Charge transport of doped polypyrrole films with various counter anions

Full text: We have measured the electrical conductivity and thermopower as a function of temperature of electrochemically synthesized polypyrrole films with various counter anions, e.g. dodecylbenzene sulfonate (DDBS-=C12H25C6H4SO3-), perchlorate (ClO4-), dodecylsulfate (DDS-=C12H25O4S-), hexafluorophosphate (PF6-), and p-toluenesulfonate (p-TsO- =CH3C6H4SO3-). For all the doped polypyrrole films, the resistivity increases with decreasing temperature and exhibits a nonmetallic behaviour; the thermopower decreases with decreasing temperature and shows nearly linear temperature dependence below 280 K, which is slightly different from the reported results of nearly constant TEP above 230 K. The analysis of the temperature dependence of normalized conductivity shows that samples with different counter anions vary in favouring the fits to the (T0/T)v-law (v =1/2, 1/3, and 1/4), e.g., PPy-DDBS favours v=1/2, PPy-DDS v=1/4, PPy-pTsO (3 min. deposition) v=1/2, and PPy-pTsO (5 min. deposition) v=1/3. Furthermore, the strong temperature dependence of the normalized conductivity, e.g., σ(295K)/σ(5K) = 104 -105, suggests that these samples are far from the metal-nonmetal transition regime, where the variable range hopping process is valid. However, the absolute value of thermopower (S ≤ 15 μV/K) at 280 K is comparable to or even smaller than the 'new' polyacetylene. Therefore, it seems to be inadequate to describe the transport by using the variable range hopping process without taking into consideration the microstructure and morphology of doped polypyrrole samples