Copper Phthalocyanine (CuPc) Thin Films and Nanostructures - Growth and Device Applications

The development and utilisation of phthalocyanine (Pc) materials for practical applications has been subject of intense research due to their desirable optical, electrical, and recently discovered magnetic properties. Typically, in crystalline organic materials the charge transport, light absorption and magnetisation are anisotropic and strongly depend on structure, morphology and molecular orientation. Hence, an understanding of growth mechanism and controlled fabrication of thin films and nanostructures is essential for tailoring properties desirable for specific applications. e.g., for OFETs, crystalline thin films or one-dimensional nanostructures. Recently, vast advancement has been made in developing functional organic films including sublimation in (ultra) high vacuum using organic molecular beam deposition (OMBD). This environment can provide the essential material purity and structural reproducibility required in future high performance optoelectronic device applications, but is unfortunately costly. In this thesis, we introduce a lesser-known technique, organic vapour phase deposition (OVPD), operating at lower cost but still maintaining high purity. The morphology, crystallinity, spectroscopic characteristics and structure of copper phthalocyanine (CuPc) thin films and nanostructures have been investigated and their dependence on deposition conditions, i.e., substrate temperature and substrate type has been studied. We compare films obtained by OMBD and OVPD and find different morphological and structural changes; the surface morphology changes from granular to larger nano-fibrous and nano-whiskers with increasing substrate temperatures. In OMBD, the structure in a small proportion of the film changes from α-CuPc to β-CuPc at a substrate temperature of 200 oC. In the case of OVPD films, extensive study of the influence of parameters such as deposition pressure, deposition time and source to substrate distance is performed and variation in film morphology, texture, structural composition and molecular orientation is observed. We find that by successive growth of films produced by OMBD and OVPD, the molecular orientation can be controlled by the first “seed” layer. With further processing and optimisation, it is hoped that this could be used to create interpenetrating networks of different organic materials and optimal molecular orientation. We also demonstrate the fabrication using OVPD of high density CuPc nanowires with typical diameters between 10 - 100 nm, high directionality, and exceptional aspect ratios. We show that these nanowires are of a new crystal phase, named eta-CuPc. Lastly, OFETs fabricated with OMBD and OVPD grown CuPc thin films and nanowires are characterised. The current on/off ratio, mobilities and threshold voltage for thin films produced by the two methods are comparable and similar to what has been reported in literature. In contrast, OFETs with CuPc nanowires show remarkable improvement in turn-on voltage, while mobilities also seem to improve dramatically, although this is difficult to quantify. The challenges in growing CuPc nanowires directly on FETs with precise control of position and directionality are reviewed. The key issues that need to be resolved for future applications of these one-dimensional nanostructures are identified and are subject of on-going research. To conclude, this work has made important contribution in the efforts to develop, improve and enhance the deposition methods for fabricating functional thin films and nanostructures of CuPc material for use in organic electronic devices