Conducting polymers polaron-bipolaron band model

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

Polaron — Polarons are charged quasiparticles with spin lf. This term has been introduced by physicists as one of the possible solutions to the equations of the relevant defect model of solids in order to describe an electron in a dielectric polarizing its environment (electron-phonon coupling), electrically situated below the conduction band, and transported together with its polarized environment. Polarons and -> bipolarons are the charge carriers in oxidized or reduced (doped) -> conducting polymers. A polaron is defined as a neutral and a charged -> soliton in the same... 

The concept of electric conduction in 7r-conjugated polymers used to be explained in terms of polaron , bipolaron , soliton , and band model [14e]. The iodine-doping of crystalline PTh prepared by the... 

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. 

Simply put, a conductive organic polymer, or for that matter any organic polymer, is not a metal, although many of the theories used to explain the electrical nature of ICPs are based on our understanding of conduction mechanisms in metals and semiconductors. Band theory, polarons, bipolarons, thermopower, etc., are but a few examples of the models borrowed from solid-state chemistry and physics to explain the observed electronic behavior of these materials. Lattice distortions that... 

A statistical mechanics model of the interface formation between metals and conjugated polymers provides additional insight into the charge transfer that occurs between metal atoms and DP-PPV (32,33). This model also helps to understand the results obtained following the deposition of Au. In this model, one considers the positions of the Fermi level of the metal, the valence and conduction bands of the polymer and polaron and bipolaron states relative to the vacuum level. It is found that if the Fermi level of the metal atom is either above or below the energy of the polaron or bipolaron states, then charge transfer... 

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