Effect of electrode material on AC electrical conductivity of organic zinc phthalocyanine semiconducting thin films

The AC electrical conductivity of thermally evaporated zinc phthalocyanine, ZnPc, semiconducting thin films was measured in the temperature range of 180-430 K and frequency between 0.1 and 20 kHz. Gold and aluminum contact electrodes were employed and both showed ohmic contacts. In both cases, the conductivity showed strong dependence on both temperature and frequency. Such dependence is related to the relevant temperature and frequency range under consideration. However, for both electrode materials, the AC conductivity σ(ω) is found to vary with angular frequency, ω, as ωs, with the index s ≤ 1 suggesting a dominant hopping conduction process at low temperatures (<250 K) and high frequency. At higher temperatures and lower frequencies free band conduction was observed in samples with Al-electrodes. The AC conductivity in the present work did not increase monotonically with temperature. For samples with Al-electrodes, σ decreased with temperature above 330 K, while for samples with Au-electrodes it decreased with temperature above 430 K. The former decrease in σ was attributed to oxygen exhaustion, while the latter was ascribed to α-β-phase transition.