The macroscopic alignment of conjugated polymers with low grain boundary is essential to carrier transport. We investigated the formation mechanism of the aligned films of the donor-acceptor copolymer of 3,6-bis(thiophen-2-yl)-N,N'-bis(2-octyl-1-dodecyl)-1,4-dioxo-pyrrolo [3,4-c]pyrrole and thieno- [3,2-b]thiophene (PDBT-TT) by solvent vapor enhanced drop-casting (SVE DC). The dichroic ratio (DR) of the SVE DC films was 4.83 which were higher than the drop-casted (DC) films (1.43). There were large areas of bright stripe-like zones in the optic microscopy morphology and parallel fibers in the microscopic morphology of the SVE DC films. The diffraction arcs of TEM images of the SVE DC films were nearly two relatively narrow spots. These phenomena suggested the SVE DC films were in alignment (several hundreds of micrometers). The stripe-like zones were formed along the contact line receding and were observed in situ during the SVE DC films formation. The parallel fibers were formed along the molecular backbones due to the molecular backbones were sheared to rearrange in alignment along the direction of the contact line receding and had enough time to assemble to aligned fibers and improve the crystallinity when the contact line receded slow (1 mu m/s). The crystallinity and alignment were improved simultaneously via SVE DC. (C) 2016 Published by Elsevier Ltd.