Structural transitions of nanocrystalline domains in regioregular poly(3-hexyl thiophene) thin films

The effects of solution processing and thermal annealing on thin film morphology and crystalline structures of regioregular poly(3-hexyl thiophene) (RR P3HT) are studied in terms of molecular weight (M_w). Using grazing-incidence X-ray diffraction, π-conjugated planes in drop-cast films from chloroform solutions are found to be preferentially oriented parallel to the substrates regardless of M_w. However, the mesoscale nanocrystalline morphology of the drop-cast films is significantly affected by M_w, exhibiting a distinctive morphological transition from short nanorods to long nanofibrils above a critical number-averaged M_w (∼ 3.6 kDa). This is probably due to the change in a conformation change from an extended-chain to a folded-chain, as M_w of RR P3HT increases. In contrast, spin-casting of high M_w RR P3HT produces less ordered films with a lower crystallinity and mixed parallel/perpendicular orientations of π-conjugated planes. The crystallinity and parallel π-conjugated orientation of RR P3HT in spin-cast films could be improved by thermal treatments at high-temperatures either (1) above the glass transition temperature or (2) above the melting temperature of RR 3PHT followed by recrystallization upon cooling under vacuum. However, the charge mobility of the spin-cast films for a field-effect transistor application is still lower than that of the drop-cast films. This would be attributed to the chain oxidation and the development of distinct grain boundaries between RR P3HT nanofibrils during the thermal treatments.