(Opto)electronic performance in emerging material systems is often governed by nano- and mesoscale morphology. Conductive atomic force microscopy (C-AFM) is a versatile tool for probing (opto)electronic properties at these length...

To reduce the harm of electronic waste, there is impetus to expand the use of biodegradable substrates, biocompatible materials, and battery-free devices. In this study, a piezoelectric paper composite is produced by integrating polyvinylidene fluoride-trifluoroethylene [P-(VDF-TrFE)] with the conducting polymer poly-(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) and wood fiber paper. It is shown that the combined effect of PEDOT:PSS and paper leads to an enhanced out-of-plane piezoelectric coefficient in P-(VDF-TrFE), without the need for electric poling. The enhanced piezoelectric performance arises from a synergistic templating effect of PEDOT:PSS and paper that induces a sharp increase in the population of edge-on β phase P-(VDF-TrFE) crystallites. The templating effect relies on supramolecular interactions between PSS and paper that lead to PEDOT enrichment and a linear PEDOT conformation at the interface with P-(VDF-TrFE). Computational modeling reveals that linear PEDOT promotes β phase PVDF growth over the α phase due to a greater availability of hydrogen bonds. The synergistic templating of P-(VDF-TrFE) on PEDOT:PSS-coated paper opens avenues for tuning the properties of piezoelectric paper.

Conductive atomic force microscopy (C-AFM) is a widely used tool for studying the charge transport properties of organic semiconductor films with nanoscale resolution. Local hole current is commonly measured by electrically contacting the film with a high work function C-AFM probe on top and an underlying electrode coated with a hole transport layer. The two voltage polarities, corresponding to the probe injection and substrate injection of holes, are both found in the C-AFM literature; nevertheless, there has been a lack of consideration about the possible influence of voltage polarity on image contrast and charge transport mechanisms. By analyzing local hole current maps and current-voltage curves for three organic semiconductors (a small molecule and two polymers), we find that probe and substrate injection leads to drastically different hole current maps and charge transport mechanisms. Specifically, the substrate injection of holes exhibits ohmic characteristics at low voltages and space-charge-limited current behavior at elevated voltages. Conversely, the probe injection of holes leads to injection-limited current that is sensitive to the state of the probe- sample interface. These measurements provide a blueprint for ensuring that C-AFM measurements are unambiguously probing the bulk properties of organic semiconductor films.

Molecular orientation and stacking motif have a major impact on charge transport within bulk heterojunction (BHJ) organic solar cell active layers. Unlike typical core pi-stacking organic semiconductors, fullerenes and the non-fullerene acceptor 2,2 '-((2Z,2 ' Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro[1,2,5]thiadiazolo[3,4-e]thieno[2 '',3 '':4 ',5 ']thieno[2 ',3 ':4,5]pyrrolo[3,2-g]thieno[2 ',3 ':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6) exhibit highly interconnected three-dimensional packing arrangements. Technical challenges, however, have hindered direct experimental probing of the electrical connectivity within the acceptor phase of BHJs. Through the development of conductive atomic force microscopy (C-AFM) protocols, this study investigates local electron transport and lateral current spreading within the acceptor phase of fullerene- and Y6-based BHJs. These measurements reveal remarkable lateral electrical connectivity, evidenced by an interconnected filamentary electrical network in C-AFM current maps and lateral current spreading radii that are more than three times greater than those in the donor phase. The effective current spreading radius for Y6 was 278 nm at -4 V, versus 182 nm in the fullerene [6,6]phenyl-C-71-butyric acid methyl ester (PC71BM), owing to the interlocked, electronically coupled three-dimensional network formed by the curved Y6 molecules. These findings point to the promise of molecular stacking arrangements with three-dimensional electronic coupling as a means of promoting efficient electron and hole collection in BHJ organic solar cells.

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)‐4 H ‐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‐C 71 ‐butyric acid methyl ester ( p ‐DTS(FBTTh 2 ) 2 :PC 71 BM), 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.

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 solution processed organic bulk heterojunction solar cells. These results implicate the need to balance in-plane and out-of-plane pathways for efficient charge percolation in bulk heterojunctions.

We report on localized nonlinear lattice deformation and nanoscale structural rearrangement in methylammonium lead triiodide films triggered by the combined action of light and voltage. These effects, revealed by second harmonic piezoresponse force microscopy, are connected with organic cation motion, implicating localized cation migration as a key contributor to perovskite optoelectronic device instability under operating conditions.

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.

Although hybrid organic–inorganic halide perovskite solar cells have achieved extraordinary improvements over the past few years, questions remain about the role of grain boundaries. This article reports on the nanoscale point‐by‐point current–voltage mapping of photovoltaic characteristics in inverted methylammonium lead triiodide perovskite solar cells. These measurements reveal an increased open‐circuit voltage and shunt resistance, along with a suppressed short‐circuit photocurrent at grain boundaries and nearby regions. Support from nanoscale ionic strain and surface potential mapping suggests that local ion accumulation and downward band bending can facilitate charge separation, but hinder charge collection at grain boundaries.

Optically transparent and highly conductive poly(3,4-ethylenedioxythiophene) (PEDOT) thin films were grown through vapor phase polymerization on (3-Mercaptopropyl)trimethoxysilane functionalized 3,4-Ethylenedioxythiophene (MPTMS functionalized EDOT) grafted glass substrates. Compared to bare glass, the EDOT grafted surface led to enhancements in both electrical conductivity and adhesion of PEDOT thin films. A quinoid-rich structure with increased crystallinity and a further enhanced conductivity was induced by post-deposition sulfuric acid doping. X-ray diffraction showed different orientations of the PEDOT crystals grown on substrates with and without EDOT grafting. The highest conductivity of 2690 S/cm, with an average optical transmittance of 95.4 % in the visible range, was achieved when PEDOT was vapor phase polymerized on EDOT grafted substrates and doped with 98 % sulfuric acid. Photostability was tested using a xenon arc light source and characterized by attenuated total reflection Fourier-transform infrared spectroscopy, showing that photoinduced degradation is associated with a decrease in C=C double bond content.

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 pi-pi 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.

Electronics based on paper substrates can be foldable, inexpensive, and biodegradable, making such systems promising for low-cost sensors, smart packaging, and medical diagnostics. In this work, we saturate tissue paper with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) by using a simple and scalable process and construct pressure sensors that exhibit an enhanced response when the active material is folded or stacked. Nanoscale pressure actuation and current mapping reveals a sensing mechanism that takes advantage of the fibrous microstructure of the paper and relies on the formation and expansion of electrical contacts between fibers in adjacent paper layers as pressure is applied. The resulting paper-based pressure sensors respond to an impulse within 20 ms and are robust, showing only a 4.6% decrease in the operating current after 30 000 load/unload cycles. Pressure distribution mapping was achieved by using a sensor array with a stacked architecture, whereas folding was used to demonstrate multistate switching and to detect conformational change in a three-dimensional origami system. These strategies of folding and layering paper saturated with functional materials open up new avenues for building multifunctional paper electronics.

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 nano scale 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 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.

Efficient solution-based growth of electronic materials is needed to enable low-cost wearable electronics, medical sensors, soft robotics, and energy harvesting technologies. In this study, a wetting-mediated two-phase dip coating method is developed to deposit high-performance organic semiconductor stripes at pre-specified locations on rigid, flexible, and three-dimensional substrates. This approach produces highly-oriented 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) crystallites, leading to hole mobilities up to 0.83 cm(2) V-1 s(-1) along the crystallite axis. The deposition method requires minimal amounts of starting material (4 mL per centimeter of substrate length) and can easily be scaled for deposition onto large (meter-scale) substrates. Wetting-mediated two-phase dip coating also enables the targeted deposition of organic semiconductors onto folded and complex three-dimensional substrates creating new opportunities for unconventional electronic applications.

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(2ethylhexyl)-4H-silolo [3,2-b:4,5-b] dithiophene-2,6-diyl)bis (6-fluoro-4-(5-hexyl-[2,2-bithiophen]-S-Abenzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)(2)) blended with phenyl C-71-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 pi-pi stacking within the donor phase. Furthermore, as the amount of in-plane pi-pi 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.

A highly conductive graphene derivative was produced by using a low-defect form of graphene oxide, oxo-G, in conjunction with voltage-reduction, a simple and environmentally-benign procedure for removing oxygen-containing functional groups. A low temperature coefficient of resistance was achieved, making this material promising for temperature-stable electronics and sensors.

Hybrid solar cells based on semiconducting polymers and metal oxides offer the possibility of combining the best attributes of both constituents to achieve efficient and low-cost solar energy harvesting. Active layers consisting of ZnO nanorods and semiconducting polymer poly(3-hexylthiophene), P3HT, have been studied extensively over the past decade. This type of solar cell, however, still exhibits a relatively low performance and poor device-to-device reproducibility. For insight into the performance bottlenecks in P3HT:ZnO nanorod solar cells, we employ a point-by-point current voltage mapping method using conductive atomic force microscopy to probe the local photovoltaic properties of hybrid P3HT:ZnO nanorod active layers with nanoscale spatial resolution. We observe that the short-circuit current density, open-circuit voltage, fill factor, and power conversion efficiency are highly heterogeneous and sensitive to the local thickness of the P3HT hole transport layer that sits atop the active layer, with local power conversion efficiencies reaching as high as double those seen in macroscopic devices. We further demonstrate that this hybrid system exhibits a high charge photogeneration rate, which approaches that in high-performance organic solar cells; however, photocurrent is limited by a low charge collection probability under short-circuit conditions. These experiments suggest the potential for significant performance gains in hybrid organic-inorganic nanorod solar cells through improvements in active layer uniformity and charge extraction efficiency.