Structure Formation during Organic Molecular Beam Deposition

Abstract

Pure and blended thin films of copper phthalocyanine (CuPc), Buckminster fullerene (C60) and coronene (Cor) molecules were deposited in vacuum onto standard silicon wafers and served as model systems for organic layers, as they are applied in organic photovoltaics (OPVs), organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs). The blends were prepared by co-deposition, i.e. simultaneous evaporation of two molecular species. The influence of substrate temperature, deposition rate and mixing on the formation of crystal structures and surface profiles was investigated by various x-ray scattering techniques, as well as by atomic force and scanning electron microscopy. The non-linear formation of surface roughness was observed in real-time during the growth by means of in-situ x-ray reflectivity. Depending on the molecular species, three different growth modes were found: The growth of distinct islands, wetting of the substrate and layer plus island growth. Higher substrate temperatures resulted in larger islands and larger crystalline domains at lower island densities. A similar effect was observed when reducing the deposition rate. Faster diffusion and a lower flux of impinging molecules accounts for the improved molecular self-assembly. Mixing of two molecular species lead to smooth CuPc-C60 blends at room temperature and extremely rough CuPc-C60 blends at 400 K. The domain sizes were significantly reduced in blends and long CuPc needles protruding from the thin film appeared. Although a theoretical description of the structure formation is challenging, the studies have shown that a systematic analysis enables to tailor the physical properties of organic thin films by a suitable choice of growth parameters, which is advantageous for technical applications.