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

• Synthesis, mesomorphic and electrical properties of a thienothiophene derivative are presented.
• Charge carrier transport was investigated in bulk, vacuum and solution deposited thin films.
• Charge carrier transport was studied by temperature-dependent time-of-flight and in field-effect transistors.
• The results show the potential of the melt-processing route for the high mobility organic semiconducting layer preparation.

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.

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