Direct Observation of Two-Step, Stratified Crystallization and Morphology in Conjugated Polymer Thin Films

A two-step stratified crystallization process has been directly observed during cooling of poly(3-hexylthiophene) (P3HT) thin films of thickness h = 20-250 nm: a thin (<20 nm) layer at the free surface crystallizes similar to 25 degrees C higher than the bulk crystallization temperature (TC,bulk), whereas the rest of the film (i.e., the underlayer) crystallizes near TC,bulk. In situ measurements of films with and without a free surface unequivocally ascribe the high-temperature crystallization to a surface-induced process, which correlates with the formation of large birefringent structures and highly oriented edge-on crystallites at the air-polymer interface. In contrast, crystallization of the bulklike underlayer leads to mostly edge-on-oriented crystallites in thinner films and becomes increasingly isotropic in thicker films. For h < 20 nm, free-surface effects dominate and only hightemperature crystallization is observed. These results highlight the potential of melt crystallization to tailor morphology and orientation across thin film thickness for specific electronic applications.