| Summary: | Thin films of organic semiconducting materials, such as copper phthalocyanine (CuPc) and C60, can be used in photovoltaic devices. The interface between these materials is the site of exciton dissociation, and thus a key region of interest in their study. The processes that occur within these films and at interfaces are governed by the local morphology and structure. Studying these films and interfaces at high spatial resolution has previously been challenging given their soft nature and scale. Using electron transparent cross-sections prepared with a focussed ion beam (FIB), high resolution transmission electron microscopy (HRTEM) has been used to probe the local crystallography of three archetypical organic photovoltaic device structures grown on silicon and indium tin oxide (ITO). In HRTEM images lattice fringes of unprecedented clarity are observed, validating the optimised FIB method. HRTEM examination of device structure cross-sections on silicon reveals lattice fringes throughout pure films of CuPc and C60. The structure of the CuPc thin film can be correlated with bulk characterisation methods however, the observation of stacking faults demonstrates film non-uniformity. Lattice fringes in C60 films show an orientation preference with respect to the interface, which allows conclusions to be made about C60 when grown on molecular films. Mixed films show no lattice fringes. Structures grown on ITO are more complex than those on silicon, which is attributed the relatively rougher growth surface. Due to this rougher surface, the morphological changes occurring result in reduced crystallinity, a conclusion supported by bulk characterisation methods. The cross-sectional methodology has been extended to thicker films, revealing the presence of structural deviations that lie parallel to the surface. Scanning transmission electron microscopy, in combination with energy dispersive X-ray spectroscopy, high resolution quantitative compositional mapping reveals the morphology of the interface for the structures studied. This been correlated with the morphology of single CuPc film surfaces, with the conclusion that morphology of the CuPc surface remains unchanged after C60 film growth.
|
|---|
