Electrical contacts on organic crystals: TTF-TCNQ

Abstract

Room temperature plots of the electrical potential measured on the surfaces of TTF-TCNQ crystals which carry current are presented. It is shown that silver paint contacts behave electrically as though thin resistive layers ( ⩽ 20 mhos mm 2 conductance) were inserted between the crystal and the current-injecting electrodes. The contact resistance is microscopically nonuniform, and can lead to erroneous results in 4-probe conductivity measurements. By measurement and calculation (using the method of image charges) it is demonstrated that measurement techniques which introduce nonuniform current densities into highly anisotropic crystals are subject to error unless the contacts are kept very small. The principal conductivities of TTF-TCNQ at room temperature were found to be: σ b = 440 ± 15, σ c ≈ 13, σ a , = 1.30 ± 0.05 Ω −1 cm −1, independent of contact effects.