Efficient Generation of Model Bulk Heterojunction Morphologies for Organic Photovoltaic Device Modeling

Kinetic Monte Carlo (KMC) simulations have been previously used to model and understand a wide range of behaviors in bulk heterojunction (BHJ) organic photovoltaic devices, from fundamental mechanisms to full device performance. One particularly unique and valuable aspect of this type of modeling technique is the ability to explicitly implement models for the bicontinuous nanostructured morphology present in these devices. For this purpose, an Ising-based method for creating model BHJ morphologies has become prevalent. However, this technique can be computationally expensive, and a detailed characterization of this method has not yet been published. Here, we perform a thorough characterization of this method and describe how to efficiently generate controlled model BHJ morphologies. We show how the interaction energy affects the tortuosity of the interconnected domains and the resulting charge transport behavior in KMC simulations. We also demonstrate how to dramatically reduce calculation time by several orders of magnitude without detrimentally affecting the resulting morphologies. In the end, we propose standard conditions for generating model morphologies and introduce a new open-source software tool. These developments to the Ising method provide a strong foundation for future simulation and modeling of BHJ organic photovoltaic devices that will lead to a more detailed understanding of the important link between morphological features and device performance.Comment: Main article: 9 pages, 6 figures, Supplementary Information: 6 pages, 6 figure

Rupture of a Two-Dimensional Alkane Crystal

We have studied the breaking of a two-dimensional alkane crystal above the disordered melt using an oscillating bubble rheometer. Surface tension changes abruptly during the expansion and contraction cycle. We postulate that this is due to rupture of the 2D crystal at grain boundaries. The magnitude of the abrupt change in surface tension decreases with a decrease in the rate of change of bubble surface area with a power law exponent of 0.8. The interfacial area formed after rupture decreases with a decrease in rate. These results provide new insights in understanding defect-mediated rupture in confined geometry

Confinement-Induced Ordering of Alkanes Between An Elastomer and a Solid Surface

We have studied the molecular structure of liquid alkanes confined between a flexible elastomeric poly(dimethyl siloxane) lens and a rigid sapphire substrate using surface-sensitive infrared-visible sum frequency generation spectroscopy. The reduction in the gauche defects suggests ordering of liquid alkanes under confinement. The cooling of confined liquid below the freezing temperature leads to crystallization with alkane molecules lying on the substrate with the symmetry axis parallel to the surface normal. This structure is very different from the bulk alkane crystals next to sapphire or air interfaces

Melting at Alkyl Side Chain Comb Polymer Interfaces

Infrared visible sum frequency generation (SFG) spectroscopy has been used to study structure and melting transition temperatures of alkyl-side chain-acrylate comb polymers at air and solid interfaces. At the air interface, the SFG spectra show methyl bands and two transitions are observed: the first, near the bulk melting temperature, T-m, and the second 10-20degreesC higher than T-m. The shorter the alkyl side chain, the larger the difference between the two transition temperatures. In contrast, methylene bands are observed at sapphire interface with a single transition near T-m (C18)

Relaxation of a Rubbed Polystyrene Surface

The relaxation dynamics of a rubbed polystyrene (PS) surface have been characterized using infrared-visible sum frequency generation spectroscopy (SFG). The SFG results were compared with previous relaxation of retardation measurements, and the results show that the rubbed PS surface has the same T(g) as the bulk where T(g) is defined as tau(T(g))=5 s, however, the surface has a lower activation energy (DeltaE) and a larger stretching exponent (beta(KWW)) than bulk PS. This indicates that the surface region relaxes more quickly than the bulk. The thickness of this region of lower DeltaE and larger beta(KWW) is estimated to be roughly 12 nm

Humidity-induced glass transition of a polyelectrolyte brush creates switchable friction in air

Polymer brushes have found extensive applications as responsive surfaces, particularly in achieving tunable friction in solvent environments. Despite recent interest in extending this technology to air environments, little is known about the impact of vapor absorption on friction. Considering polyelectrolyte brushes, we report findings that reveal, with increasing relative humidity, a trend of frictional shear forces decaying over two orders of magnitude only after achieving a critical humidity. However, water absorption, structure, and swelling of the brushes followed continuous trends with increasing humidity. In contrast, a humidity-induced glass transition occurred which caused a shift from dry to fluid-like sliding with the water-swollen brush acting as the lubricant. Below the glass transition, friction is large without regard to water concentration; thus, friction transitions sharply. This switching mechanism, shown to be a general property of the hygroscopic polymer, provides new opportunities for switchable friction or other surface actuation from vapor stimuli.Comment: 24 pages, 6 figure

Developing noniridescent structural color on flexible substrates with high bending resistance

Nanostructured materials producing structural colors have great potential in replacing toxic metals or organic pigments. Electrophoretic deposition (EPD) is a promising method for producing these materials on a large scale, but it requires improvements in brightness, saturation, and mechanical stability. Herein, we use EPD assembly to codeposit silica (SiO2) particles with precursors of synthetic melanin, polydopamine (PDA), to produce mechanically robust, wide-angle structurally colored coatings. We use spectrophotometry to show that PDA precursors enhance the saturation of structural colors and nanoscratch testing to demonstrate that they stabilize particles within the EPD coatings. Stabilization by PDA precursors allows us to coat flexible substrates that can sustain bending stresses, opening an avenue for electroprinting on flexible materials

A Device for Surface Study of Confined Micron Thin Films in a Total Internal Reflection Geometry

A device to probe the molecular structure of materials next to a solid interface in a thin film geometry has been developed. The device can produce controlled thicknesses as small as 1 mum with parallelity better than 0.003degrees. We have shown that the thickness and the parallelity of the film produced between two optical surfaces can be quantified using white light and monochromatic light interferometry, respectively. In addition, this apparatus allows the study of these films in a static state or under shear using spectroscopic techniques involving transmission or reflection measurements. (C) 2002 American Institute of Physics

Understanding Rubber Friction in the Presence of Water Using Sum-Frequency Generation Spectroscopy

Infrared-visible sum-frequency-generation spectroscopy (SFG) was used to study the molecular structure of water between a poly(dimethylsiloxane) (PDMS) and a sapphire substrate. The observation of SFG peaks associated with the dangling surface hydroxyl groups (3690 cm(-1)) and water bands (3000-3400 cm(-1)) indicates that the contact spot between the PDMS lens and the sapphire substrate is heterogeneous. Within the contact spot there are regions where the methyl groups of the PDMS chains are in direct contact with the surface hydroxyl groups on the sapphire substrate. In the other regions, a thin water layer is trapped between the two surfaces with spectral features that are different from that of the unconfined water next to the sapphire or the PDMS surface. The higher adhesion and friction values observed in these experiments, compared to those expected for a uniform thin layer of water trapped between the PDMS and the sapphire substrate, are consistent with the hypothesis that the contact spot is heterogeneous. These results have important implications in understanding the sliding behavior of wet, deformable hydrophobic materials on hydrophilic substrates