Bipolarons or polaron pairs in conducting polymers: Equilibrium and kinetics

Abstract

Equilibrium and kinetics are compared for conjugated polymers, supposing the charged states are either polarons (Ps) and bipolarons (BPs) or polarons and polaron pairs (PPs). For low concentrations equilibrium and kinetics are virtually indistinguishable. Both are essentially different for high concentrations due to the fact that the extension of the bipolaron is almost the same as that of one polaron whereas the polaron pair has the extension of two polarons. As a result, the polaron concentration, i.e. the spin concentration, shows for a system with bipolarons a maximum and decreases for continuing increase of the potential. This dependence corresponds to results of ESR spectroelectrochemistry. In contrast, for the system with PPs both P and PP concentrations saturate at high potentials. Thus, the PP model is only reasonable if an additional subsequent second oxidation step is possible. The kinetics of the formation and dissociation of PPs or BPs, is essentially the same for concentrations that can be achieved in accumulation layers in organic MOS devices. The estimated relaxation times point to the formation and dissociation of either BPs or PPs as processes that can either cause or influence hysteresis effects in polymeric MOS capacitors, which can be described by an interface charge, depending on the sweep direction in the capacitance-voltage measurements.