Electrical Properties of Phthalocyanines

Abstract

The electrical conductivity of single crystals of metal-free, copper, nickel, and cobalt phthalocyanines were measured over temperature ranges extending from 50° to 390°C. The weighted mean values of the activation energies obtained were: 1.71 ev for the metal-free, 1.60 ev for the cobalt, and for the nickel, and 1.64 ev for the copper substituted compounds. No significant difference in the activation energy of the metal-free compound was observed between directions parallel and perpendicular to the (001) face of the crystal. The results were well reproducible and showed no significant variation for different crystals. It is concluded that these compounds are, above room temperature, intrinsic semiconductors and that the introduction of a metal atom into the molecule produces comparatively little change in the activation energy of conduction. A thermoelectric power of +50 μv/°C was measured for metal-free and for copper phthalocyanine, indicating that the majority carriers are holes. Since the values of the constant σ0 were found to be in the order of 100 Ω—1 cm—1 it is concluded that their mobilities are rather low. Optical transmission spectra are reported for these compounds in solution as well as in the form of thin evaporated films. The energy of the first electronic transition evident in the film spectra agrees with the thermal activation energy for electrical conduction. There is little difference in the positions of the absorption maxima observed in evaporated films and in solution, although the intensities are altered. The photoconductive response resembles the spectral transmission characteristics of the compounds. The quantum efficiency is low.