465694 Direct, Single-Step Alignment of Solution-Sheared Donor-Acceptor Polymer Thin Films and Factors Influencing Their Deposition
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 – 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 films’ 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.