Interrogating Polymorphism in Conjugated Poly(thieno)thiophene Thin Films for Field-Effect Transistors

The ability to modulate the polymorphism of semiconducting conjugated polymers (CPs) renders the scrutiny of the correlation between their molecular structure and charge transport characteristics. Surprisingly, investigation into the control over polymorphism to boost the charge mobility is few and limited in scope. Herein, we report, for the first time, two distinct polymorphisms in conjugated poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) thin films, the transformation between them via thermal and solvent vapor annealing, and the correlation of the respective polymorphism to charge transport properties. Specifically, a series of PBTTTs with different alkyl side chains (i.e., hexyl, octyl, decyl, and dodecyl; denoted PBTTT-6, PBTTT-8, PBTTT-10, and PBTTT-12, respectively) and molecular weights (MWs) are synthesized and their polymorphisms are examined by varying both the intrinsic (i.e., alkyl side-chain lengths and MWs) and extrinsic (i.e., thermal and solvent vapor annealing) parameters. Initially, type II (i.e., possessing a smaller interlayer spacing along the alkyl side-chain direction) exists in all as-cast PBTTT films. Upon thermal annealing at different temperatures (150 and 250 degrees C), PBTTT-6 and PBTTT-8 with shorter alkyl side chains and low MWs favor the transition from type II to type I (i.e., experiencing a larger interlayer spacing). Intriguingly, the type I PBTTT can be readily reverted to type II via subsequent solvent vapor annealing, and such polymorphism transition is highly reversible. The kinetics of the polymorphism transformation as well as the accompanied change in molecular stacking are elucidated. Finally, the charge transport properties of PBTTT thin films are found to depend sensitively on their crystalline structures, including the polymorphism change. This study provides insights into the control over the PBTTT polymorphisms and their intertransition via synergetic intrinsic molecular engineering and extrinsic post treatments.