Abstract
The ac conduction properties of thermally evaporated films of α-nickel phthalocyanine (α-NiPc) were studied in situ and ex situ employing symmetric gold ohmic electrodes in the frequency range of 20−106 Hz at various temperature regimes. ac conductivity was identified to be via a hopping-type mechanism in the lower temperature region and via a free-band conduction in the high temperature region. Upon exposure of the films to dry air, the low frequency ac conductivity was found to increase by 2 orders of magnitude, which was attributed to oxygen absorption within NiPc. The doping effect was partially reversed by thermal annealing of the films under high vacuum. Measurements on the dependence of capacitance and loss tangent (tanδ) on frequency were consistent and quantitatively explained by invoking an equivalent circuit model. Oxygen doping was found to increase the low frequency capacitance of NiPc. The phenomenon was understood in terms of reduction in the value of device internal resistance induced by oxygen absorption.