Selecting for Surface-Induced vs Bulk Crystallization in P3HT Thin Films: Distinct Morphology, Orientation, and Linear Growth Rates

By removal of the air–polymer interface in poly(3-hexylthiopene) (P3HT) thin films, bulk crystallization morphology is attained, where highly absorbing "bulk objects" grow throughout the film thickness at T ∼ TC,BULK and exhibit no preferential crystalline orientation. In contrast, films with a free surface develop highly edge-on oriented 2D spherulites ("circulites") at the air–polymer interface at T ≫ TC,BULK, which then promote edge-on oriented, featureless crystallization in the underlayer near TC,BULK. Interestingly, circulites exhibit ∼10× faster linear growth rates than bulk objects─despite both morphologies consisting of the same crystal form─at temperatures where chain mobility is not expected to dominate growth rates, suggesting that another mechanism may be at play. Overall, these results provide insight into the role of the free surface in the crystallization process and indicate that the presence or absence of an air–polymer interface can be used to control the semicrystalline morphology and orientation of P3HT thin films.