Solution Shearing of Conjugated Polymer with Highly Aligned Nanofibrillar Structures for Organic Field-Effect Transistors

Control of polymer crystallinity and chain orientation typically rely on template-assisted techniques, mechanical rubbing and directional solidification, which require complex equipment setup and process optimization such as shearing speed or substrate temperature. Therefore, a simple solution coating method of organic semiconductors becomes attractive with the aim of industrial-scale roll-to-roll production. Here, we propose the use of solution shearing to induce long-range alignment in poly(3-hexylthiophene) (P3HT) thin films by simply tuning the aggregation of P3HT in solution. The synergistic combination of ultraviolet-irradiation and solution aging facilitates the self-assembly of P3HT into long nanofibrillar structures prior the coating process. The length of these nanofibers can be modulated via the sequential application of both solution treatments. The surface morphologies of deposited films reveal that the degree of polymer alignment was greatly improved with increased polymer assembly, which can be controlled by the solution aging time. The development of birefringence and increased dichroic ratio determined from polarized optical microscopy and UV-Vis, respectively, provide further support for the macroscopic ordering and polymer chain alignment. According to image analysis, which provides insight into the orientation distribution of polymer chains, the P3HT nanofibers are shown to align in the solution shearing direction, leading to significant charge carrier mobility anisotropy. The correlations between highly oriented nanofibrillar structures and their two dimensional charge transport properties were systematically investigated by shearing the pretreated solution parallel and perpendicular to the active channel. Spin coated thin films, which exhibit randomly oriented nanofibrillar structures will also be discussed. This facile solution coating method demonstrated an effective approach to investigate the charge transport behavior within highly oriented crystalline domains, which can be obtained by directly controlling their intrinsic solution properties without the need for extrinsic techniques such as grooved substrates or blade patterning.