Transport and in situ ESR studies on polymer film composed of quinquethiophenes bridged by monosilanylene units

Apparent mobilities of positive charge carriers in a poly[(ethoxypropylsilanylene)quinque(2,5-thienylene)] (MS5T-OEt) film are evaluated over a wide range of doping levels. The mobility is found to decrease monotonously with the increase in doping level up to a few percents, in contrast to four order-of-magnitude increase of mobilities reported earlier for the poly(3-methylthiophene) (PMT) film. This clear difference indicates that the two kinds of diamagnetic species, cation radical π-dimers in the MS5T-OEt film and bipolarons in the PMT film, are related to the electrical conduction in completely different manners. A rather small difference between mobilities in almost dedoped MS5T-OEt and PMT films is discussed from a viewpoint of an interchain or intrachain transport of polarons. Anomalous changes of mobilities are observed in a relatively high doping region of the MS5T-OEt film. An in situ ESR study performed to characterize charge-carrying species in the MS5T-OEt film demonstrates that there are oligothienylene segments where polarons are isolated so as not to form π-dimers. A theoretical treatment is developed to evaluate a molar fraction of the isolated oligothienylene segments.