Two-step, stratified crystallization in conjugated polymer thin films: the role of interfacial effects.

It is well known that crystallization of classical polymers is extremely sensitive to molecular characteristics and processing conditions, so even small changes can result in significantly different morphologies and properties. Likewise, charge transport in conjugated polymers—which are typically processed into thin films—critically depends on their semicrystalline structure, but the complex interplay between film interfaces, processing conditions, chain structure, and crystallization remains elusive and hinders the development of a predictive model to attain optimal properties. Here, we investigate the role of interfaces on solvent-free crystallization of poly-3-hexylthiophene (P3HT). In particular, we reveal a two-step crystallization process in P3HT films 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. We will discuss the potential of using interfacial effects to direct crystallization and morphology in films, as well as the role of molecular weight, regioregularity, and self-seeding.