No abstract

The molecular orientation of π‐conjugated polymers is a significant factor for the electrical properties of polymer‐based printable devices, because π‐conjugated polymers exhibit a notable fast carrier transport property along their main chain. Although various coating processes are suggested, the challenge to realize high‐degree alignment by one‐step coating without pre‐ or post‐treatments still remains. In this paper, it is reported on the fabrication of an extremely highly aligned thin film by a slow bar coating process using a donor–acceptor π‐conjugated polymer with great aggregability. The 2D order parameter achieved is 0.9, which is the highest obtained among the alignment methods with a one‐step coating process. Furthermore, the direction of the main chain is selectively oriented parallel or perpendicular to the coating direction depending on the solution concentration with high orientation degree. This unique orientation phenomenon seems to be related to the nematic liquid crystallinity of poly[2,5‐(2‐octyldodecyl)‐3,6‐diketopyrrolopyrrole‐alt‐5,5‐(2,5‐di(thien‐2‐yl)thieno [3,2‐b]thiophene)] aggregates formed by the slow coating process. The orientation dynamics is discussed by taking the structure and behavior of the aggregates in the solution into account.

It is important to understand the film formation mechanism of π-conjugated polymers to realize their high-performance electronic devices. In this study, the dynamics of the preaggregate, which affects the conformation and alignment of a typical donor−acceptor (D-A) π-conjugated polymer during film formation, was investigated by two approaches based on a smallangle X-ray scattering (SAXS) method. SAXS analysis of a highly concentrated solution, assuming the state before becoming the solid-state thin film, provided insight into the aggregation phenomena of the D-A π-conjugated polymer during the film formation process. In solution, the small rod-like structure of the polymer seems to form a large preaggregate, the shape of which almost coincided with that of the aggregate in the solid-state thin film. Besides, in situ grading incident SAXS (in situ GISAXS) analysis in the actual film formation process from the solution state revealed a microstructural change of the aggregate over time.

A fast meniscus-guided coating for ultrahighly oriented thin films of a typical donor–acceptor π-conjugated polymer, poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno[3,2-b]thiophene)](PDPP-DTT) was realized. A coating speed higher than 100 mm/s, which was regarded as a Landau–Levich regime, was applicable. The 2D order parameter (S 2) of the thin films changed by selecting the solvent and adjusting the initial concentration of the solution, and the large elongated rodlike preaggregates formed particularly in chlorobenzene contributed to the high orientation in the solid film state, resulting in the highest value of S 2 = 0.87. Focused on the PDPP-DTT preaggregate formation in the solution, the SAXS analysis was carried out to investigate the shape and size of the preaggregates. The mechanism of the molecular orientation was discussed by taking the preaggregates and the solution flow under the coating process into account.