Modifying field-effect transistor response in a conjugated polymer upon the addition of radical dopants

Abstract

As open-shell moieties are increasingly integrated into organic electronic devices, there remains a need to establish the interactions that occur between these oxidation–reduction-active (redox-active) radical species and commonly-used conjugated polymers. In this report, we show that the addition of the stable radical galvinoxyl to the conjugated polymer poly(3-hexylthiophene) (P3HT) alters the thin film transistor response from semiconducting to conducting as well as modestly enhances the electrical conductivity. This interaction is not seen with other radical species. While an increase in charge carrier concentration is observed, the interaction does not seem to be otherwise consistent with a simple charge-transfer doping mechanism, due to the mismatched reduction and oxidation potentials of the two species. Additionally, no freeze-out of charge carriers is observed at lower temperatures. It is also not due to parallel conduction through the radical fraction of the bulk composite, as the radical species is non-conductive. Hole mobility is enhanced at lower concentrations of the radical, but it decreases at higher concentrations due to the reduced fraction of conductive material in the polymer bulk. Despite the increase in mobility at lower concentrations, the activation energy for charge transport is increased by the presence of the radical. This suggests that the radical is likely filling trap states within the P3HT for the composite thin film while not affecting their energetic distribution.