Imaging and electro-optical characterization of donor net-works for diluted donor organic solar cells with D18:L8-BO

Donor dilution in organic solar cells can significantly improve their semitransparency, making them interesting candidates for building integrated photovoltaics and tandem devices.[1,2] How-ever, the increase in visible light transmission due to donor reduction results in efficiency losses. To gain further insight into the origin of this efficiency drop we investigated the morphology changes of active layers based on D18:L8-BO using scanning transmission electron micros-copy (STEM) and electron energy loss spectroscopy (EELS). Donor and acceptor domains provide insufficient mass contrast, but STEM-EELS is capable of separating the two phases based on small differences in elemental concentration of sulfur and nitrogen. Six ratios be-tween 1:1.5 and 1:100 donor to acceptor are investigated. Mapping the elemental ratio of sulfur vs. nitrogen allows clear separation of donor and acceptor domains revealing a donor network formation in bulk heterojunctions (BHJ). Changes in these networks depending on donor/ac-ceptor ratio and their influence on the device physics are investigated in detail. While the charge carrier mobilities are quite unaffected by donor dilution a significant decrease is shown in the charge carrier collection and exciton dissociation efficiency with increasing donor dilution. The investigation and understanding of donor networks in donor diluted solar cells facilitates optimization of organic solar cells resulting in higher efficiencies and improved transmission of visible light subsequently extending the applications of organic photovoltaics.