Charge transport in thin films of semiconducting oligothiophenes

Electrical measurements have been carried out at various temperatures (T) and electric fields (F) on field-effect transistor (FET) devices having thin films of dimethyl-substituted oligothiophenes of tetramer to hexamer. The mobility is strongly dependent on temperature and electric field. The temperature dependence of the mobility (μ) follows the simple Arrhenius relationship: μ is proportional to exp(-W/kT) where W is an activation energy. The activation energy decreases with increasing electric fields as ${\mathit{F}}^{1/2}$. Electron spin resonance and x-ray studies indicate that the charged form of the oligothiophenes can be characterized as molecular polarons. Consequently, we have analyzed the charge transport results using a polaron theory. On the basis of the FET results correlated with the optical data, we show that the charge transport is described in terms of the hopping of polarons.