Organic semiconductors(OSs) have stirred huge commercial interest due to their potential applications in electronic and optoelectronic devices such as field effect transistors, photovoltaic cells, and organic light-emitting diodes. Major benefits of OSs over conventional semiconductors include mechanical flexibility, low temperature processing, very low cost, and ease of fabrication in large area electronic devices on plastic and paper substrates. Liquid crystals (LCs) are particularly interesting classes of OSs, both from the standpoints of fundamental physics and practical applications. Systems we studied include a thiophene-benzene-thiophene-based smectic (1,4-di-(5-n-tridecylthien-2-yl)-benzene). This material exhibited polaron band behavior with very impressive hole transport (> 0.1 cm2/Vs with the smectic-F phase templating large domains of more ordered phases with very large mobilities. The mobilities are high enough to be of practical interest.
Another project involved calamitic LCs with pyridine-thiophene-thiophene-pyridine cores (5, 5’-di-(alkyl-pyridin-yl)-2, 2’ bithiophenes). We found both electron and hole mobilities to be strongly electric field dependent but very weakly dependent on temperature. Pyridine-based LCs often exhibit very high order smectic phases and are therefore of interest as OSs. However, the mobilities of these materials were found quite low, even in high-order phases. We were able to describe some part of our data using Basseler’s theory of hopping conduction in disordered systems.
We also studied charge transport in a triphenylene-based discotic LC (1-nitro-2, 3, 6, 7, 10, 11-hexakis (pentyloxy) triphenylene). This material showed strong temperature and field dependent hole mobilities described by disorder dominated one-dimensional hopping. Since the columnar phase exists over a wide range of temperatures, such photo-conducting materials may be very useful for applications in electronics.
Finally, we developed a technique to measure charge carrier mobility in freely suspended films of LCs in high vacuum. Here, the external field can be coupled easily to the molecular order, no electrodes contact the sample, and extremely high voltages can be applied. Also, both hole and electron mobility (which depends on high purity and absence of oxygen), and samples with a very wide range of thickness may be studied.