Two-step, stratified crystallization in conjugated polymer thin films: The role of interfaces and molecular characteristics.

Charge transport in conjugated polymers films is dictated by 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 and reveal a two-step crystallization process where the free surface induces distinct, highly edge-on oriented crystallization which occurs at temperatures much higher than for the underlayer and which remains confined within 20 nm of the air-polymer interface. By covering the films, nucleation and growth of "bulk objects" with overall isotropic orientation can be directly observed. 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 regioregularity and molecular weight impact crystallization, and that the 2-step crystallization can be observed in other materials as well. These results provide insights on the potential of using interfacial effects to direct crystallization and morphology in conjugated films.