Hopping transport in disordered aromatic solids: a re-interpretation of mobility measurements on PKV and TNF

Published drift mobility measurements on amorphous PVK, TNF, TNF:PVK mixtures and dispersions of TNF in polyester strongly suggest that transport is by small-polaron hopping controlled by charged traps. The existence of a characteristic temperature at which the extrapolated mobility loses its field dependence is ascribed to the temperature dependence of the permittivity. With the aid of a first-order model of Coulomb-trapping in hopping systems it is argued that the transport activation energy at room temperature is only 0.4-0.45 eV, not 0.65 eV as suggested by the experiments, and that the trap-free hopping mobility is of the order 10-3 cm2 V-1 s-1. The experiments suggest that the pre-factor mu 0 in the mobility of pure PVK or TNF is of the order 1 cm2 V-1 s1, and this is shown to be a plausible value for aromatic hopping systems.