Bipolarons mobility

The authors postulated that the conductivity is proportional to the number of carriers in the film and that the mobility of polarons is equal to that of bipolarons, hence the conductivity is independent of carrier type. Thus, the conductivity increases steadily as polarons, and then both polarons and bipolarons, are generated but should attain a steady value when polaron recombination to give half as many bipolarons becomes important. 

Bipolarons are mobile electron pairs. They have been identified in a small temperature interval above the semiconductor-metal transition in Ti407 In this case, they represent mobile electron-pair bonds in a mixed-valence compound, the electrons condensing into metal-metal homopolar bonds that are ordered at low temperatures, but become disordered and mobile at somewhat higher temperatures. 

Mobile defects are Frenkel78 excitons, Mott-Wannier excitons, polarons, bipolarons, polaritons, and solitons. 

Figure 5.3. The reduction (doping) of trans-polyacetylene. In the ideal case, the mobility of the charged states allows the bipolaron to collapse into a pair...

P3HT. But at the same time the mobility was redueed by about one order of magnitude by the purifieation. Reduetion of the hysteresis does not neeessarily mean that impurity indueed traps must be eausal for the hysteresis. The relative stability of polarons and bipolarons depend also on the presenee of impurities and ean be modified in this manner. Also for the eapaeitor with pentaeene, prepared from a soluble preeursor, the hysteresis is rather small. For sueh small moleeules one ean rule out a bipolaron meehanism, and apparently neither traps nor mobile ions lead to hysteresis. 

Several mechanisms for the hysteresis effeets have been proposed in the literature, in most cases as verbal statements without mathematical formulation and numerical evaluation. Typical statements are that trap recharging or mobile ions are likely to cause the hysteresis. Additionally we have proposed that formation and dissociation of bipolarons in the accumulation layer can cause this effeet. 

Finally, it is shown that a process involving mobile ions can also contribute to hysteresis. After fast formation of complexes between a polaron and a counter ion, subsequent capture of a further polaron leads to complexes formed by a bipolaron and a counter ion. They can attract slowly moving ions. The Langevin bimolecular reaction rate for this process again yields time constants on the time scale of the measurements. 

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