Toward Precision Control of Nanofiber Orientation in Conjugated Polymer Thin Films: Impact on Charge Transport

The deposition of conjugated polymers is typically subject to chain-entanglement effects, which can severely hinder chain unfolding, alignment, and pi-pi stacking during rapid solution-coating processes. Here, long-range ordering and highly aligned poly(3-hexylthiophene) (P3HT) thin films were demonstrated by preprocessing the polymer solution with ultraviolet (UV) irradiation/solution aging and then depositing via the blade-coating method, which is compatible with roll-to-roll printing processes. The surface morphologies and optical anisotropy of deposited films revealed that the degree of chain alignment was greatly improved with increased levels of polymer assembly that can be precisely controlled by solution-aging time. The correlations between oriented nanofibrillar structures and their charge-transport anisotropy were further systematically investigated by blade coating pretreated solutions parallel and perpendicular to the direction of the source and drain electrodes. Interestingly, charge transport across the well-aligned P3HT nanofibers was more efficient than along the long-axis of nanofibrillar structures owing to enhanced intramolecular charge transport and tie-chains. The facile and scalable solution-coating method investigated here suggests an effective approach to induce anisotropic crystalline structures, which are readily obtained by directly controlling their intrinsic solution properties without the need for extrinsic techniques such as surface templating or shearing blade patterning.