Frequency dependence of the electrical properties of coordination complexes of transition metals
Measurements are reported of the dielectric permittivity Îµ and conductivity σ of two organic molecular solids, nickel-bisquinoxaline-2,3-dithiol (NiQDT) and copper-phthalocyanine (CuPC) in the frequency range 102 < ƒ < 1010 Hz. The results show a strong dispersion of Îµ and σ with frequency and fall broadly into two regions: at low frequencies Îµ falls with increasing ƒ and σ is approximately constant, while at high frequencies Îµ is constant while σ∝ƒn with n < 1. It is pointed out that although the barrier-dominated Maxwell-Wagner mechanism cannot be ruled out, a very likely explanation of the behaviour in the higher frequency range is in terms of electronic carriers hopping either within or between molecules. The low frequency behaviour is consistent with a separate form of hopping characterized by a wider distribution of hopping time constants or it may be due to some form of interfacial barriers blocking the passage of electronic or ionic carriers.