Hole Transport in Doubly-Doped Polystyrene

Abstract

Hole mobilities have been measured in polystyrene films doped with a mixture of two very similar triphenylamine derivatives at an overall concentration of 20% by weight. The data were analyzed in the framework of a formalism based on disorder due to Bässler and coworkers. The key predictions of the formalism concern the field and temperature dependencies of the mobility from which the key parameters σ, the energy width of the hopping site manifold, and ∑, the positional disorder parameter, can be determined. The experimental results from this study are in good agreement with the predictions of the formalism, although some of them are difficult to explain on arguments based on disorder. This is especially true for the energetic disorder parameter σ that was found to be smaller for films that contained 5% of the higher oxidation potential compound (an antitrap thus) compared to polystyrene layers doped with 20% of the pure compounds. But even the trap case cannot be fully explained on the basis of trapping arguments or the dipolar disorder model. Therefore, the results confirm earlier findings that doubly-doped polymers show some other contribution to charge transport than merely disorder related effects.