WEB Imaging of polymer:fullerene bulk-heterojunctions in a scanning electron microscope: methodology aspects and nanomorphology by correlative SEM and STEM
Lowering the electron energy from values ≥ 80 keV in standard STEM experiments to ≤ 30 keV in scanning electron microscopes equipped with a STEM detector leads to the suppression of knock-on damage and contrast enhancement due to the increased scattering probability. Another advantage of scanning electron microscopes is the inherent availability of surface topography imaging by secondary electron (SE) SEM imaging, which can be used in a correlative manner in combination with STEM. As a prototype specimen, the PTB7:PC71BM system is a well-studied material system for bulk-heterojunction absorber layers of organic solar cells, which consist of an interpenetrating network of domains of donors (PTB7) and acceptors (PC71BM). To understand the contrast formation in this photoactive layer, the experimental STEM intensities for pure PTB7 and PC71BM were simultaneously analyzed in a wedge sample with well-known thickness under different E0. The normalized STEM intensities as function of layer thickness are plotted. It shows that contrast inversion between PTB7 and PC71BM appears in HAADF-STEM at a given thickness and strongly affects by E0, which is not observed in BF-STEM. Then, the presented STEM methodology with local topography are used to distinguish PTB7 and PC71BM in DIO-processed absorber layers. The results show that the nanomophology depends strongly on the DIO concentration.
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