FREQUENCY-DEPENDENCE OF ELECTRONIC CONDUCTION PARAMETERS IN EVAPORATED THIN-FILMS OF COBALT PHTHALOCYANINE

A.c. dark current measurements were taken on purified cobalt phthalocyanine (CoPc) thin films in the frequency range 10(2)-2 x 10(4) Hz and in the temperature range 163-433 K. Measurements revealed that the a.c. conductivity sigma(omega) varied as omega(s), where the index s was variable and always less than unity, generally increasing with increasing frequency and decreasing with increasing temperature. A.c. activation energy measurements at fixed frequencies showed that at temperatures below approximately 230 K the conductivity is frequency dependent with very low activation energy, while at higher temperatures a frequency independent activation energy of approximately 0.54 eV was observed. These results were interpreted in terms of hopping through a band of localized states at lower temperatures and by free; band conduction at higher temperatures. Measurements of capacitance and loss tangent showed a well defined decrease with increasing frequency and an increase with increasing temperature. An interpretation is presented, based on existing theory, for the case of a thermally activated process when using ohmic contacts. A comparison of samples before and after heat treatment at 450 K showed a decrease in both conductivity and capacitance. This reduction was ascribed to desorption of oxygen, which acts as an acceptor level, as previously inferred from d.c. measurements.