DC CONDUCTION IN ELECTROCHEMICALLY SYNTHESIZED POLYPYRROLE FILMS

DC conductivity measurements were performed by modified four-probe rig on electrochemically synthesized polypyrrole films at a temperature r ange of -30 degrees C to 120 degrees C. Conductivity increased with te mperature. The temperature dependence of conductivity was very high fo r lightly doped polymers, decreasing as the doping level increased. Th e model used to describe the conduction process was the conduction mod el originally developed for amorphous silicon by Mott and Davis. When applied to conducting polymers, it assumes that electron transport ori ginates from localized or fixed states within the polymer chain. The c harge transfer between the chains takes place by hopping, referred to as phonon-assisted hopping, between two localized states. Plots of DC conductivity versus temperature can be parametrized by Mott's Variable Range Hopping conduction model. The DC conductivity of polypyrrole fi lms doped from light to intermediate levels with p-toluene sulphonic a cid were measured in the temperature range of 77K to 300K. The localiz ation length of localized electrons was assumed to be 3 Angstrom which is approximately equal to the length of the pyrrole monomer. Mott par ameters of polypyrrole films doped with p-TS were evaluated at 300K an d 10K. Results were found to be consistent with the Mott's requirement that alpha R >> 1. Theoretical values of alpha and N(E-F) have been d etermined at approximately 10(8) cm(-1) and 10(19)-10(20) cm(-3) eV(-1 ), respectively. Hence according to Mott parameters determined by the experimental data for the p-TS doped polypyrrole samples. Mott paramet ers are seen to have a better agreement with those expected from disor dered systems, particularly for lightly doped samples, indicating the suitability of Mott's model to these samples. The average hopping dist ance R decreased from 16 Angstrom to 4.4 Angstrom with the increase in the doping level from 0.006 M to 0.03 M at 300K, whereas at 10K, R de creased from 37 Angstrom to 10 Angstrom over the same dopant range.