Dielectric Properties of Some Powdered Organic Semiconductors

The use of electrical conductivity measurements to characterize the charge density and/or mobility parameters of a material is discussed. The common practice of using simple, pressed electrodes with dc measurements is felt to be grossly inferior to four-probe techniques. Even this refinement, however, is subject to considerable error for powdered samples. Many workers have used fairly simple ac techniques to eliminate interparticle contact resistances in organic semiconductors; this approach is shown to have been misapplied. It is possible, however, to measure what appears to be a specific conductivity (independent of packing pressure or filling factor) by more elaborate ac analysis. Measurements of the effective parallel resistance and capacitance are given for anthracene (powder and single-crystal), metal-free β-phthalocyanine, and a β-carotene tri-iodide complex for a frequency range of 0.05 cps to 300 Mc/sec.