<i>J-V</i> curves of P3HT:PCBM solar devices.

<p>(a) The average <i>J-V</i> curves for devices with transparent electrodes consisting of PEDOT:PSS (<i>N</i> = 7), PEDOT:PSS and grids patterned by polypropylene knife (<i>N</i> = 6), and PEDOT:PSS and grids patterned by steel razor (<i>N</i> = 4). (b) The <i>J-V</i> curves for the highest efficiency cells from the sample sets from (a). (c) The <i>J-V</i> curves for the highest efficiency larger cells (∼0.5 cm² compared to ∼0.1 cm² in (a) and (b)).</p

Dulling of cutting tools.

<p>(a) SEM micrographs showing the progression of the dulling of a razor used in Method II, from out of box to 50 cycles. (b) The dulling of the razor plateaus at around 50 cycles.</p

Schematic diagrams summarizing the two implementations of abrasion lithography.

<p>(a) Method I uses mechanical abrasion with a sharp tool to pattern water-soluble thin films. (b) Method II produces patterns by direct abrasion of glass substrates.</p

Images of transparent electrodes fabricated by abrasion lithography.

<p>(a) Photograph showing the high transparency of grids produced by Method II. A reflection in the bottom left corner of the glass substrate shows the copper wires. (b) Optical micrograph showing the wires produced by Method II.</p

Optical micrographs of junctions of copper and copper/nickel microwires.

<p>(a) A microwire junction fabricated by Method I. Surface roughness caused by the razor inadvertently abrading the substrate is apparent. (b) A microwire junction fabricated by patterned a PAA film with a polypropylene picnic knife, which was too soft to abrade the glass substrate, and thus the microwires appear to have a smoother topography. (c) A microwire junction patterned by direct abrasion of glass by a steel razor. Significant roughness generated by the razor is clearly visible.</p

Efficient Characterization of Bulk Heterojunction Films by Mapping Gradients by Reversible Contact with Liquid Metal Top Electrodes

The ways in which organic solar cells (OSCs) are measured and characterized are inefficient: many substrates must be coated with expensive or otherwise precious materials to test the effects of a single variable in processing. This serial, sample-by-sample approach also takes significant amounts of time on the part of the researcher. Combinatorial approaches to research OSCs generally do not permit microstructural characterization on the actual films from which photovoltaic measurements were made, or they require specialized equipment that is not widely available. This paper describes the formation of one- and two-dimensional gradients in morphology and thickness. Gradients in morphology are formed using gradient annealing, and gradients in thickness are formed using asymmetric spin coating. Use of a liquid metal top electrode, eutectic gallium–indium (EGaIn), allows reversible contact with the organic semiconductor film. Reversibility of contact permits subsequent characterization of the specific areas of the semiconductor film from which the photovoltaic parameters are obtained. Microstructural data from UV–vis experiments extracted using the weakly interacting H-aggregate model, along with atomic force microscopy, are correlated to the photovoltaic performance. The technique is used first on the model bulk heterojunction system comprising regioregular poly­(3-hexylthiophene) (P3HT) and the soluble fullerene derivative [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM). To demonstrate that the process can be used to optimize the thickness and annealing temperature using only small (≤10 mg) amounts of polymer, the technique was then applied to a bulk heterojunction blend comprising a difficult-to-obtain low-bandgap polymer. The combination of the use of gradients and a nondamaging top electrode allows for significant reduction in the amount of materials and time required to understand the effects of processing parameters and morphology on the performance of OSCs

Measuring the Glass Transition Temperature of Conjugated Polymer Films with Ultraviolet–Visible Spectroscopy

The glass transition temperature (<i>T</i>_g) of a conjugated polymer can be used to predict its morphological stability and mechanical properties. Despite the importance of this parameter in applications from organic solar cells to wearable electronics, it is not easy to measure. The <i>T</i>_g is often too weak to detect using conventional differential scanning calorimetry (DSC). Alternative methodse.g., variable temperature ellipsometryrequire specialized equipment. This paper describes a technique for measuring the <i>T</i>_g of thin films of semicrystalline conjugated polymers using only a hot plate and an ultraviolet–visible (UV–vis) spectrometer. UV–vis spectroscopy is used to measure changes in the absorption spectrum due to molecular-scale rearrangement of polymers when heated past <i>T</i>_g, corresponding to the onset of the formation of photophysical aggregates. A deviation metric, defined as the sum of the squared deviation in absorbance between as-cast and annealed films, is used to quantify shifts in the absorption spectra. The glass transition is observed as a change in slope in a plot of the deviation metric versus temperature. To demonstrate the usefulness of this technique, a variety of semiconducting polymers are tested: P3BT, PBTTT-C14, F8BT, PDTSTPD, PTB7, PCDTBT, TQ1, and MEH-PPV. These polymers represent a range of solid-state morphologies, from highly ordered to predominantly amorphous. A successful measurement of <i>T</i>_g depends on the ability of the polymer to form photophysical aggregates. The results obtained using this method for P3BT, PBTTT-C14, F8BT, and PDTSTPD are in agreement with values of <i>T</i>_g that have been reported in the literature. Molecular dynamics simulations are used to show how the morphology evolves upon annealing: above the <i>T</i>_g, an initially kinetically trapped morphology undergoes structural rearrangement to assume a more thermodynamically preferred structure. The temperature at which onset of this rearrangement occurs in the simulation is concomitant with the spectroscopically determined value of <i>T</i>_g

Metallic Nanoislands on Graphene as Highly Sensitive Transducers of Mechanical, Biological, and Optical Signals

This article describes an effect based on the wetting transparency of graphene; the morphology of a metallic film (≤20 nm) when deposited on graphene by evaporation depends strongly on the identity of the substrate supporting the graphene. This control permits the formation of a range of geometries, such as tightly packed nanospheres, nanocrystals, and island-like formations with controllable gaps down to 3 nm. These graphene-supported structures can be transferred to any surface and function as ultrasensitive mechanical signal transducers with high sensitivity and range (at least 4 orders of magnitude of strain) for applications in structural health monitoring, electronic skin, measurement of the contractions of cardiomyocytes, and substrates for surface-enhanced Raman scattering (SERS, including on the tips of optical fibers). These composite films can thus be treated as a platform technology for multimodal sensing. Moreover, they are low profile, mechanically robust, semitransparent and have the potential for reproducible manufacturing over large areas

Asymmetric Colloidal Janus Particle Formation Is Core-Size-Dependent

Colloidal particles with asymmetric surface chemistry (Janus particles) have unique bifunctional properties. The size of these particles is an important determinant for their applications in diverse fields from drug delivery to chemical catalysis. The size of Janus particles, with a core surface coated with carboxylate and a partially encapsulating silica shell, depends upon several factors, including the core size and the concentration of carboxylate coating. The role of the carboxylate coating on the Janus particle size is well-understood; however, the role of the core size is not well defined. The role of the carboxylated polystyrene (cPS) core size on the cPS–silica Janus particle morphology (its size and shape) was examined by testing two different silica sizes and five different cPS core sizes. Results from electron microscopy (EM) and dynamic light scattering (DLS) analysis indicate that the composite cPS–silica particle acquires two distinct shapes: (i) when the size of the cPS core is much smaller than the non-cPS silica (b-SiO₂) sphere, partially encapsulated Janus particles are formed, and (ii) when the cPS core is larger than or equal to the b-SiO₂ sphere, a raspberry-like structure rather than a Janus particle is formed. The cPS–silica Janus particles of ∼100–500 nm size were obtained when the size of the cPS core was much smaller than the non-cPS silica (b-SiO₂) sphere. These scalable nanoscale Janus particles will have wide application in a multifunctional delivery platform and catalysis