The effects of annealing on the AC electrical properties of cobalt phthalocyanine thin films

A comparative study of the AC electrical properties of cobalt phthalocyanine (CoPc) thin films before and after annealing at 493 K (220 degrees C) has been performed. The films were prepared by evaporation using purified material and measurements were made in the frequency range 10(2)-2 x 10(4) Hz and for temperatures of 150-500 K. The AC component of conductivity showed an omega(s) dependence, where omega is the angular frequency and s is an index less than unity. Features characteristic of free band conductivity at higher temperatures and hopping through a band of localized states at lower temperatures were observed. Hopping appeared to be via multiple hopping through clusters, rather than by a two-site process. After annealing there was an increase in the value of the index s at low temperatures and behaviour consistent with desorption of oxygen impurities and/or a partial phase transformation from the alpha- to the beta-form of CoPc. Capacitance and loss tangent normally showed a decrease with increasing frequency and an increase with increasing temperature, with a decrease in absolute values and an improvement in the quality of data after annealing. These results are also consistent with existing equivalent circuit models, with the reduced values after annealing resulting from oxygen desorption and phase transformation behaviour. It is clear that measurements taken after annealing are necessary in order to reveal the intrinsic properties of evaporated phthalocyanine thin films.