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Erfan Mohammadi1 Fengjiao Zhang1 Ying Diao1

1, University of Illinois at Urbana Champaign, Urbana, Illinois, United States

Printed electronics based on solution processable semiconducting polymers has emerged as a burgeoning technology that promises to revolutionize electronics manufacturing from transistors, sensors, solar cells, light-emitting diodes (LEDs), to medical devices. Unlike traditional electronic manufacturing that requires elevated temperature and high vacuum, organic electronics can be printed at near ambient conditions on flexible substrates to produce light-weighted, biointegrated electronics at low cost and large scale. It is well-known that substrate surface properties have a profound impact on morphology of thin films solution coated atop and the resulting solid-state properties. Therefore, it is important to establish general design rules to better understand surface-induced crystallization and develop material-agnostic methods for controlling morphology across multiple length scales at once during coating, not only to enable large-scale manufacturing of high-performance devices, but also for elucidating charge transport mechanism in conjugated polymers.

We have demonstrated that by modulating the free energy barrier to heterogeneous nucleation multiscale morphology of donor-accepter (D-A) conjugated polymers can be controlled during meniscus-guided solution coating [1, 2]. Lower nucleation free energy is associated with faster polymer crystallization addressing the disparity in time scales of polymer assembly and high-throughput coating. By conducting in-depth morphology characterizations, we concluded that surfaces with lower free energy barrier would increase 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 transistors along the polymer backbone as well as the pi-pi stacking direction. We further developed a free energy model, for the systems following classical nucleation theory, relating the substrate surface energy to the penalty of heterogeneous nucleation from solution in the thin film geometry. The model correctly predicts the experimental trend. The introduced generic model introduced 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.

References:
1- Zhang, F.*; Mohammadi, E.*; Luo, X.; Strzalka, J.; Mei, J.; Diao, Y. Lanmguir 2017, 8, 16070.
2- Mohammadi, E.; Zhao, C.; Zhang, F.; Qu, G.; Meng, Y.; Zhao, X.; Mei, J.; Zuo, J.; Shukla, D.; Diao, Y. Nat. Comm. 2017, 8, 16070.

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