Orientation Enhancement in Donor-Acceptor π-Conjugated Polymer Thin Films by a Cooled High-Speed Bar-Coating Method

π-Conjugated polymers are promising materials for next-generation flexible and lightweight electronics owing to their solution processability and excellent charge transport properties, where molecular orientation is critical for efficient charge transport. However, maintaining high molecular orientation at high coating speeds required for industrial production remains challenging, particularly when using volatile, low-boiling-point solvents. In this study, we employed a cooled, high-speed bar-coating method at 50 mm s^-1 to fabricate highly oriented films of poly{2,2'-[(2,5-bis(2-octyldodecyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)dithiophene-5,5'-diyl]-alt-thieno[3,2-b]thiophene-2,5-diyl} (PDPP-DTT) from low-boiling-point solvents. For volatile solvents such as chloroform (bp: 61 °C) and trichloroethylene (bp: 87 °C), substrate cooling suppressed rapid solvent evaporation and random crystallite nucleation, enabling highly oriented films. Time-resolved polarized UV-Vis absorption spectroscopy further revealed that molecular orientation develops during the solidification process, suggesting the formation of a lyotropic liquid crystalline phase guided by a weakly oriented template layer. The degree of orientation strongly depended on both solvent volatility and substrate temperature. While cooling was essential for volatile solvents, excessive cooling of less volatile solvents, such as chlorobenzene (bp: 132 °C), resulted in reduced orientation. These results highlight the trade-off between suppressing random nucleation and avoiding orientation relaxation. By balancing solvent volatility with substrate temperature control, this strategy enables scalable, low-temperature fabrication of high-performance polymer films compatible with flexible substrates.