DEPENDENCE OF AC ELECTRICAL PARAMETERS ON FREQUENCY AND TEMPERATURE IN ZINC PHTHALOCYANINE THIN-FILMS

Measurements are made of the ac properties of thermally-evaporated thin films of zinc phthalocyanine using ohmic gold electrodes. These are performed over a frequency range of 100 Hz to 20 kHz at temperatures in the range 90 to 470 K, sufficient to initiate a phase transition from the alpha-form to the beta-form. The ac conductivity sigma(omega) is found to vary with angular frequency omega as omega(s), where the index s less-than-or-equal-to 1, suggesting a dominant hopping process at lower temperatures and higher frequencies. At higher temperatures and lower frequencies free band conductivity with activation energy approximately 0.29 eV is observed. A decrease in conductivity at all frequencies is observed at temperatures above 430 K suggesting a partial alpha- to beta-phase transition. Capacitance and loss tangent are found to decrease with increasing frequency and increase with increasing temperature, in good qualitative agreement with existing equivalent circuit models assuming ohmic contacts. After prolonged heat treatment both conductivity and capacitance diminish, particularly when the phase transition temperature is exceeded. This behaviour is ascribed to the desorption of oxygen acceptors by the heat treatment, as also observed in both the dc and ac conduction behaviour of copper phthalocyanine films.