Role of the Air-Polymer Interface on the Crystallization Behavior of Poly(3-hexylthiophene)

Poly(3-hexylthiophene) (P3HT) is a conjugated polymer—which are electrically conductive plastics that show promise in electronic devices such as field effect transistors, solar cells, and flexible sensors. Such devices require thin films where interfaces play a crucial role in structure formation and final device properties; however, their exact influence is not well understood. During the melt-crystallization of P3HT, a two-step crystallization process is observed. Surface-induced highly edge-on oriented crystallites and spherulite-like structures formed in a ~20 nm layer at the air-polymer interface at temperatures much higher than the bulk crystallization temperature (~25 °C higher than T_C,BULK) indicative of surface freezing. Between the substrate and crystallized surface (i.e. the underlayer), crystallites form near T_C,BULK with much lower edge-on orientation than at the air surface. Without the air-polymer interface (i.e. when the sample is coated in aluminum), only 1 crystallization event is observed: crystallization occurs near T_C,BULK with isotropic orientation (even in very thin ~40 nm films) and no spherulites are observed. Overall, surface freezing is directly observed in P3HT and is accompanied by significant changes in crystallization and the final molecular orientation and morphology which are expected to have large implications for device performance.