Influence of temperature and frequency on the electrical conductivity and the dielectric properties of nickel phthalocyanine

The temperature and frequency dependence of the AC conductivity sigma(ac)(omega), the dielectric constant epsilon'(omega) and the dielectric loss epsilon ''(omega) were studied on pellet samples of nickel phthalocyanine (NiPc) with evaporated ohmic Au electrodes in the frequency range from 20 kHz to 10 MHz and within the temperature range from 303 to 600 K. The DC conductivity sigma(dc) has also been measured in the considered range of temperature. Two temperatures-induced changes in the thermal activation energy Delta E have been observed. For T <= 435 K, Delta E-1 = 0.322 eV; for 435 K <= T <= 525 K, Delta E-2 = 0.497 eV; for T >= 525 K, Delta E-3 = 0.703 eV. These variations in the activation energy were attributed to a partial phase transformation from alpha- to beta-NiPc phase and as a change from extrinsic to intrinsic conduction mechanism. The AC conductivity sigma(ac)(omega) showed temperature independence and it has been found to vary with angular frequency as omega(s) with the index s <= 1 suggesting a hopping conduction mechanism at low temperatures and high frequency. At higher temperatures and lower frequencies a free-band conduction mechanism was observed. Both the dielectric constant epsilon'(omega) and the dielectric loss epsilon ''(omega) increased with temperature and decreased with frequency in the investigated ranges. Such characteristics, reveal that the tested organic NiPc exists in the form of molecular dipoles which remain frozen at low temperature, whereas at higher temperatures, when the dipoles attaining rotational freedom, the dielectric constant was found to decrease with increasing frequency and increase with increasing temperature. The increase in the dielectric loss epsilon ''(omega) with increasing temperature at low frequencies can be understood in terms of an increase in DC conductivity. (C) 2006 Elsevier B.V. All rights reserved.