Shear alignment and 2D charge transport of tilted smectic liquid crystalline phases – XRD and FET studies

Abstract

In 2001 Hanna et al. reported time of flight (ToF) experiments on the liquid-crystalline 5,5′′-dioctyl-2,2′:5′,2′′-terthiophene (8-TTP-8) and observed high charge carrier mobilities which made liquid-crystalline terthiophenes a promising class of materials for organic semiconductors. We now report detailed investigations on the structures of the smectic 8-TTP-8 phases – smectic C, smectic F and crystal G – and their impact on 2D charge carrier mobility in field-effect transistors (FETs). We found shear alignment to be a very simple and effective method to align the smectic phases as an active layer in FETs. Depending on the fluidity of the smectic phase the effective FET mobilities are far less reduced in comparison to the ToF mobilities than in the case of 1D columnar materials. The shear alignment in orthogonal directions further allowed the measurement of the anisotropy of mobility along and normal to the director tilt direction of the smectic phases. The anisotropy was found to be surprisingly high (namely μ_‖/μ_⊥ = 5 in crystal G) even though the director tilt angle is about 15 degrees rather low. The large anisotropy of the charge carrier mobility indicates that the quadrupolar ordering in the tilted smectic phases is connected to a face-on orientation of the aromatic terthiophene cores, which effectively enhances the charge transport in the tilt direction.