Simultaneous voltammetric and in situ conductivity studies of n-doping of polythiophene films with tetraalkylammonium, alkali, and alkaline-earth cations

Polythiophene (PTh) films, both p- and n-doped, have been fully characterized in a solution of TEABF₄ (tetraethylammonium tetrafluoroborate), in propylene carbonate (PC) by a simultaneous application of cyclic voltammetry and in situ conductometry. We observed a considerable charge-trapping effect during n-doping of PTh with TEA⁺ cations. The release of the trapped, negatively charged polarons becomes possible only at the onset of the subsequent p-doping of this polymer, accompanied by an increase in conductivity. The interaction between the trapped, negatively charged polarons and mobile, positively charged polarons resulted in their neutralization. This effect was accompanied by the simultaneous exaltation of the voltammetric current and the transient decrease in conductivity. Correction for the trapped charged allowed us to estimate the mobility of the free negatively charged polarons: μ = 2.4 × 10^-3 cm²/V s (doping level with TEA⁺, cations, X = 0.022). At the same doping level with BF₄^--anions, positively charged polarons exhibited similar mobility, μ = 2.8 × 10^-3 cm²/V s. This similarity is in good agreement with the reported electron spin resonance characteristics of PTh. We have studied the influence of a low content of cations (with different ionic radii) in the mother TEABF₄ solution on the deterioration of the n-type redox-capacity and conductivity increasing in the sequence: TEA⁺ < K⁺ ≪ Li⁺ < Ba²⁺. This remarkable effect is discussed in terms of competition between the solvation of small cations in PC and the hard-soft pinning of negatively charged polarons. This pinning seems to be partially reversible, as revealed by the sub-sequent n-redoping in a clean TEABF₄ solution.