High field electronic conduction processes in copper phthalocyanine thin films using lead and gold electrode combinations

Abstract

The electrical properties of copper phthalocyanine (CuPc) thin films have been studied using lead and gold electrode combinations. Samples with two gold electrodes showed ohmic conduction and space-charge-limited conductivity, in agreement with previous work. Samples having one electrode of each metal showed three separate regions of space-charge-limited conductivity under forward bias (gold positive), but with considerably lower current density when the non-substrate (top) electrode was lead. Diode behaviour was also observed in samples where the lead electrode was on the substrate (bottom) side of the CuPc film. Under reverse bias the injecting lead electrode was found to act as a blocking contact with barrier height approximately 1 eV. The barrier widths were approximately 40 nm and 100 nm when the lead electrode was on the bottom and top sides respectively of the CuPc films. At higher voltage levels the Poole-Frenkel effect was observed when lead was the top electrode. Sample rectification ratios were also dependent on the electrode positions, having a value of several hundred with lead as the bottom electrode and considerably lower values as the top electrode. In the latter case the rectification ratio increased by a factor of about 10 on annealing and exposure to air. The results are consistent with the establishment of a barrier region when the non-substrate side of the CuPc films is in contact with a lead electrode.