Control of Rubrene Polymorphs via Polymer Binders: Applications in Organic Field-Effect Transistors

Abstract

In this report, the crystallization mechanisms in solution-processed, annealed thin films of semiconducting small molecule 5,6,11,12-tetraphenylnaphthacene (rubrene) blended with three different amorphous polymers (polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(4-vinylpyridine) (P4VP)) are investigated. The results show that the degree of phase separation, the exact crystal structure, and the electronic properties of the blend films depend strongly on the choice of polymer binder. While rubrene films crystallized from blends with PS and P4VP consist of crystalline spherulites in a mostly orthorhombic crystal structure, rubrene in PMMA blends contains a significant fraction of triclinic phase and is generally more disordered. Structural characterizations also reveal a high degree of vertical phase separation in PS and P4VP films, which is attributed to residual solvent effects in the case of rubrene/PS films and a preferential hydrophilic interaction with the Si/SiO2 interface for rubrene/P4VP films. This type of phase separation is shown to be critical for crystallization and lead to improved field-effect mobilities. Finally, this processing technique easily allows for patterning of transistors using chemically modified substrates, which is useful for large-scale device fabrication.