Conduction in lead phthalocyanine films with aluminum electrodes

Electronic conduction has been investigated in evaporated lead phthalocyanine thin films with aluminum electrodes. In contrast to previously investigated samples where the injecting electrode was a gold ohmic contact, these samples did not exhibit space-charge-limited conductivity, but showed carrier excitation via a field-lowering mechanism with a log J∝V1/2 current density-voltage (J-V) characteristic. Both polarities showed two regions in the J-V characteristics having different slopes, leading to the conclusion that conduction is via Schottky and Poole-Frenkel emission at lower and higher voltages, respectively. The Schottky barrier height was 1.03 eV and the depletion region thickness was typically 100 nm for forward bias and 77 nm for reverse bias. In the presence of air there was a decrease in conductivity, which was attributed to the effects of oxygen establishing an interfacial region at the electrodes. The reverse bias conductivity was higher than that for forward bias when exposed to air; this effect was associated with different values of the hole injection efficiency at the two electrodes consequent on the effects of the oxygen exposure.