LOW-FREQUENCY AC CONDUCTION IN LIGHTLY DOPED POLYPYRROLE FILMS

The total measured ac conductivity sigma(omega)m of lightly doped polypyrrole films has been measured in the frequency range 100 Hz-10 MHz and in the temperature range 77-350 K. In the low-temperature region the measured ac conductivity is almost independent of temperature but shows a strong dependence on frequency and can be described by a relation sigma(omega) = A-omega-s where the exponent s has been observed to be less than unity. In the high-temperature region the frequency dependence becomes weak at low frequencies but remains strongly frequency dependent at high frequencies. The weak frequency dependence is due to the contribution of dc conductivity to the measured ac conductivity. A clear Debye-type loss peak is observed by substracting the contribution of dc conductivity. The frequency dependence of conductivity remains less than quadratic at low frequencies indicating thereby some distribution of relaxation times. This is confirmed by the measurement of dielectric constant as a function of frequency and temperature. The frequency and temperature dependence of ac conductivity can be qualitatively explained by the proposed model which considers the contributions from two mechanisms, one giving a linear dependence of conductivity on frequency and the other having distribution of relaxation times giving rise to broad dielectric loss peak.