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Colocalized Nanoscale Electrical and Compositional Mapping of Organic Solar Cells

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journal contribution
posted on 20.06.2019, 00:00 by Jeremy S. Mehta, Pravini S. Fernando, John L. Grazul, Jeffrey M. Mativetsky
Organic photovoltaics based on the bulk heterojunction is an emergent technology with the potential to enable low-cost, lightweight, and mechanically flexible energy conversion applications. Organic photovoltaic performance is intimately linked to the heterogeneous nanoscale structuring of the active layer. Means of directly assessing the interplay between local nanoscale structure and local nanoscale function are lacking, however. This work combines the complementary strengths of energy-filtered transmission electron microscopy and conductive atomic force microscopy to perform colocalized nanoscale measurements of bulk heterojunction chemical composition and charge carrier mobility in a high-performance small molecule organic photovoltaic system. We find that the nanoscale donor concentration and hole mobility maps are uncorrelated, unlike device-scale measurements that show a strong dependence of hole mobility on donor concentration. These results challenge standard interpretations of nanoscale structural and electrical maps, e.g., that a high local donor concentration implies a high local hole mobility and vice versa. Our results demonstrate instead that factors such as local phase continuity have a greater impact on charge transport than the local amount of each phase. These results also support an emerging picture for small molecule bulk heterojunctions in which electrical connectivity within finely mixed domains plays a decisive role in charge migration. The devised colocalized approach can be generalized to a broad range of transmission electron microscope and atomic force microscope modes, opening vast opportunities for nanoscale structure–function mapping of materials.