AC conductivity and dielectric measurements of metal-free phthalocyanine thin films dispersed in polycarbonate

The dielectric constant and the dielectric loss of thin films of metal-free phthalocyanine dispersed in polycarbonate using ohmic gold electrodes are investigated in the frequency range 20-10(5) Hz and within the temperature range 300-388 K. The frequency dependence of the impedance spectra plotted in the complex plane shows semicircles. The Cole-Cole diagrams have been used to determine the molecular relaxation time, tau, The temperature dependence of tau is expressed by thermally activated process. The AC conductivity sigma(AC)(omega) is found to vary as omega(s) with the index s less than or equal to 1, indicating a dominant hopping process at low temperatures. From the temperature dependence of AC conductivity, free carrier conduction with mean activation energy of 0.33 eV is observed at higher temperatures. Capacitance and loss tangent are found to decrease with increasing frequency and increase with increasing temperature. Such characteristics are found to be in good qualitative agreement with existing equivalent circuit model assuming ohmic contacts. (C) 1999 Elsevier Science B.V. All rights reserved.