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We studied the molecular packing and crystallization of a highly regio-regular semiconducting polymer poly(3-hexylthiophene) (P3HT) on both single layer graphene and silicon as a function of temperature, during cooling from the melt. The onset of crystallization, crystallites' size, orientation, and kinetics of formation were measured in situ by synchrotron grazing incidence X-ray diffraction (GIXD) during cooling and revealed a very different crystallization process on each surface. A favored crystalline orientation with out of plane π-π stacking formed at a temperature of 200 °C on graphene, whereas the first crystallites formed with an edge-on orientation at 185 °C on silicon. The crystallization of face-on lamellae revealed two surprising effects during cooling: (a) a constant low value of the π-π spacing below 60 °C; and (b) a reduction by half in the coherence length of face-on lamellae from 100 to 30 °C, which corresponded with the weakening of the 2nd or 3rd order of the in-plane (k00) diffraction peak. The final ratio of face-on to edge-on orientations was 40% on graphene, and 2% on silicon, revealing the very different crystallization mechanisms. These results provide a better understanding of how surfaces with different chemistries and intermolecular interactions with the polythiophene polymer chains lead to different crystallization processes and crystallites orientations for specific electronic applications.

In situ probing of the crystallization kinetics of rr-P3HT on single layer graphene as a function of temperature