Nanoscale analysis of molecular photovoltaic thin film structures and interfaces

摘要

Thin films of organic semiconducting materials, such as copper phthalocyanine (CuPc) and udC60, can be used in photovoltaic devices. The interface between these materials is the site of udexciton dissociation, and thus a key region of interest in their study. The processes that udoccur within these films and at interfaces are governed by the local morphology and udstructure. Studying these films and interfaces at high spatial resolution has previously been udchallenging given their soft nature and scale.udUsing electron transparent cross-sections prepared with a focussed ion beam (FIB), high udresolution transmission electron microscopy (HRTEM) has been used to probe the local udcrystallography of three archetypical organic photovoltaic device structures grown on silicon udand indium tin oxide (ITO). In HRTEM images lattice fringes of unprecedented clarity are udobserved, validating the optimised FIB method.udHRTEM examination of device structure cross-sections on silicon reveals lattice fringes udthroughout pure films of CuPc and C60. The structure of the CuPc thin film can be correlated udwith bulk characterisation methods however, the observation of stacking faults uddemonstrates film non-uniformity. Lattice fringes in C60 films show an orientation udpreference with respect to the interface, which allows conclusions to be made about C60udwhen grown on molecular films. Mixed films show no lattice fringes.udStructures grown on ITO are more complex than those on silicon, which is attributed the udrelatively rougher growth surface. Due to this rougher surface, the morphological changes udoccurring result in reduced crystallinity, a conclusion supported by bulk characterisation methods. The cross-sectional methodology has been extended to thicker films, revealing the udpresence of structural deviations that lie parallel to the surface.udScanning transmission electron microscopy, in combination with energy dispersive X-ray udspectroscopy, high resolution quantitative compositional mapping reveals the morphology udof the interface for the structures studied. This been correlated with the morphology of udsingle CuPc film surfaces, with the conclusion that morphology of the CuPc surface remains udunchanged after C60 film growth.