  Electron energy-loss spectroscopy (EELS) is a powerful tool for imaging
chemical variations at the nanoscale. Here, we investigate a polymer/organic
small molecule-blend used as absorber layer in an organic solar cell and employ
EELS for distinguishing polymer donor and small molecule acceptor domains in
the nanostructured blend based on elemental maps of light elements, such as
nitrogen, sulfur or fluorine. Especially for beam sensitive samples, the
electron dose needs to be limited, therefore optimized acquisition and data
processing strategies are required. We compare data acquired on a post-column
energy filter with a direct electron detection camera to data from a
conventional CCD camera on the same filter and we investigate the impact of
statistical data processing methods (principal components analysis, PCA) on
acquired spectra and elemental maps extracted from spectrum images. Our work
shows, that the quality of spectra on a direct electron detection camera is far
superior to conventional CCD imaging, and thereby allows clear identification
of ionization edges and the fine structure of these edges. For the quality of
the elemental maps, the application of PCA is essential to allow a clear
separation between the donor and acceptor phase in the bulk heterojunction
absorber layer of a non-fullerene organic solar cell.
