Polaron-bipolaron transition

In all of these, certain features, common to many but by no means all CPs, are of note. Firstly, all show a strong absorption at zero or low doping levels, usually in the near-UV, due to the valence -> conduction band ( tt ) transition, separated from the polaron/bipolaron transitions by a characteristic isosbestic point. With increasing doping, this transition diminishes, with die intermediate polaron transition not always visible. At high doping levels, the bipolaron bands blend into one broad band stretching from the far-Visible (ca. 0.6 p) to the near-IR (up to 2.5 p). 

The polaron and bipolaron transitions are more or less reproduced by VEll calculations [172]. In summary, 4T is the shortest chain length which can stabilize one charge, 6T the shortest chain to bear two charges. This is not true for substituted oligothiophenes in which oligomers with n = 4 and 5 could also be doubly oxidized [33,207,210]. 

M. R. Fernandes, J. R. Garcia, M. S. Schultz, and F. C. Nart. Polaron and bipolaron transitions in doped poly(p-phenylene vinylene) films. Thin Solid Films, 474(1-2) 279-284, March 2005. 

Fernandes MR, Garcia JR, Schultz MS, Nart FC. Polaron and bipolaron transitions in doped... 

Next, we discuss the electronic transitions due to polarons, bipolarons, and soli-tons in a polymer chain on the basis of a theoretical study reported by Fesser et al. [48] on a continuum electron-phonon-coupled model. The electronic energy levels of a neutral infinite polymer and those of polarons, bipolarons, and solitons are shown schematically in Figure 4-5. 

At present it would appear that the polaron-bipolaron model is fully accepted by the scientific community. The main conductivity features of PPPs can easily be explained on the basis of this band energy scheme, and several experiments based on spectroscopic determinations have confirmed the theory. In particular, electron energy loss spectroscopy (EELS) of PPPs doped by AsFs [219] clearly showed the presence of two peaks near 1 eV and 2 eV, corresponding to transitions from the valence band to two states in the gap [219b], in fair agreement with the predictions of Bredas et al. [224]. 

Fig. 21.7 (a) Band diagram scheme for n-type doped PPP according to the polaronic-bipolaronic model, (b) Transitions in the Mott-Davis model of an amorphous semiconductor. 

The polaron and bipolaron transitions shown in Fig. 22.3 can also be used to calculate the important parameters U, Ueff and J , defined earlier in Fig. 22.2, using Eqs. (l)-(4). This is possible because the polaron s SOMO level is singly occupied and is therefore equivalent to 5 in Fig. 22.2, whereas BP levels are either unoccupied (BP ) or doubly occupied (BP ) and therefore... 

Table 4. VEH-calculated polaron and bipolaron transition energies and related oscil-...

Write out definitions of the terms polaron, bipolaron, soliton, antisoliton. Identify commonalities and differences, and spin-charge relationships. Discuss all possible optical transitions possible for each. 

The simplest spectroelectrochemical measurement which yields information on electrochromic properties of CPs is the UV-Vis-NIR spectroelectrochemical curve, an in-situ or sometimes ex-situ measurement of the transmission-mode UV-Vis-NIR spectrum of the CP at various applied potentials. Such a measurement, which we shall hereinafter abbreviate as a SPEL curve (or just SPEL), is depicted in Fig. 3-1 this figure is a re-representation of the optical spectra of poly(pyrrole) (P(Py)) discussed in Chapter 2, with an abscissa in terms of wavelength, and represents a particularly well-behaved CP system. To recap again here, the single, prominent valence conduction (tt tt ) band transition in the pristine polymer (at ca. 388 nm) is accompanied by three additional polaron based transitions at low doping level (ca. 590 nm, 885 nm, 1,771 nm), which finally evolve into two bipolaron based bands (ca. 459 nm, 1,240 nm). 

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