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

Conjugated polymers—plastics capable of charge transport—are promising candidates for electronic devices such as field effect transistors, solar cells, and flexible sensors. In thin films, interfaces (such as the air-polymer interface) are known to play an important role in structure formation and final device properties, but the exact relationship is not well understood. To understand how a free surface interface affects morphology, orientation, and final properties across the thickness of P3HT (a model conjugated polymer) thin films, this study combined in-situ UV-vis spectroscopy and grazing incidence wide angle X-ray scattering (GIWAXS). A two-step crystallization process was revealed: first, surface-induced highly edge-on oriented spherulite-like structures formed in a ~20 nm layer at the air-polymer interface at T ~25 °C higher than the bulk crystallization temperature (T_C,BULK) regardless of the film thickness. Then, the remainder of the film crystallized near T_C,BULK with orientation that depended on film thickness. Overall, the results demonstrate that the air-polymer interface has a significant effect on the crystallization and final orientation of P3HT which is expected to have large implications for device performance.