Precrystalline P3HT nanowires: growth-controllable solution processing and effective molecular packing transfer to thin film

Abstract

Solution-processable precrystalline nanowires (NWs) of conjugated polymers (CPs) have garnered significant attention in fundamental research based on crystallization-driven self-assembly and in the roll-to-roll fabrication of optoelectronic devices, such as organic field-effect transistors, diodes, and solar cells. The highly crystalline NWs manufactured using a suitable solvent system ensure the improved performance of the solution-processable devices. Herein, we report a comprehensive understanding of how solubility-considering binary solvent selectivity and their mixing rates affect the growth of poly(3-hexylthiophene) (P3HT) into NWs and the transfer of the crystallinity and molecular orientation of precrystalline NWs determined in solutions to thin film. The crystal growth rate was controlled by varying the mixing rate of a good solvent (chloroform, CF) and various orthogonal solvents (marginal-, poor-, and non-solvents) with respect to the P3HT. Highly crystalline P3HT NWs were successfully obtained by slow diffusion of orthogonal solvents into a good solvent, even when non-solvents with vastly different Hansen solubility parameters to P3HT, such as acetonitrile and methanol, were used. The crystallinity and molecular orientation of precrystalline NWs were determined by the solvent quality and mixing rate, and their transfer efficacy was rationalized by considering the difference in the surface energies and boiling points between binary solvents. The large difference between CF and acetonitrile led to the formation of rapid, massive large aggregates of P3HT with precrystalline P3HT NWs after CF evaporation, providing the thin film deterioration. As a result, the well-defined NWs with high crystallinity, long conjugation length, and highly ordered edge-on molecular orientation were successfully fabricated in a mixed solvent of chloroform and hexane showing small differences in boiling point and surface tension, exhibiting the high charge-carrier mobilities in thin film devices.