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 °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.