Electrical Properties of CuPc Films Prepared by Plasma-Activated Evaporation in N₂ and NO_x Environments

CuPc films with good mechanical and chemical resistance were prepared by the plasma-activated evaporation method and the electrical properties of the films were measured in pure N₂ and in 100 ppm NO_x environments. In N₂ environment, the dc conductivity of the film depends on the temperature as e^-E/kT while the ac conductivity is independent of temperature and proportional to ωⁿ with the exponent n∼0.95. Adsorption of NO_x gas affects mainly the dc conductivity by forming acceptor states to generate holes, but negligibly affects the ac conductivity. The increment of dc conductivity is related to the amount of adsorbed NO_x gas molecules, which depends on the temperature and the film morphology. The impedance of CuPc thin film can be reasonably expressed as that of an equivalent circuit consisting of two resistor-capacitor pairs in series corresponding to the grain and the grain boundary. As the exposure time in NO_x environment increases, the resistances of both grain and grain boundary decrease, while the capacitances do not vary with the exposure time.