Positive bipolaron

If two polarons of like sign are formed close to each other, a bipolaron is formed. Two energy levels are created by a bipolaron in the bandgap. They are both occupied either by two electrons (for a negative bipolaron) or by two holes, i.e. they are empty (in the case of a positive bipolaron). The bipolaron has no spin. The bipolaron may not be stable because of the repulsion of the two polarons which constitute the bipolaron. However the dopant ions in the neighborhood stabilize the bipolaron. 

Figure 5. Ener level shematics at the metal/polymer interface. The formation of bipolaron lattice at the interface depends on the relative positions of the metal Fermi level and the bipolarons. At the interface, Ca will tend to form negative bipolarons (a), A1 will not form bipolarons (b) and Au will tend to form positive bipolarons (c). Although bipolarons extend over several monomeric units, they span only 4 units in this diagram (33). We iUustrated the bipolarons with PPV chains.

Recently, the results of photoinduced absorption studies (i.e. excitation spectroscopy) of the emeraldine base polymer have been reported.22-24 Photoinduced absorptions were observed23 24 t 0.9eV, 1.4eV and 3.0 eV, accompanied by photo-induced bleaching at 1.8eV and at energies above 3.5 eV. The 1.4 and 3.0 eV photoinduced absorptions have been interpreted, by analogy with the absorption spectrum of the emeraldine salt, to the photoproduction of polarons in emeraldine base.24 Stafstrom et al25 concluded that positive and negative polarons,as well as positive bipolarons account very well for the 1.4 and 3.0 eV photoinduced absorptions. No specific explanation has been proposed for the 0.9 eV photoinduced peak. 

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). 

FIGURE 21.15 Schematic drawing of the energy level alignment at interfaces of PFO and different metallic substrates. In all cases, the Fermi level of the substrate ( p) is situated between the negative and positive bipolaron levels. 

Figure 8.13 Energy diagram showing the new levels appearing in the gap for a positive polaron (P) and a positive bipolaron (BP) regarding a neutral form. Reprinted with permission from J. Casado, M. Z. Zgierski, R. C. Hicks, D. J. T. Myles, P. M. Viruela, E. Orti, M. C. Ruiz Delgado, V. Hernandez and J. T. Lopez Navarrete, Mesitylthio-oligothiophenes in various redox states. Molecular and electronic views as offered by spectroscopy and theory, J. Phys. Chem. A, 109, 11275-11284 (2005), Copyright 2005 American Chemical Society...

Figure 4-3. Schematic structures of self-localized excitations in poly( p-phenylene) and trans-po y-acetylene. (a) Positive polaron (b) negative polaron (c) positive bipolaron (d) negative bipolaron (e) positive polaron (f) negative polaron (g) neutral soliton (h) positive soliton (i) negative soli-ton . D, donor A, acceptor -I-, positive charge negative charge , unpaired electron.

Figure 4-5. Schematic electronic band structures, (a) Neutral polymer (b) positive polaron ic) negative polaron (d) positive bipolaron (e) negative bipolaron (f) neutral soliton (g) positive soliton (h negative soliton. CB, conduction band VB, valence band , electron arrow, electronic transition.

The electronic structure of a positive bipolaron is shown in Figure 4-5d. Since the geometric changes for a bipolaron are larger than those for a polaron, localized electronic levels appearing in the band gap for a bipolaron are farther away from the band edges than for a polaron. Two electrons are removed from the bipolaron... 

Fig. 5. Schematic structures of self-localized excitations in polythiophene (a) a positive polaron, (b) a positive bipolaron, and (c) two positive polarons.

When a positive bipolaron is formed, two electrons are removed from the - >o level (Fig. 18d). Thus, a positive bipolaron is expected to have the following intragap transitions ... 

Figure 11.6 The conjugated polymer poly(p-phenylene) A can be oxidized (p-doped) to a radical cation (positive polaron) B or a dication (positive bipolaron) C. The charged excitations are localized along the polymer chain, but may be forced to move when driven by an electric field. If the polymer is reduced (n-doped) instead of oxidized, negatively charged states are generated, viz., radical anions (negative polarons) and dianions (negative bipolarons).

Schematic drawing illustrating these aspects in case of NbsGe is presented in Fig. 27.6. The Fu phonon mode covers out-of phase stretching vibration of two perpendicular Nb chains in two planes - see Fig. 27. Id. For simplicity, drawing of only a single chain of Nb atoms in a plane (e.g. b-c plane) is sketched in Fig. 27.6. For equilibrium high-symmetry structure (Req) on the crude-adiabatic level, the highest electron density is localized at equilibrium position of Nb atoms in a chain - Fig. 27.6a. For distorted nuclear geometry (Rd,cr) in the Fn mode, electron density is polarized and the highest value is shifted into the inter-site positions-bipolarons are formed. The Fig. 27.6b corresponds to compression period in stretching vibration of Nbl-Nb2 which induces increase of Nbl-Nb2 inter-site electron density and decreases of Nb2-Nb3 electron density. For an expansion period. Fig. 27.6c, situation is opposite. Inter-site electron density is decreased for Nbl-Nb2 and increased for Nb2-Nb3. On the lattice scale, increase and decrease of electron density is periodic. On the adiabatic level, alternation of electron density is bound to vibrations at equilibrium nuclear positions (Fig. 27.6a-c).

Draw expected positive bipolaron and positive polaron structures for poly(aniline). 

Bipolarons are formed by the combination of two polarons with the same charge. The bipolaron also has two levels in the energy gap. In the case of a negative bipolaron both levels are fully occupied and for a positive bipolaron both levels are empty. In either case the spin is zero. Because of their charge, bipolarons are assumed to be in close proximity to their coimterions. 

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