Interfacial Design to Direct Crystallization in Thin Films of Conjugated Polymers

Charge transport in polymer-based thin films is determined by the morphology that develops during processing, but the complex interplay between structure development, processing conditions, and film interfaces remains elusive. Here, we investigate the role of interfacial effects on crystallization and structure development in polymer-based films of poly-3-alkylthiophene (P3AT) films. We show that P3AT films with an air-polymer interface show a two-step crystallization process, where the free surface induces highly edge-on oriented crystallization of the top ~ 20 nm at temperatures much higher than the bulk-like crystallization of the rest of the film. Therefore, surface freezing is general phenomenon in P3ATs, but the characteristic free surface-induced “circulite” structures are not always observable due to changes in nucleation density at the air-polymer interface. Additionally, we show that overall crystalline orientation can be manipulated over a wide range of orientations through interfacial design, and that interfacial effects are also key to structure development and melting in multi-component films.