Long range ordering in poly(3-hexylthiophene) fluids and films: Implications for organic electronics

Abstract

Understanding and controlling polymer semiconductor morphology is critical to achieving high charge carrier mobility and enabling widespread use of organic electronic devices such as organic photovoltaics, organic light-emitting diodes and organic field-effect transistors (OFETs). In the studies described herein, micro- and macro-scale ordering of model conjugated polymer poly(3-hexylthiophene) (P3HT) were achieved both in the solution state and film state and the impacts on charge transport were explored. The self-assembly of P3HT through solution aging over time formed long nanofibers that can align in the fluid state, as monitored by polarized optical microscopy, and polarized Raman spectroscopy. When induced by ultrasonication, the fibers that formed were significantly shorter than with aging and it was discovered that the length of these fibers played a critical role in their ability to align, and maintain alignment in the solid state. The microstructural features formed through these solution processing techniques, as determined by atomic force microscopy, were correlated with enhanced field-effect mobility as measured by OFETs. More controlled alignment of P3HT nanofibers was then explored through blade coating, a technique less wasteful than spin coating and more amenable to roll-to-roll fabrication. P3HT nanofibers were successfully aligned over large areas (tens of millimeters), enabling the investigation of macro-scale orientational ordering on charge transport anisotropy, and giving insight into the important roles of both intra- and intermolecular charge transport in P3HT nanofiber films for organic electronics applications.