurements are not inconsistent with the PAM predictions. 

Meyer et al. [75] studied the TES spectral range of different bulk ZnO samples in detail to obtain the binding energies of various donors. They have observed the splitting of TES lines into 2s and 2p states as a result of the effects of anisotropy and the polar interaction with optical phonons in polar hexagonal semiconductors. The effects of anisotropy and the polaron interactions were combined by employing the second-order perturbation theory and the results of numerical calculations of the... 

Consequently the absorption at 3.6 eV determined in this study can be ascribed to a bandgap transition for the conducting form of polypyrrole. Bredas and Street(54) suggested that absorptions at 0.7, 1.4 and 2.1 eV present for polypyrrole doped at low levels, were characteristic of polaron species. Furthermore, as the doping levels increased, features at 1.0 and 2.7 eV developed which were taken to be indicative of bipolaron formation. Batz et al,(35) used HREELS to study doped polypyrrole films, and they noted that a low doping levels polarons caused loss features to be established at 2.0 and 2.5 eV. This structure was attributed to the fact that isolated polarons interacted with each other to produce a band with an appreciable width. 

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