Frequency-dependent transport in the pure and irradiation-disordered organic semiconductor N-methyl-(4-methyl) pyridinium (7,7,8,8-tetracyano-p-quinodimethanide).

Abstract

The introduction of controlled disorder in crystals of N-methyl-(4-methyl) pyridinium (7,7,8,8-tetracyano-p-quinodimethanide) by fast-electron irradiation provides information on the charge-transport properties. ac microwave conductivities at different frequencies (4.6, 9.3, and 23.5 GHz) of pure and irradiated samples are presented and discussed together with previously reported dc transport properties. The ac microwave conductivity measured along the long axis of the crystal increases with frequency and differs by a few orders of magnitude from ${\ensuremath{\sigma}}_{\mathrm{dc}}$. This effect, present even in pure samples at low temperatures, develops gradually as the disorder is increased by irradiation. These results cannot be interpreted in terms of a simple Mott-Hubbard model with a strong on-site Coulomb interaction. A model involving the contribution of the localized-state transport is proposed to account for the frequency-dependent conductivity. We have shown that for high-disorder limit ${\ensuremath{\sigma}}_{\mathrm{ac}}$ varies as A${\ensuremath{\omega}}^{s}$ with s1 and A proportional to T as predicted by the Mott-Davis model.