Abstract
To understand and optimize the performance of thin-film electronic devices incorporating crystalline organic semiconductors, it is important to consider the impact of their structural anisotropy on the charge transport. Here we report on the charge mobility anisotropy in 6,13-bis(triisopropylsilylethynyl) (TIPS) and 6,13-bis(triethylsilylethynyl) (TES) pentacene field effect transistors, in which microstructure is controlled by solution processing conditions. Thin-film structures that range from millimetre size, crystalline domains to macroscopic, high-aspect-ratio (similar to 1 mu m wide and >1 cm long) needles are systematically produced by controlling the substrate displacement rate during zone-cast deposition. Through precise control of the microstructure we experimentally explore the differences in charge transport anisotropy between TIPS-and TES-pentacene molecules. Aligned needles of TIPS- pentacene result in a mobility anisotropy (mu(parallel to) /mu(perpendicular to)) of similar to 20 (mobility of similar to 0.7 cm(2) V-1 s(-1)) whereas TES-pentacene produce an order of magnitude lower mobility (similar to 0.06 cm(2) V (-1) s(-1)) but much higher mobility anisotropy (>45). Such significant changes in absolute mobility and mobility anisotropy are attributed to their different packing structures, which permit 2D charge transport in TIPS-pentacene and 1D transport in TES-pentacene. Bulky TIPS-side groups (diameter similar to 7.5 angstrom) force a brick-wall type packing structure, whereas TES- side groups (diameter similar to 6.6 angstrom) pack in a 1D slipped-stack. Furthermore, through precise control of the molecular alignment, the impact of crystal orientation on charge transport is investigated. TIPS-pentacene achieves the highest mobility when the angle between the needle long-axis and charge transport directions is similar to 35 degrees, whereas in TES-pentacene it is much closer to 0 degrees. These results are supported by theoretical simulations.