Ultrapure Films of Polythiophene Derivatives are Born on a Substrate by Liquid Flow

Almost all films of conjugated polymers are composed of mixtures of two different structures called H- or J-aggregates, which are side-by-side and head-to-tail structures, respectively, between polymer chains. The mixture of H- and J-aggregates is responsible for the mixed optical and/or electrical properties of a conjugated polymer film. If a highly purified conjugated polymer film composed of only a single species of aggregate was obtained, its properties would enable the realization of new soft electronics, such as flexible solar cells, wearable sensors, and artificial skin. However, there have been no reports on a method to prepare such a highly purified polymer film so far. Here, we show a facile method to obtain an ultrapure 99.8% H-aggregate polymer film by dropping a solution of poly(3-hexylthiophene) (P3HT) on a substrate at room temperature, inspired by the coffee-ring effect. According to contact angle measurements, photoluminescence and Raman spectroscopy, and X-ray diffraction, a critical step for achieving the ultrapure H-aggregate film was a two-step drying process of the polymer solution droplet on a specific substrate. The pinning of the droplet caused this drying process and yielded a flow-induced extension of the P3HT chains on the substrate, followed by the formation of ultrapure H-aggregates. We also found a 6-fold enhancement in carrier density using the ultrapure (99.8%) H-aggregate P3HT film. Furthermore, five conjugated polymers of polythiophene derivatives, used as state-of-the-art polymer solar cells, revealed the same trends to ultrapure H-aggregates, whose structure enhances performance of solar cell. The proposed method could provide a new approach to make solution-processable films for next-generation soft and flexible electronics.