(272c) Direct, Single-Step Alignment of Solution-Sheared Donor-Acceptor Polymer Thin Films and Factors Influencing Their Deposition

Organic semiconductors (OSCs) have properties that allow them to fill an application space beyond silicon. Continuous fabrication processes, such as roll-to-roll printing, are highly desirable for lowering the industrial and economic barriers that impede a more widespread deployment of organic electronics in the marketplace. Solution shearing is a deposition technique developed by our group that not only can be easily adapted to such processes, but also has been demonstrated to produce highly aligned thin films of small molecule OSCs. Such alignment of OSCs in the active layers of electronic devices can impart desirable properties, such as polarized light emission or absorption, reduced device cross-talk due to charge transport, and enhanced charge transport due to better ordering.

While the alignment of small molecule OSCs has been widely demonstrated, we have also observed alignment in a donor-acceptor polymer based on a diketopyrrolopyrrole-terthiophene backbone deposited directly from solution without surface templating or complex pre- or postdeposition processing ­â?? a dichroic ratio up to ~7 was achieved with the solvent tetralin. The degree of alignment in the thin films were tuned by controlling the coating speed and was found to be a competition between the shear alignment of polymer chains in solution and the complex thin film drying process. Unlike previous reports, no charge transport anisotropy was observed because of the filmsâ?? meshlike morphology, small crystallite size relative to the channel length, and the likelihood of more grain boundaries perpendicular to the coating direction. Coupled with the observed anisotropy in X-ray diffraction data, these results are consistent with polymer alignment in both the amorphous and crystalline regions of the films.

The shear speed at which maximum dichroism is achieved can be tuned with deposition parameters like temperature and substrate treatment. Longer polymer alkyl side chains were found to reduce the degree of alignment, and a change in molecular weight showed negligible effects on alignment. Solution shearing, our work shows, can be used to tune polymer film alignment in a single-step process without relying on substrate patterning or postdeposition treatments.