Structure and electrical conduction properties of phthalocyanine thin films

The structure and the dc and ac electrical conduction properties of evaporated phthalocyanine thin films are critically reviewed. Results of various structural studies on phthalocyanine single crystals and thin films performed using X-ray diffraction methods are described, and reported unit cell dimensions are given for several phthalocyanines in both the metastable alpha and the stable beta forms; reported unit cell dimensions for both the monoclinic and triclinic forms of the atypical lead phthalocyanine, are given. Recent work on the alpha to beta phase transformations and growth in phthalocyanine films, the dependence of structure and preferential orientation on film thickness and substrate temperature, and observations of the monoclinic and triclinic phases in lead phthalocyanine films are also discussed. The importance of the type of electrical contacts applied and the existence of surface states in sandwich structures of phthalocyanine films are stressed, and a summary of the various dc conduction processes observed is given, together with the relevant theoretical conductivity equations. Selected results on samples showing ohmic and space-charge-limited conductivity, the Poole-Frenkel effect, variable-range hopping, tunnelling, the Schottky effect and diode-type conductivity are compared, the present state of knowledge is summarized and suggestions for further work are proposed. Ac measurements on phthalocyanine films using both blocking and ohmic contacts are described, together with an account of equivalent circuit models adopted to account for the dependences of capacitance and loss tangent on frequency and temperature. Several sets of measurements of ac conductivity as a function of frequency, which usually show a power-law dependence, are compared and associated with hopping conduction in the lower temperature range. Free-band conductivity at higher temperatures is also reviewed. Finally, constraints placed on the making of electrical measurements by the necessary use of materials less pure than those of the inorganic materials customarily used in semiconductor technology and the tendency for phthalocyanine films to absorb oxygen is emphasized. The use of improved materials deposition and characterization in future work is recommended.