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3D Spatial Mapping of the Nanomorphology of Polymer:Fullerene Blends by Highly Selective, Homogeneous Copper Staining

Bulk-heterojunction absorber layers of organic solar cells (OSCs) consist of three-dimensional (3D) interpenetrating networks of conjugated polymer and fullerene (donor/acceptor) domains [1]. The power conversion efficiency of OSCs depends strongly the intrinsic physical properties such as the exciton diffusion length and the charge carrier dissociation at interfaces of donors and acceptors. It requires a nanomorphology with domains sizes that are on the order of the exciton diffusion length. Thus, the nanomorphology has to be carefully characterized for correlation with solar cell performance. However, owing to their similar material properties, distinction of conjugated polymers and fullerenes in bulk-heterojunctions is always hampered by low contrast in electron microscopy. A common method for contrast enhancement is selective staining where one phase of a material mixture is labeled by heavier elements. This increases the difference of the average atomic number Z and yields Z-contrast imaging. Staining agents like OsO 4 /RuO 4 ,[2] and uranylacetate [3] are commonly used. However, the staining process can lead to swelling of nanoscaled phases, and, more seriously, these staining agents are highly toxic. To overcome these issues, we describe