Control of Molecular Orientation in Polydiketopyrrolopyrrole Copolymers via Diffusive Noncovalent Interactions

In thin films of semiconductor polymers, the polymer chains often exhibit distinct orientation with respect to the substrate. The planar π-face of the backbone typically orients either in an edge-on or face-on manner. Generally, an edge-on alignment is thought to be favorable for transport in thin film transistors, whereas face-on alignment is considered to improve vertical transport as desired in solar cells. However, molecular orientation is among the very few parameters that usually cannot be controlled when tailoring new semiconducting polymers. Here we show for an important class of semiconducting polymer that both the mode of orientation as well as the degree of alignment can be well-controlled by exploiting diffusive noncovalent interactions along the backbone. Studying polydiketopyrrolopyrroles (PDPPs) as a case study, by strategically varying chemical structure, we demonstrate systematic variation in molecular orientation with degree of chain planarization resulting from different degrees of diffusive noncovalent interactions. This observation opens the possibility of controlling and optimizing the orientation of semiconducting polymer chains in thin films by rational design. We anticipate our findings to open the door to new high-performance organic semiconductors with the additional benefit of tailored orientation that fits the desired application.