Understanding of Face-On Crystallites Transitioning to Edge-On Crystallites in Thiophene-Based Conjugated Polymers

The orientation of crystallites (face-on or edge-on, in regard to the plane of the substrate) in semicrystalline conjugated polymer thin films has a significant impact on charge transport properties of such thin films. It has been observed in a number of conjugated polymers where predominantly face-on orientation in the as-cast film can transition to edge-on upon cooling from the melt (melt-annealing). However, why and how do face-on crystallites transition to edge-on during melt-annealing remain unclear. To answer these fundamental questions, we designed and synthesized a random copolymer (RP-3T) by inserting unsubstituted terthiophenes into regioregular poly(3-hexylthiophene)s. RP-3T shows a predominantly face-on orientation in an as-cast film but transitioning to an almost exclusively edge-on orientation upon annealing at 190 °C, a temperature below the Tm (194 °C). Through a detailed investigation on changes in the microstructure and orientation of crystallites in RP-3T with annealing temperature, we find that face-on crystallites in RP-3T start to disappear at 170 °C (notably lower than the Tm), while edge-on crystallites continue to grow as the annealing temperature increases toward the Tm. This finding provides a reasonable explanation for the transition of face-on orientation to edge-on orientation during thermal annealing in RP-3T, in particular, at a temperature below the Tm.