Nanotechnologies / Materials
Flexible device, Solar cell, Thin-film printing technology

Ultra-highly oriented thin-film coating based on rodlike-preaggregate
formation of π-conjugated polymer in solution
Division of Electrical, Electronic and Infocommunications Engineering, Graduate School of Engineering
Associate Professor Akihiko Fujii
https://researchmap.jp/read0054804?lang=en

Nanotechnologies / Materials

Abstract
A ultra-highly oriented thin film of a donor– acceptor conjugatedpolymer, poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt5,5-(2,5-di(thien-2-yl)thieno[3,2-b]thiophene)] (PDPP-DTT), whichtends to form a rod-shaped aggregate in solution, was fabricatedby bar-coating method. When the coating speed was 20 μm/s orless, a very highly oriented thin film with quite high 2D orientation
parameter S ≈ 0.9 was obtained. The alignment direction of thepolymer main chain strongly depended on the solution concentration. When the solution concentrations were 1 g/L and 3 g/L, themain chains were uniformly aligned parallel and perpendicular tothe coating direction, respectively. The molecular orientation mechanism was proposed by taking the aggregation phenomenon in thesolution into account.

new insights about the dynamics of the film structure formation including molecular conformation and orientation.

▍ Background & Results
By the recent progress of material development, the high solubility and processability of organic semiconducting materials
have been realized, and the thin film fabrication by some solution
processes for the printed electronics are enabled. For developing
organic semiconductor devices with excellent carrier transport
properties, the direct fabrication of highly oriented thin films fromthe solutions onto substrates is practically required. Herein, We
focused on PDPP-DTT, which is a candidate solution-processedmaterial and demonstrates the carrier mobility exceeding 10 cm2/Vs, and selected a bar-coating technique as a uniaxial solution
process to fabricate highly oriented thin films.
The order parameter of molecular orientation as well as film
thickness could be controlled by the sweep rate, and reached 0.9,
which is the highest value for π-conjugated polymers among onestep coating methods. Besides, the direction of the main chain was
selectively oriented parallel or perpendicular to the sweep direction, which depended upon the solution concentration under thelow sweep rate like 20 μm/s. Such unique orientation phenomena
should be related to the nematic liquid crystallinity of PDPP-DTT
aggregates formed under the slow coating process. The orientation
dynamics was interpreted by the existence of rodlike PDPP-DTT
preaggregates in the solution and their behavior due to the extension flow and shearing flow. Furthermore, the hole mobility depending upon the orientation direction, namely, the electrical anisotropy
was demonstrated.

▍ Significance of the research and Future perspective
In the PDPP-DTT solution, the small rodlike structure of the polymer chains forms the preaggregate, and the size of the preaggregate becomes larger as the concentration increased. The ultra-highmolecular orientation and its direction must be strongly related withthe rodlike preaggregate formation in the coating solution. Such athin-film coating process based on rodlike preaggregate formation
would be applicable to other π-conjugated polymers, and provide

P a t e n t
Yabuuchi, Yuta; Minowa, Yu; Fujii, Akihiko et al. Direction-Selectable Ultra-Highly Oriented State of Donor-Acceptor Conjugated Polymer Induced by Slow
Bar-Coating Process. Advanced Electronic Materials. 2021, 7, 2100313-1-8, doi: 10.1002/aelm.202100313
Yabuuchi, Yuta; Minowa, Yu; Fujii, Akihiko et al. Dynamics of Preaggregation and Film Formation of Donor-Acceptor π-Conjugated Polymers. ACS
Tr e a t i s e
Materials Letters. 2022, 4, p.205-211, doi: 10.1021/acsmaterialslett.1c00734
Minowa, Yu; Yabuuchi, Yuta; Fujii, Akihiko et al. Fast-Coating Process Based on Elongated Rodlike Preaggregate for Highly Oriented Thin Film of Donor–Acceptor Conjugated Polymer. ACS Applied Materials & Interfaces. 2022, 14, p.50112−50119, doi: 10.1021/acsami.2c13516
URL http://opal.eei.eng.osaka-u.ac.jp/K e y w o r d π-conjugated polymer, organic semiconductor, molecular orientation, printed electronics