Influence of temperature and frequency on the electrical parameters of thermally evaporated metal-free phthalocyanine H2Pc thin films

The ac electrical properties of metal-free phthalocyanine (H2PC) thin films have been studied in the frequency range from 10(2) to 2 x 10(4) Hz and in the temperature range from 150 to 475 K. The ac conductivity sigma was found to vary as omega(s) with the index s less than or equal to 1. Although these general values of s appear to be consistent with a hopping process, the present Q values do not increase monotonically with temperature. At low frequency, the capacitance and loss tangent were found to be constant over the entire frequency range, in good qualitative agreement with the equivalent circuit model consisting of an inherent capacitance in parallel with a resistive element. Moreover, at constant frequency, the two parameters increased with increasing temperature up to approximately 300 K. Above this temperature, another sharp decrease in both capacitance and loss tangent was obtained. This type of behavior was interpreted in terms of nomadic (delocalized) polarization, which leads to an increase in the dielectric constant. The drastic decrease of the capacitance and loss tangent observed above room temperature is thought to be related to the decrease in the dielectric constant, which results from the inability of the domains to hold the increases in free charge carrier concentration due to the increase of temperature.