The nanoscale interpenetrating electron donor–acceptor network in organic bulk heterojunction (BHJ) solar cells results in efficient charge photogeneration but creates complex 3D pathways for charge transport. At present, little is known about the extent to which out‐of‐plane charge flow relies on lateral electrical connectivity. In this work, a procedure, based on conductive atomic force microscopy, is introduced to quantify lateral current spreading during out‐of‐plane charge transport. Using the developed approach, the dependence of lateral spreading on BHJ phase separation, composition, and molecule type (small molecule vs polymer) is studied. In the small‐molecule BHJ, 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):(6,6)‐Phenyl‐C71‐butyric acid methyl ester (p‐DTS(FBTTh2)2:PC71BM), an increase is observed in lateral hole current spreading as the population of donor crystallites, bearing an edge‐on molecular orientation, is increased. When integrated into BHJs, the polymer donor poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) leads to greater lateral hole current spreading and more spatially uniform charge transport than the small‐molecule donor, owing to in‐plane charge transport along the polymer backbone. Through the newly introduced electrical characterization scheme, these experiments bring to light the role of lateral electrical connectivity in assisting charge navigation across BHJs.
The nanoconfinement of organic semiconductors in nanoporous media presents a means of manipulating molecular assembly and optoelectronic properties. This work introduces a solution-infiltration process with slow solvent evaporation for filling nanoporous anodic aluminum oxide templates with crystalline organic semiconductors. This approach is used to systematically study the dependence of crystal growth on nanopore size for four organic semiconductors, including planar small molecules, a fullerene, and a polymer. The planar molecules exhibit preferential π–π stacking along the pore axis in addition to a second co-existing growth orientation, indicating competition between fast-growth directions as nuclei attempt to reach a critical cluster size. Size-dependent effects were seen in the crystallinity, crystal orientation, and molecular aggregation of these compounds.
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.
By systematically varying the molecular orientation of poly(3-hexylthiophene-2,5-diyl) (P3HT) in P3HT:fullerene bulk heterojunctions, we show that a mixed face-on and edge-on texture can be beneficial for out-of-plane charge flow in...
Nanoconfinement of organic semiconductors in nanoporous templates is promising for manipulating polymorphism and molecular orientation while forming nanowires with controlled dimensions. To harness the potential advantages of templated organic semiconductor nanowires, there is a need to understand the factors that influence final nanowire structure. Little is known, however, about the extent to which nanowire morphology and internal structure are impacted by nanowire release from the boundary conditions imposed by the template. To address this knowledge gap, we assessed the morphological, crystallographic, and photophysical changes that occur in three common organic semiconductors in response to nanoporous template removal [(Bis(triisopropylsilylethynyl)pentacene (TIPS-Pn), (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), and poly(3-hexylthiophene) (P3HT)]. Although the nanowires comprising planar small molecules maintained their cylindrical shape following template removal, the investigated polymer, P3HT, exhibited extensive nanowire fusing for pore sizes of 55 nm and below, leading to a networked structure. All three systems presented preferred crystallite orientations that persisted in the freed nanowires. Nanowires generally exhibited an increasingly J-like aggregation character following template removal. Collectively, these results reveal that subtle molecular rearrangement takes place in the small molecule systems, while significant structural rearrangement can occur in polymer systems in response to template removal.
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