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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.

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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.

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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.

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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.

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Solution-processed nanowires are promising for low-cost and flexible electronics. When depositing nanowires from solution, due to stacking of the nanowires, an air gap exists between the substrate and much of the active material. Here, using confocal Raman spectroscopy, we quantify the thickness of the air gap in transistors comprising organic semiconductor nanowires. The average air gap thickness is found to be unexpectedly large, being at least three times larger than the nanowire diameter, leading to a significant impact on transistor performance. The air gap acts as an additional dielectric layer that reduces the accumulation of charge carriers due to a gate voltage. Conventional determination of the charge carrier mobility ignores the presence of an air gap, resulting in an overestimate of charge carrier accumulation and an underestimate of charge carrier mobility. It is shown that the larger the air gap, the larger the mobility correction (which can be greater than an order of magnitude) and the larger the degradation in on-off current ratio. These results demonstrate the importance of minimizing the air gap and of taking the air gap into consideration when analyzing the electrical performance of transistors consisting of stacked nanowires. This finding is applicable to all types of stacked one-dimensional materials including organic and inorganic nanowires, and carbon nanotubes.

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The competition between exciton dissociation and charge transport in organic solar cells comprising poly(3-hexylthiophene) [P3HT] and phenyl-C61-butyric acid methyl ester [PCBM] is investigated by correlated scanning confocal photoluminescence and photocurrent microscopies. Contrary to the general expectation that higher photoluminescence quenching is indicative of higher photocurrent, microscale mapping of bulk-heterojunction solar-cell devices shows that photoluminescence quenching and photocurrent can be inversely proportional to one another. To understand this phenomenon, we construct a model system by selectively laminating a PCBM layer onto a P3HT film to form a PCBM/P3HT planar junction on half of the device and a P3HT single junction on the other half. Upon thermal annealing to allow for interdiffusion of PCBM into P3HT, an inverse relationship between photoluminescence quenching and photocurrent is observed at the boundary between the PCBM/P3HT junction and P3HT layer. Incorporation of PCBM in P3HT works to increase photoluminescence quenching, consistent with efficient charge separation, but conductive atomic force microscopy measurements reveal that PCBM acts to decrease P3HT hole mobility, limiting the efficiency of charge transport. This suggests that photoluminescence-quenching measurements should be used with caution in evaluating new organic materials for organic solar cells.

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Having recently surpassed 10% efficiency, solar cells based on organic molecules are poised to become a viable low-cost clean energy source with the added advantages of mechanical flexibility and light weight. The best-performing organic solar cells rely on a nanostructured active layer morphology consisting of a complex organization of electron donating and electron accepting molecules. Although much progress has been made in designing new donor and acceptor molecules, rational control over active layer morphology remains a central challenge. Long-term device stability is another important consideration that needs to be addressed. This review highlights supramolecular strategies for generating highly stable nanostructured organic photovoltaic active materials by design.

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Self-assembled organic fibers combine facile solution processing with the performance benefits of single crystals. Here, the first evidence is shown of band-like transport in an n-type solution-processed small molecule system, a limited role of shallow traps, and a single fiber electron mobility that is several orders of magnitude higher than that measured in fiber ensembles or spin-cast films.

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Efficient out-of-plane charge transport is required in vertical device architectures, such as organic solar cells and organic light emitting diodes. Here, we show that graphene, transferred onto different technologically-relevant substrates, can be used to induce face-on molecular stacking and improve out-of-plane hole transport in copper phthalocyanine thin films.

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The nanoscale boundaries formed when neighboring spherulites impinge in polycrystalline, solution-processed organic semiconductor thin films act as bottlenecks to charge transport, significantly reducing organic thin-film transistor mobility in devices comprising spherulitic thin films as the active layers. These interspherulite boundaries (ISBs) are structurally complex, with varying angles of molecular orientation mismatch along their lengths. We have successfully engineered exclusively low- and exclusively high-angle ISBs to elucidate how the angle of molecular orientation mismatch at ISBs affects their resistivities in triethylsilylethynyl anthradithiophene thin films. Conductive AFM and four-probe measurements reveal that current flow is unaffected by the presence of low-angle ISBs, whereas current flow is significantly disrupted across high-angle ISBs. In the latter case, we estimate the resistivity to be 22 MΩµm(2)/width of the ISB, only less than a quarter of the resistivity measured across low-angle grain boundaries in thermally evaporated sexithiophene thin films. This discrepancy in resistivities across ISBs in solution-processed organic semiconductor thin films and grain boundaries in thermally evaporated organic semiconductor thin films likely arises from inherent differences in the nature of film formation in the respective systems.

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Control over the molecular orientation in organic thin films is demonstrated with precise in-plane spatial resolution over large areas. By exploiting the differential crystallization rates on substrates with different surface energies, the radial symmetry of spherulitic growth can be disrupted by preferentially selecting the molecular orientations that promote growth along the paths of the underlying patterns.

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Threaded molecular wires are shown to feature tunable properties. A new rotaxane based on a quaterthiophene threaded through a single ß-cyclodextrin exhibits delocalization of the aromatic system that is also extended onto the central phenyl rings of the m-terphenylene end-groups. The rotaxane can undergo self-assembly that is better than the analogous bithiophene derivative, due to the increased π-π interactions.

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We report a comparative study on the self-assembly from solution and electrical characterization of n-type semiconducting fibres obtained from five different perylenebis(dicarboximide) (PDI) derivatives. In particular we investigated the role of the nature of the alkyl chain covalently linked to the N,N' sites of the PDI in modulating the molecular solubility and aggregation capacity. We explored the morphologies of the self-assembled architectures physisorbed on dielectric surfaces and in particular how they can be modified by tuning the deposition and post-deposition procedures, i.e. by modulating the kinetics of the self-assembly process. To this end, alongside the conventional spin-coating, solvent vapour annealing (SVA) and solvent induced precipitation (SIP) have been employed. Both approaches led to fibres having widths of several hundred nanometres and lengths up to tens of micrometres. SVA formed isolated fibres which were tens of nanometres high, flat, and tapered at the ends. Conversely, SIP fibres exhibited nearly matching heights and widths, but organized into bundles. Despite these morphological differences, the same intermolecular packing is found by XRD in each type of structure, albeit with differing degrees of long-range order. The study of the electrical characteristics of the obtained low dimensional nano-assemblies has been accomplished by fabricating and characterizing organic field-effect transistors.

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We report on the formation of photoconductive self-assembled fibres by solvent induced precipitation of a HBC-PMI donor-acceptor dyad. Kelvin Probe Force Microscopy revealed that upon illumination with white light the surface potential of the fibres shifted to negative values due to a build-up of negative charge. When integrated in a field-effect transistor (FET) configuration, the devices can be turned 'on' much more efficiently using light than conventional bias triggered field-effect, suggesting that these structures could be used for the fabrication of light sensing devices. Such a double gating represents an important step towards bi-functional organic FETs, in which the current through the junction can be modulated both optically (by photoexcitation) and electrically (by gate control).

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Here, we describe a simple approach to fabricate graphene-based field-effect-transistors (FETs), starting from aqueous solutions of graphene-oxide (GO), processed entirely under ambient conditions. The process relies on the site-selective reduction of GO sheets deposited in between or on the surface of micro/nanoelectrodes. The same electrodes are first used for voltage-induced electrochemical GO reduction, and then as the source and drain contacts of FETs, allowing for the straightforward production and characterization of ambipolar graphene devices. With the use of nanoelectrodes, we could reduce different selected areas belonging to one single sheet as well.

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Conductive atomic force microscopy (C-AFM) has been used to correlate the detailed structural and electrical characteristics of graphene derived from graphene oxide. Uniform large currents were measured over areas exceeding tens of micrometers in few-layer films, supporting the use of graphene as a transparent electrode material. Moreover, defects such as electrical discontinuities were easily detected. Multilayer films were found to have a higher conductivity per layer than single layers. It is also shown that a local AFM-tip-induced electrochemical reduction process can be used to pattern conductive pathways on otherwise-insulating graphene oxide. Transistors with micrometer-scale tip-reduced graphene channels that featured ambipolar transport and an 8 order of magnitude increase in current density upon reduction were successfully fabricated.

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Conductance switching associated with the photoisomerization of azobenzene-based (Azo) molecules was observed in nanoscopic metal-molecule-metal junctions. The junctions were formed by using a conducting atomic force microscope (C-AFM) approach, where a metallic AFM tip was used to electrically contact a gold-supported Azo self-assembled monolayer. The measured 30-fold increase in conductance is consistent with the expected decrease in tunneling barrier length resulting from the conformational change of the Azo molecule.

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Measuring the electronic transport properties of single molecules and molecular nanostructuresis an interesting and challenging new frontier from both a fundamental as well as technologicalperspective. Conducting atomic force microscopy (C-AFM) represents an attractive line of approachgiven its ability to position a sharp electrical probe with nanometer-scale precision and a controllednano-Newton-range force. Moreover, the combination of AFM imaging and C-AFM electrical characterizationenables investigation of the relationship between structure and function in molecular architectures.The aim of the present review is twofold: (1) to introduce the C-AFM method, alongside a discussionof experimental practices, capabilities and limitations, and (2) to provide an overview of the applicationof C-AFM to different types of molecular systems. These include alkane-based and oligomer-based self-assembledmonolayers, molecular crystals, conducting polymer films, molecular wires (e.g. carbon nanotubes),and electrically active biomolecules. We will also discuss C-AFM approaches that allow single moleculemeasurements as well as other recent developments.