Elucidating the Effect of Interfacial Interactions on Crystal Orientations in Thin Films of Polythiophenes

Molecular orientation is crucial for improving the efficiency of organic electronic devices. In many ordered materials, orientation is achieved by directional crystallization on a substrate but usually not in semicrystalline conjugated polymers. Specifically, a full face-on molecular alignment of poly(3-hexylthiophene) (P3HT) on a substrate has not been realized so far. Here, it is found that P3HT crystallized on graphene exhibits a double-layered edge-on and face-on crystal orientation with edge-on crystals formed on the top surface. This finding is interpreted as a result of two competing interfacial interactions of P3HT chains with graphene and vacuum. By modifying the side-chain chemical composition in poly[3-(6-bromohexyl)]thiophene, the influence of the interface to vacuum can be reduced, resulting in full face-on orientation in films with a thickness of up to 26 nm. These results demonstrate that directed crystallization can be used to control the orientation of semicrystalline functional polymers in thin films given that the interaction with both interfaces is taken into account.