Interpretation of the thermal activation energy of conduction for molecular semiconductor thin films with blocking contacts

Abstract

We report the in situ electric properties of lead phthalocyanine two-terminal devices. The application of a strong bias voltage induced current rectification and shifted the activation energy of conduction ±0.1 eV from the equilibrium value. The rectification and activation energy splitting could be removed by annealing the device at 175 °C. The results are explained by postulating that the device current is injection limited. In the proposed model the measured activation energy represents an energy barrier height for charge injection which may increase or decrease according to the amount of trapped space charge at the electrode interface. Further consideration of this model suggests that the trap filling level in the phthalocyanine film electrode must be in thermal equilibrium with, and effectively pinned to, the Fermi level of the metal electrodes.