Physics Department News

Replacing rare and expensive inorganic materials in today'’s electronics is becoming increasingly important as this vast industry grows. One option is organic semiconductors (OSCs) – they are cheap to make, disposable and can easily form the thin films required for applications such as solar cells and thin-film transistors. However, their performance is inferior to traditional crystalline inorganic semiconductors; most importantly, their charge carrier mobility is typically orders of magnitude lower. This is often dependent on the structure of the material: the more crystalline the OSC film, the higher its mobility. The structure of the film is dependent on deposition conditions, but also on the substrate. Crystalline substrates are conventionally used to epitaxially grow inorganic thin films, but this increases the cost. Further, it is not easy to integrate this approach with flexible substrates and there are strict criteria over which materials can grow epitaxially on which substrates.

Graphene is showing great promise as an OSC-compatible electrode/substrate: it is conductive, transparent, flexible and impermeable. Our work here shows another potential benefit from using graphene as a substrate: graphene’s flat, homogeneous surface can encourage the growth of large OSC crystals without relying on epitaxy. By combining X-ray diffraction with scanning probe microscopy and high resolution transmission electron microscopy, we show that large single-crystals of VOPc grow on graphene if the substrate is heated during deposition, but that this large-grain growth is not linked to epitaxy: there appears to be no orientation preference of the VOPc crystals relative to the graphene underneath. Conducting atomic-force microscopy measurements show that the more crystalline films display improved electrical performance. As the improved crystallinity of the film is not dependent on epitaxy, it should be generically true for a wide range of OSCs on graphene.

• Publication: http://onlinelibrary.wiley.com/doi/10.1002/adfm.201503594/abstract

• DOI: doi/10.1002/adfm.201503594/