In-Plane Pathways Facilitate Out-of-Plane Charge Transport in Organic Solar Cell Active Layers

Solution-processed organic bulk heterojunctions are promising for enabling low-cost, lightweight, and mechanically flexible solar cells. While the nanostructured interpenetrating donor–acceptor morphology in bulk heterojunctions leads to efficient charge photogeneration, the locally varying composition, crystallinity, and electrical connectivity result in a complex landscape for charge transport. This work examines the structural features that govern out-of-plane charge transport in a high-performance small molecule:fullerene organic photovoltaic system, 7,7-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b]dithiophene-2,6-diyl)bis(6-fluoro-4-(5-hexyl-[2,2-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) blended with phenyl-C71-butyric acid methyl ester (PC71BM). Active layer composition and degree of donor–acceptor phase separation were systematically varied and characterized electrically by conductive atomic-force-microscope-based charge carrier mobility mapping and structurally by grazing incid...