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 incidence X-ray diffraction. These experiments reveal that the strongest predictor of out-of-plane hole mobility across all morphologies is the amount of in-plane II-II stacking within the donor phase. Furthermore, as the amount of in-plane II-II stacking increases, the width of moderate-hole-mobility regions concentrated around high-hole-mobility hot spots increases in nanoscale hole-mobility maps, resulting from lateral access of charge from the surrounding regions to the hot spots. These findings challenge the notion that out-of-plane transport in bulk heterojunctions is predominantly dependent on out-of-plane pathways and instead suggests the importance of balancing in-plane and out-of-plane transport components.