Selective Nucleation of Poly(3-hexyl thiophene) Nanofibers on Multilayer Graphene Substrates

Abstract

We demonstrate that graphene surfaces provide highly selective nucleation of poly(3-hexyl thiophene) (P3HT) nanofibers (NFs) from supersaturated solutions. Solvent conditions are identified that give rise to a wide hysteresis between crystallization and melting centered around room temperature, yielding metastable solutions that are stable against homogeneous nucleation for long periods of time but that allow for heterogeneous nucleation by graphene. Selective growth of P3HT crystals is found for multilayer graphene (MLG) supported on either Si or ITO substrates, with nucleation kinetics that are more rapid for MLG on Si but slower in both cases than for highly oriented pyrolytic graphite (HOPG). Although the NFs grow vertically from the substrate with face-on orientation of P3HT chains, we observe edgeon orientation in dried films, presumably due to capillary forces that cause collapse of the NFs onto the substrate during solvent evaporation. S olution-based assembly of conjugated organic materials represents a simple and scalable route to tailored crystalline nanostructures, offering potential pathways to low cost electronic devices with improved performance. 1 While conventional techniques to enhance crystallinity within device active layers (e.g., vapor or thermal annealing) often lead to structural coarsening, solution-based self-assembly can deliver highly crystalline structures with controlled nanoscale morphologies. 2 In the case of polymer-based heterojunction photovoltaic cells, which benefit from interpenetrating arrangements of crystalline domains of two materials, 2,3 one-dimensional (1D) crystalline nanostructures assembled in solution have been used to enhance device performance. 4,5 Moreover, attaining the proper orientation of conjugated molecules within active layers can enhance charge mobility along the relevant direction (i.e., toward the electrodes), offering further improvements in performance. 6−9 For conjugated polymers, high charge mobility is realized along the polymer chain backbone as well as through overlapping π orbitals between adjacent chains. 7 Therefore, an "edge-on" orientation (with the π planes of the polymer oriented perpendicular to the substrate) is beneficial for field effect transistors, which require high in-plane mobility values, 6 while "face-on" orientation (backbone π planes parallel to the substrate) is sought to increase out-of-plane charge transport in photovoltaic devices. 8 Techniques for nanostructure assembly to provide the desired molecular ordering and orientation include the use of dip coating, 10 substrate rubbing, 11 zone casting, 12 and substratedirected epitaxial growth. 13,14 Regioregular poly(3-hexyl thiophene) (P3HT) is among the most extensively studied conjugated polymers, selected for its high mobility (0.1 cm 2 V −1 s −1 ), ease of synthesis, and tendency to assemble into crystalline nanofibers (NFs) suitable for electronic devices. 4,15,16 Graphene 17 and carbon nanotubes 18 represent attractive platforms for directing organization of P3HT into...

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