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 highperformance small molecule:fullerene organic photovoltaic system, 7,7-( 42 ) blended with phenyl-C 71 -butyric acid methyl ester (PC 71 BM). 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 π−π stacking within the donor phase. Furthermore, as the amount of in-plane π−π 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.

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The crystallization of a series of triisopropylsilylethynyl (TIPS)-derivatized acene-based organic semiconductors drop cast from solution onto substrates was investigated as a function of the size of their conjugated cores. When drop cast onto a substrate, the molecules in TIPSpentacene crystals adopt a "horizontal" orientation, with the long axis of the pentacene core parallel to the substrate surface. For crystals comprising molecules with dibenzopyrene, anthanthrene, and pyranthrene cores, two-dimensional X-ray diffraction patterns revealed the existence of a second population of crystals adopting a "vertical" molecular orientation with the long axis of the acene core perpendicular to the substrate surface. The ratio of the population of TIPS-pyranthrene crystals with molecules adopting horizontal versus vertical orientations was controlled by varying the surface energy of the underlying substrate. These crystals displayed orientationdependent linear birefringence and linear dichroism, as observed by differential polarizing optical microscopy. Conductive atomic force microscopy (C-AFM) revealed a 42-fold improvement in out-of-plane hole mobility through crystals adopting the vertical molecular orientation compared to those adopting the horizontal molecular orientation.

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.

Graphene oxide (GO) has garnered attention for its tunable chemical, electrical, and optical properties. An integral part of the efforts to manipulate and improve the performance of GO is the ability to reliably characterize its complex structure. Raman spectroscopy and confocal Raman mapping are widely used for insight into the extent of GO's nanoscale graphene-like domains, the degree of lattice order, and its sheet stacking structure. It has also been reported, however, that laser sources, similar to those used for Raman spectroscopy, can be used to intentionally reduce and ablate GO. In light of this, it is unclear how invasive Raman measurements of GO are and how reliable published Raman data is. In this study, we employ Raman laser doses spanning 4 orders of magnitude to investigate the impact of Raman measurements on GO structure. We find that GO undergoes reduction at all practical laser doses, with the degree of reduction increasing with dose. Lattice damage and ablation dominate at high laser doses. Based on our findings, we encourage the use of a minimal laser dose (8 × 10 7 J/m 2 or below) for Raman measurements of GO. Despite the resulting loss in signal, these conditions limit sample modification and measurement inaccuracies.