Charge transport in a regioregular poly(3-octylthiophene) film

Apparent mobilities of positive charge carriers in a regioregular poly(3-oc tylthiophene) (POT) film at various doping levels are evaluated by an elect rochemical technique combining potential-step chronocoulometry and an in si tu conductivity measurement. The carrier mobility decreases from 5 x 10(-3) cm(2) V-1 s(-1) to 5 x 10(-4) cm(2) V-1 s(-1) as the doping level increase s from 0.014% to 1%. Further electrochemical doping results in a dramatic i ncrease in the mobility, reaching 0.5 cm(2) V-1 s(-1) at a maximum doping l evel of 23%. The decrease of the apparent carrier mobilities in the low dop ing range is ascribed to both scattering of polarons by ionized dopants and the formation of immobile pi -dimers which are favored in the highly stack ed POT film. The marked increase of mobilities at doping levels beyond 2% i s explained by the formation of bipolarons, followed by the evolution of th e metal-like conduction.