(417d) Critical Role of Surface Energy in Guiding Crystallization of Solution-Coated Polymer Semiconductor Thin Films

Solution processable semiconducting polymers have demonstrated potential applications in a diverse range of applications from printed electronics to medical devices, thanks to rapid materials innovation and six orders of magnitude improvement in charge carrier mobilities over the last thirty years. It is well-established that the electronic properties of semiconducting polymers depend drastically on morphological parameters across all length scales from intermolecular ordering to the degree of macroscale alignment. Therefore, it is critical to understand and control macromolecular assembly across length scales during non-equilibrium processing conditions of high-throughput solution printing/coating, not only to enable large-scale manufacturing of high-performance devices, but also for elucidating charge transport mechanism in conjugated polymers.

Substrate surface properties has been under intense investigation given the central role of interface-directed assembly in the manufacturing of modern electronics and energetic materials. In this talk, we discuss the critical role of surface energy in guiding surface-induced crystallization of solution-coated donor-acceptor polymer semiconductor thin films. Substrate surface energy was modulated systematically and by performing comprehensive morphology and device characterizations we established the surface energy – thin film morphology – charge transport property relationship. We discovered that surfaces with lower surface energy increased thin film crystallinity, degree of molecular ordering and extent of domain alignment in synergy with unidirectional flow. Notably, the enhanced morphology led to a significant increase in the charge carrier mobility in organic field-effect. We hypothesize that substrate surface energy influences pairwise interfacial properties between the substrate, the OSC and the solvent which in turn modulate free energy barrier to heterogeneous nucleation and ensuing polymer crystallization. Correspondingly, we further developed a generic free energy model for heterogeneous nucleation in thin films following classical nucleation theory. The model considered all pair-wise interfacial free energies between the substrate, the solute, and the solvent which correctly predicted the observed trend in surface-energy-dependent thin film morphology. Lower surface energy led to lower nucleation free energy which is associated with expedited polymer crystallization. and leaves longer time for growth and reorientation in the coating flow within the short time frame of solution coating. The introduced model is a significant step towards establishing design rules and understanding the critical role of substrates in determining morphology of solution coated thin films. Our methodology and mechanistic understanding have broad implications, given the importance of surface-induced crystallization across many disciplines.