Influence of pi-conjugation length on mobilities of charge carriers in conducting polymers

Mobilities of charge carriers in thin films of a series of copolymers with repeat units consisting of oligothiophenes bridged by Si atoms are measured over a range of oxidation stages in order to study the influence of pi-conjugation length on the mobility. The number of thiophene units in oligothiophenes ranges from five to nine, the polymers being designated accordingly by PS5T to PS9T. Mobilities of charge carriers in these polymer films, where intrachain hopping transport is restricted within each oligothienylene unit, are constant in the low doping regions below 1%. As the doping level increases, mobilities for polymers with longer pi-conjugation lengths start to rise at lower doping levels. The mobility enhancement follows also an increasing order of the pi-conjugation length, although the 30 times enhancement of mobility for PS9T is still smaller than the four orders-of-magnitude increase of mobility observed earlier for electrochemically synthesized poly(3-methylthiophene). The results are discussed with electrochemical and spectroelectrochemical measurements from the viewpoint of the involvement of polarons, pi-dimers, and bipolarons as possible charge carriers in the conduction mechanisms.