Organic semiconductors show promise in the development of a new age of electronics that are inexpensive, flexible, wearable, and bio-compatable. A great amount of work has been done to engineer and characterize new organic semiconductors in organic thin film transistors (OTFTs), resulting in charge carrier mobility values greater than 10cm2/Vs. The performances of these devices still fall well short of their silicon counterparts mostly due to molecular morphology, grain size, and carrier concentration. One solution to these problems is to use the intrinsic order found in liquid crystal (LC) mesophases. Previous work shows that benzothieno[3,2-b]benzothiophene (BTBT) can be used to produce high-performing OFTFs. This is due large in part to the paramorphic SmE to crystal transition commonly seen in these materials that induces long-range molecular order within the crystal structure. In this work, we synthesized and characterized single-tailed BTBT molecules that contain a paramorphic SmE to crystal transition in single-step solution-processed OTFTs. Further, the addition of a pentafluorobenzene thiol (PFBT) self-assembled monolayer (SAM) to the gold electrodes improved charge injection, reducing the device threshold voltage and increasing the on/off ratio. It is suspected that the molecular packing is responsible for high mobility values. Future work will aim to explore the use of host-guest chemistry doping and LC alignment techniques to further improve carrier concentration and charge transfer properties to improve bulk material transport properties.
The control of the molecular orientation of liquid crystals (LCs) is important in both understanding phase properties and the continuing development of new LC technologies including displays, organic transistors, and electro-optic devices. Many techniques have been developed for successfully inducing alignment of calamitic LCs, though these techniques typically do not translate to the alignment of bent-core liquid crystals (BCLCs). Some techniques have been utilized to align various phases of BCLCs, but these techniques are often unsuccessful for general alignment of multiple materials and/or multiple phases. Here, we demonstrate that glass cells treated with polydimethylsiloxane (PDMS) thin films induce high quality homeotropic alignment of multiple mesophases of four BCLCs. On cooling to the lowest temperature phase the homeotropic alignment is lost, and spherulitic growth is seen in crystal and crystal-like phases including the dark conglomerate (DC) and helical nanofilament (HNF) phases. Evidence of homeotropic alignment is observed using polarized optical microscopy. We speculate that the methyl groups on the surface of the PDMS films strongly interact with the aliphatic tails of each mesogens, resulting in homeotropic alignment.