Imaging polymer:fullerene bulk heterojunctions with X-ry ptychography*

Improving the efficiency of organic photovoltaics (OPV) requires a comprehensive understanding of the relationship between microstructure and device function. Most OPVs are blends of polymers (or small molecule) donors and fullerene derivative acceptors (or increasingly polymers or small molecules) in bulk heterojunctions (BHJs). Microstructure characterization is challenging due to the similarity of the donor and acceptors and to easily damaged nature of organics. We have demonstrated that X-ray ptychography can be used to achieve this goal and to directly image the BHJ morphology. Ptychography is a technique that scans across a sample, collecting X-ray scattering images at localized but overlapping positions, and reconstructs real space images. We have used this technique to characterize the morphology in BHJs of PTB7 and PC71BM, allowing identification of the PTB7 and PC71BM domains and how these vary with the additive 1,8-diiodooctane (DIO).