Benefits of direct electron detection and PCA for EELS investigation of organic photovoltaics materials

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 andemploy EELS for distinguishing polymer donor and small molecule acceptor domains in the nanostructured blendbased on elemental maps of light elements, such as nitrogen, sulfur or fluorine. Especially for beam sensitivesamples, the electron dose needs to be limited, therefore optimized acquisition and data processing strategies arerequired. We compare data acquired on a post-column energy filter with a direct electron detection camera todata from a conventional CCD camera on the same filter and we investigate the impact of statistical data processingmethods (principal components analysis, PCA) on acquired spectra and elemental maps extracted fromspectrum images. Our work shows, that the quality of spectra on a direct electron detection camera is far superiorto conventional CCD imaging, and thereby allows clear identification of ionization edges and the fine structure ofthese edges. For the quality of the elemental maps, the application of PCA is essential to allow a clear separationbetween the donor and acceptor phase in the bulk heterojunction absorber layer of a non-fullerene organic solarcell.