Charge carrier mobility study of a mesogenic thienothiophene derivative in bulk and thin films

A novel mesogenic 2,5-bis-(5-octylthiophene)-thieno[3,2b]thiophene (TT) derivative has been synthesized. The fused-ring thiophene, end-capped with two octylthiophenes, exhibits ordered lamellar mesophases which were characterized by polarizing optical microscopy, differential scanning calorimetry and small-angle X-ray diffraction at various temperatures. The charge transport properties were investigated by time-of-flight technique as a function of temperature. On cooling from isotropic phase, a maximum hole mobility value of 0.07 cm² V^-1 s^-1 was measured in the highly ordered mesophase of the bulk films. Field-effect transistor experiments on both solution and vacuum deposited thin films have also been performed. The solution-processed films exhibit charge carrier mobilities several orders of magnitude lower than values extracted from bulk time-of-flight curves and from vacuum deposited thin film transistors. This work provides evidence that the melt-processing route is an efficient alternative to commonly used solution-processing for fabrication of charge transporting layers from liquid crystalline semiconductors, with performances comparable to evaporation techniques.