Printing Conjugated Polymers to Order via Non-Equilibrium Assembly*

Controlled morphology evolution via directed assembly has played a central role in the development of modern electronic, optical and clean energy materials. In comparison to conventional ‘hard’ materials, polymer-based functional materials can be easily processed into diverse form factors by low-cost, high-throughput methods such as roll-to-roll printing and 3D printing. The printing conditions intimately couple with the assembly process and sensitively modulate the solid-state properties in the fabricated devices. We are particularly interested in semiconducting polymers which have demonstrated potential uses in a diverse range of applications from transistors, thermoelectrics, sensors, light-emitting diodes to solar cells etc. However, major challenges remain, in controlling the nucleation, growth and aggregation of conjugated polymers during solution printing and coating, which critically impact the printed device performance by orders of magnitude. The rapid printing process creates a complex environment with coupled physics that drive the polymer assembly far from equilibrium.

In our work, we combine printing experiments, morphology and device characterizations with governing-equation-based modeling and simulations to present new insights and strategies for controlling multi-scale assembly of semiconducting polymers. We observe unexpected flow-induced morphology and electronic transition that accompanies change in printing regimes. We elucidate that printing flow in a moving, drying meniscus can drastically alter the polymer assembly pathways by flow-induced conformation change. We further establish tools for investigating design rules for flow-induced conjugated polymer crystallization. High degree of morphology control from molecular to device scale further enables new insights into charge transport properties of semiconducting polymers and realizes advanced electronic device applications.