Effects of annealing on the trap distribution of cobalt phthalocyanine thin films†

Abstract

Current density-voltage characteristics have been obtained from evaporated cobalt phthalocyanine thin Film sandwich structures using ohmic gold contacts, both before and after annealing. Results show that at low voltages the conduction process is ohmic, while at high voltages space-charge limited conduction is present. These results are consistent with current density-voltage characteristics obtained with copper phthalocyanine thin films. Annealing resulted in little change in current density within the space-charge limited region but a significant reduction within the ohmic region. Subsequent exposure to air gave an increase in current density tending towards levels obtained with fresh samples. Analysis of the results yields a typical room temperature hole concentration of p 0 = 8 × 1017 m−3 for unannealed samples. An exponential trap distribution P(E) = P 0 exp (−E/kT t) was consistent with the higher-voltage data, where typically P 0 = 8 × 1043J−1 m−3 and T t = 885 K. Variations of parameters with heating conditions are discussed, with particular reference to the desorption of oxygen during annealing and its absorption during exposure to air.