Transport properties in oxygen-doped cobalt phthalocyanine thin film devices

The electrical properties of oxygen doped thin films of cobalt phthalocyanine sandwich structures have been studied using ohmic gold and blocking aluminium electrodes. The current density-voltage characteristics under forward bias (gold electrode positive) were found to exhibit three different regions. At low voltages the devices (Al/CoPc/Au) showed Schottky diode behaviour. The model of Cheung and Cheung was used to derive diode parameters. At higher voltage regions the results showed ohmic conduction and above a threshold voltage a power-law dependence of current density on applied voltage was observed. The latter was ascribed to space-charge-limited conduction controlled by exponential distribution of traps above the valence band edge. The hole trapping parameters were evaluated. Under reverse bias, the conduction processes were interpreted in terms of both the Poole-Frenkel and Schottky effects. Barrier heights emphasise the importance of oxygen distribution in CoPc and the existence of interfacial oxide layers in the determination of the electrical characteristics of CoPc based devices.