Homologous binary mixtures and improved hole conduction of self-assembled discotic liquid crystals

Abstract Discotic liquid crystals (DLCs) are considered promising materials for organo-electronic applications. Columnar alignment of DLCs leads to anisotropic charge transport with high charge carrier mobility. However, pure DLCs exhibit low intrinsic charge carrier density which limits bulk conductivity. This research studies the alignment and conductivity properties of small molecule triphenylene-based DLCs to develop hole transport layers for potential applications in organic semiconductor devices. Binary mixtures of homologous DLCs of the hexakis(n-alkyloxy)triphenylene series (HAT6 and HAT10) are formulated. Mesophase characteristics and columnar alignment of these mixtures are characterized using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). Alignment, orientation and order of columnar packing in the mixtures is studied using X-ray diffraction (XRD) and grazing incidence wide angle X-ray scattering (GIWAXS) measurements. It is identified that binary mixture formation strongly effects the columnar alignment in solution processed films. Furthermore, to increase charge carrier density in the DLC films a strong electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is added as a p-type dopant, followed by an extensive characterization of its doping effect. POM, DSC thermal scans, UV–visible spectroscopy, photo-luminescence spectroscopy (PL) and I-V measurements are utilized to characterize and establish the improvement of hole conduction in the doped films. It is observed that F4TCNQ-doped triphenylene DLC films exhibit two-fold increase in hole conductivity, making the materials highly relevant for charge transport applications.