Measurements and modeling of the DC temperature dependence of electrical conductivity in thin films of lead phthalocyanine

Abstract

Many of the phthalocyanines exhibit p-type conductivity, and electrical conductivity through thin films of these materials having ohmic contacts show space-charge-limited conductivity (SCLC) dominated by trap levels located within the band gap. In the present work evaporated thin films of lead phthalocyanine with ohmic gold electrodes were prepared, which showed two distinct regions in the dependence of current density J on applied voltage V. At low voltages sample conductivity was ohmic, changing at higher voltages to a square-law dependence of J on V, which is indicative of SCLC dominated by trap levels located at a single discrete energy level. The results of temperature measurements indicate three distinct regions, in each of which the hole concentrations are controlled by different activation energies. A simple model is proposed in which a single trap level is located at the same energy spacing E t from the valence band edge as a single acceptor level. This predicts three different temperature ranges, two of which correspond to those covered by the experimental results. The experimental results indicate a trap level located at an energy E t = 0.36 eV above the valence band edge and a thermal band gap E g = 1.51 eV. Using the proposed model together with data from the experimental J–V characteristics, an acceptor concentration of 4.85 × 10 19 m − 3 and a trap concentration of 5.18 × 10 25 m − 3 are indicated. Measurements of mobility based on this model yield a value of 2.6 × 10 − 4 m 2 V − 1 s − 1 , which is in close agreement with previous work.