Negative bipolaron

Fig. 4. Energy level diagrams showing possible electronic configurations for positively-charged polaron (a) and bipolaron (b) defects and (c) a schematic bipolaron band model. The negatively-charged polaron would carry three electrons and the bipolaron four. Also shown is the neutral polaron-exciton (d) which would decay to restore the chain structure.

However, while the evidence for the existence of polarons was extremely convincing, that for bipolarons was rather more problematical in that it was largely effectively negative in nature the absence of an absorption peak in the optical spectrum, the absence of a signal in epr studies on the decline of the observed signal. In essence, bipolarons had not been actually observed. This fact was remedied by the work of Christensen and Hamnett (1991) who employed ellipsometry and FTIR to study the growth and electrochemical cycling of polypyrrole in situ in aqueous solution. 

The wavefunctions from the Density Functional Theory calculations on calcium on DP7 indicate that calcium-doped DP7 accommodates a bipolaron-like defect, with two excess electrons per DP7. In the modelling of an isolated molecule with two negative charges, two new occupied states appear above the HOMO of the pristine molecule42. When the calcium counter-ion is included, however, the electrostatic interaction of the Ca2+-ion with DP72- leads to the stabilization of the two new states, so that the lower state (the HOMO-1 for Ca on DP7) lies at the same energy as the HOMO of... 

Fig. 1.12. Structure, spin configuration and optical transitions for positive and negative bipolarons...

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. 

Because of their special polymerization process [136], PDA crystals should not contain any dopant or associated residual polaron or bipolaron concentration. These negative results give some constraints on other low-lying states, especially exciton states. Triplet-state absorption from the ground state would probably be too strongly spin-forbidden to have absorption coefficients as high as 1 cm-1 [129]. As for g states, the absorption coefficient depends sensitively on how far from the intense absorption they are (for the polyene case, see Ref. 127). That they are not found in these experiments means that they are either above the main transition at 2 eV or not far below it and buried in its tail. Indeed, evidence of a weak absorption = 0.1 eV below the main transition has recently been found at low temperature [118] it would be buried in the absorption tail at higher temperatures. 

Figure 5-9. Band diagram of negatively charged bipolarons (BP ) in PPV.

---