Authors: Erfan Mohammadi
Source: IDEALS (University of Illinois Urbana-Champaign) (article, 2019-04-15)
Field: Materials Science — Polymers and Plastics

Dynamic-template-directed crystallization of semiconducting polymers for organic electronics Solution processable semiconducting polymers have been under intense investigations due to their potential applications in large-area printed electronics from solar cells to biomedical devices. However, controlling the multiscale morphology and device performance of printed polymer thin films remains a key challenge. This is largely due to the disparity in time scales of polymer assembly and high-throughput manufacturing. Designing novel substrate/ink interfaces to induce crystallization is particularly a promising strategy to address this challenge given that heterogeneous nucleation is energetically more favorable than that of the bulk.\nDynamic surfaces play a critical role in templating highly ordered complex structures in living systems (e.g., biomineralization). This strategy is rarely employed for directing assembly of the synthetic functional materials. In my thesis, I introduce the concept of dynamic templating to expedite polymer nucleation and address the kinetic mismatch between polymer crystallization and high-speed solution coating. The solid-state properties of the dynamic-templated films are comprehensively characterized and correlated to the template properties.\nI designed an ionic-liquid-based dynamic template compatible with solution coating and fabricated highly ordered polymer thin films over large area (&gt;1cm2). Dynamic-templated films showed 2 orders of magnitude increase in domain size, over 10 times higher in-plane backbone alignment and dramatic out-of-plane order compared to the static substrates. These results were independent of coating conditions and led to 2-fold enhancement of hole transport mobility. Simulation results suggest that surface reconfigurability is key to promoting template–polymer interactions. The result of such enhanced interaction is lower nucleation barrier and expedited crystallization process which is responsible for the improved morphological characteristics.\nI further established the dynamic-template-directed assembly design rules revealing the critical role of template dynamics and chemistry to promote template-polymer interactions. I demonstrated that the concept of dynamic templating can be expanded to other non-ionic templates. However, the effectiveness of the templates is governed primarily by the strength of template-polymer interactions measured directly via adsorption enthalpy. Increasing template-CP interactions leads to larger domains, higher alignment and crystallinity as well as enhanced device performance even at very high coating speeds (0.1 m.s-1). \nFinally, I designed the second-generation dynamic templates using ion gels with widely tunable dynamics via chemical composition. By modulating template properties, I systematically modulated the degree of alignment over 55 times and increased crystallinity &gt;49%. Correspondingly, charge transport was improved by up to 4 and 11 times along both polymer backbone and π-π stacking direction, respectively. Apparent hole mobilities exceeded 3.0 cm2V-1s-1 which is way higher than the minimum required performance to use in commercial displays. Intriguingly, I discovered a synergistic effect between the gel components that produces enhanced templating effect outperforming the neat components. Our experimental and computational studies suggest complementary multivalent interactions to be responsible for this synergy. The generality of this notion is confirmed by designing multicomponent templates and analyzing polymer thin film morphology. Our methodology and mechanistic understanding have broad implications, given the importance of surface-induced assembly of functional materials across disciplines. Crystallization ; Polymer ; Electronics ; Materials science ; Organic electronics ; Nanotechnology ; Organic polymer ; Process engineering ; Polymer science ; Chemical engineering ; Electrical engineering ; Engineering ; Transistor ; Composite material Conducting polymers and applications