Asymmetric electrical conductivity in oxygen-doped thin films of copper phthalocyanine, using gold and aluminium electrodes

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Abstract
Evaporated copper phthalocyanine (CuPc) thin films in the form of metal-CuPc-metal sandwich structures have been extensively studied, particularly with regard to their electrical conductivity and its variation on exposure to various gases. In the present work, CuPc thin films doped by prolonged exposure to oxygen have been incorporated into asymmetric sandwich structures using ohmic gold and blocking aluminium electrodes. DC conduction processes identified depend both on the polarity and on the electric field strength. Under forward bias (gold electrode positive), conductivity was ascribed to space-charge-limited conductivity controlled by an exponential trap distribution at lower fields and by a single dominant level at higher fields, in accordance with prior work on comparable organic films. Under reverse bias, currents were identified with Schottky emission over a potential barrier of approximate height 1 eV and thickness 200 nm. Additional work on annealed samples showed broadly similar behaviour under forward bias with a reduced Schottky barrier thickness under reverse bias. At higher voltages there is evidence of an electrode-to-bulk limited transition, with conductivity dominated by the Poole-Frenkel effect.