Charge transport in conjugated polymers films is dictated by their semicrystalline morphology, but the complex interplay between film interfaces, processing conditions, molecular characteristics, and crystallization remains elusive. Here, we investigate the role of interfaces on solvent-free crystallization of poly-3-hexylthiophene (P3HT) films. In films with an air-polymer interface, we reveal a two-step crystallization process where the free surface induces distinct, highly edge-on oriented crystallization at temperatures much higher than the rest of the film—and which remains confined within 20 nm of the air-polymer interface. However, by covering the films, a single crystallization event occurs which yields nucleation and growth of “bulk objects” with overall isotropic orientation. Unexpectedly, disc-like spherulitic structures (“circulites”) at the air-polymer interface exhibit linear growth rates that are at least ~ 10x faster than those of bulk objects—even though circulites crystallize ~ 25 °C higher than bulk objects and at temperatures where chain mobility is not expected to dominate growth rates. We also show that the specific combination of regioregularity and molecular weight of P3HT impact the crystallization process, and that free surface-induced crystallization can also be observed in other materials. These results provide insight on the potential use of interfacial effects to direct crystallization and morphology in conjugated films.
