Tailoring Polymorph, Crystal Orientation, and Large-Scale Alignment in Conjugated Polymers via Rubbing for Field-Effect Transistors

The ability to tune the polymorph and crystal orientation of semiconducting conjugated polymers provides a powerful platform for revealing processing–structure–property relationships. In contrast to the widely studied poly(3-alkylthiophene)s, their close poly(3-alkylselenophene) analogs have been much less studied. Herein, we report tailoring the polymorphs and crystal orientations in poly(3-hexylselenophene) (P3HS) of different molecular weights (MWs) via rubbing and elucidate the relationships between the crystalline structure and charge transfer characteristics for organic field-effect transistors (OFETs). In particular, the as-cast P3HS-3K film exhibits polymorph II with a dominant flat-on crystal orientation; however, it transforms into polymorph I with a face-on crystal orientation upon rubbing at elevated temperatures. Moreover, the as-cast P3HS-22K film transitions from edge-on to face-on with its polymorph I retained throughout the rubbing process. The polymorph and crystal orientation transitions in the P3HS-3K and 22K systems are accompanied by the formation of large-scale highly aligned crystalline films along the rubbing direction. The polymorph transformation kinetics during the rubbing process and associated alterations in the crystalline packing and photophysical properties are investigated. The various P3HS films exhibit OFET performance that shows a strong relationship with the attained crystalline structures. Such polymorph and crystal orientation transformations involving the rubbing technique are readily applied to other conjugated polymers in interest. This research emphasizes tailoring the crystalline structures and, thus, the charge transfer characteristics of conjugated polymers, which advances their use in optoelectronic-device applications.