Mixed molecular orientations promote charge transport in bulk heterojunction solar cells.

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.

Nanoscale light- and voltage-induced lattice strain in perovskite thin films.

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.

Paper-Based Mechanical Sensors Enabled by Folding and Stacking.

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.

Voltage-reduced low-defect graphene oxide: a high conductivity, near-zero temperature coefficient of resistance material.

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.

An air gap moderates the performance of nanowire array transistors.

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.

Mapping the Competition between Exciton Dissociation and Charge Transport in Organic Solar Cells.

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.

Supramolecular Approaches to Nanoscale Morphological Control in Organic Solar Cells.

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.

Face-on stacking and enhanced out-of-plane hole mobility in graphene-templated copper phthalocyanine.

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.

Charge transport over multiple length scales in supramolecular fiber transistors: single fiber versus ensemble performance.

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.

Quantifying resistances across nanoscale low- and high-angle interspherulite boundaries in solution-processed organic semiconductor thin films.

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.

Guiding crystallization around bends and sharp corners.

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.

A quaterthiophene-based rotaxane: synthesis, spectroscopy, and self-assembly at surfaces.

Threaded molecular wires are shown to feature tunable properties. A new rotaxane based on a quaterthiophene threaded through a single β-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.

Charge transport in fibre-based perylene-diimide transistors: effect of the alkyl substitution and processing technique.

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.

Photoconductive and supramolecularly engineered organic field-effect transistors based on fibres from donor-acceptor dyads.

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.

Graphene transistors via in situ voltage-induced reduction of graphene-oxide under ambient conditions.

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.

Local current mapping and patterning of reduced graphene oxide.

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.

Azobenzenes as light-controlled molecular electronic switches in nanoscale metal-molecule-metal junctions.

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.

Exploring electronic transport in molecular junctions by conducting atomic force microscopy.

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 mitochondrial transcription factor TFAM coordinates the assembly of multiple DNA molecules into nucleoid-like structures.

Packaging DNA into condensed structures is integral to the transmission of genomes. The mammalian mitochondrial genome (mtDNA) is a high copy, maternally inherited genome in which mutations cause a variety of multisystem disorders. In all eukaryotic cells, multiple mtDNAs are packaged with protein into spheroid bodies called nucleoids, which are the fundamental units of mtDNA segregation.