Understanding Comparable Charge Transport Between Edge-on and Face-on Polymers in a Thiazolothiazole Polymer System

The orientation of semiconducting polymers is one of the most important factors for the charge transport and thus the performance of polymer-based devices such as organic field-effect transistors and organic photovoltaics. Here, we study thiophene–thiazolothiazole polymers with different alkyl side chains, which apparently form the “edge-on” or “face-on” orientation depending on the side chain composition. Interestingly, hole mobilities observed in transistors were seemingly independent of the backbone orientation. However, in-depth X-ray analysis revealed that while the edge-on polymers evenly had the edge-on to face-on fraction throughout the film thickness, the face-on polymers had the unevenly distributed edge-on to face-on fraction: the edge-on orientation was abundant at the film–substrate interface and the face-on orientation was abundant in the bulk. Thus, in fact the edge-on was the dominant orientation at the film–substrate interface in all the polymers, which agrees well with the high hole mobility in the face-on polymers that was comparable to the edge-on polymers. Our findings provide good guidelines for in-depth understanding of structure–property relationship in semiconducting polymers and their optoelectronic devices.