Molecular Orientation in Thin Films of Poly(3-hexylthiophene) And Poly(3-(6-bromohexyl)-thiophene) Crystallized on Graphene

Recent findings have shown that crystallization of polymers on a substrate is advantageous for inducing molecular orientation, especially for semicrystalline conjugated polymers due to their anisotropic charge transport properties. Although graphite induces face-one crystal orientation in monolayers of poly(3-hexylthiophene) (P3HT), a full face-one orientation in thicker P3HT films was not realized so far. The latter is assumed to be a result of two competing interfacial orientations due to different interactions of P3HT side chains with vacuum and the graphite substrate. Here we show that modification of the chemical structure of P3HT side chains can alter the interfacial interactions and result in completely face-on oriented crystals. Specifically, we present a comparative study of the substrate induced molecular orientation of poly(3-(6-bromohexyl)-thiophene) (P3BrHT) and P3HT. The crystal orientation in ultrathin films of both polymers crystallized on a single layer graphene was explored by grazing incidence XRD. The results indicated that P3BrHT on graphene had solely face-on oriented crystals, whereas P3HT on graphene showed mixed crystal orientation with edge-on crystals formed at the top surface.