Novel techniques in the scanning electron microscope for characterising polymer-based photovoltaic materials

In this thesis, a range of new techniques are developed in the scanning electron microscope (SEM) for investigating and imaging the nanoscale morphology of polymer:fullerene blends with organic photovoltaic (OPV) applications. The primary focus of these techniques is energy-selective detection of secondary electrons (SE) emitted in the SEM, applied both to measure the energy spectrum of a sample’s SE emissions, and for high-resolution energy-filtered SEM (EFSEM) imaging with improved material contrast. The SE energy-filtering performance of a FEI Sirion SEM is evaluated, and the SE spectrum of P3HT, a popular polymer for OPV, is measured and found to demonstrate a range of spectral features. These features are believed to reflect molecular ordering in the polymer. It is also found that degradation of the P3HT film under air and light alters the SE spectrum of the sample. Based upon SE spectroscopy methods, energy-filtered SE images are then applied to image the phase-separated morphology of a P3HT:PC60BM film with increased material contrast. EFSEM images of the blend film surface are found capable of mapping the blend morphology with a lateral resolution of (0.8 ± 0.1) nm, and demonstrate approximately double the material contrast in conventional SEM images. This improved contrast allows for the direct identification of mixed phase material in the image data, a first for this particular blend system. In P3HT:PC60BM films processed for optimal performance, (25 ± 5) % of the imaged phase area is classified as mixed phase by the technique. A further imaging technique is developed using low-energy backscattered electrons (BSE) in the SEM to probe the 3-dimensional morphology of the polymer:fullerene film as well as the surface. The technique is used to compare reference P3HT:PC60BM blends with a modern, high-performance PffBT4T-2OD:PC70BM blend film. At the surface, correlation between the phase size of fullerene domains in both blend systems is found, with both films showing a most probable domain radius of 6 nm. Further, by carefully tuning the primary beam energy, BSE images are used to probe for ‘columnlike’ phases that penetrate a large fraction of the film’s thickness; a characteristic feature of optimised OPV blend morphologies. The subsurface characterisation of the two blend systems reveals the highly optimised morphology of the modern, high-performance system.