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Lipomi D.J.,Stanford University | Chong H.,Foothill College | Vosgueritchian M.,Stanford University | Mei J.,Stanford University | Bao Z.,Stanford University
Solar Energy Materials and Solar Cells

This paper describes the effect of tensile strains of up to 20% on the photovoltaic properties of organic solar cells based on two different conjugated polymers and their blends with (6,6)-phenyl C 61 butyric acid methyl ester (PCBM). The two conjugated polymers are poly(3-hexylthiophene) (P3HT) and a donor-acceptor polymer whose repeat unit comprises a diketo pyrrolo-pyrrole moiety, thiophene, thienothiophene, and thiophene (DPPT-TT). The stretchable substrate, transparent electrode, and top electrode are, respectively, poly(dimethylsiloxane) (PDMS), poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), and eutectic gallium-indium (EGaIn). Analysis of the results of mechanical testing using a buckling based metrology, atomic force and optical microscopy, and linear and cyclic applications of strain yielded six major conclusions. (i) The identity of the conjugated polymer significantly affects the response of the solar cells to strain. Blending of PCBM in bulk heterojunction films stiffens P3HT more than it stiffens DPPT-TT, as determined by buckling-based measurements of elastic moduli. (ii) The greater elastic modulus of P3HT:PCBM than that of DPPT-TT:PCBM is correlated with a greater tendency of P3HT:PCBM to fracture under strain. (iii) PEDOT:PSS has the effect of an adhesion layer of the bulk heterojunction films to PDMS; its presence increases the crack-onset strain from <2% to 8% for P3HT:PCBM and from 6% to 12% for DPPT-TT:PCBM. (iv) Cracks dominate the characteristics of the devices under strain. The differences in the response to strain between devices comprising P3HT:PCBM and DPPT-TT:PCBM are correlated to the unequal brittleness of the polymer:fullerene films. (v) Devices based on both P3HT and DPPT-TT exhibit increased open-circuit voltage with strain, which is greatest for devices containing P3HT. These effects are attributed to disruption of the gallium oxide skin that covers the top electrode, EGaIn. (vi) Wave-like buckles appear in the films upon stretching ≤ 10% for the first time. These buckles permit reversible stretchability. © 2012 Elsevier B.V. All rights reserved. Source

Bard D.,Kavli Institute for Particle Astrophysics and Cosmology | Bellis M.,Siena College | Bellis M.,Northern Illinois University | Allen M.T.,Stanford University | And 2 more authors.
Astronomy and Computing

Cosmological measurements require the calculation of nontrivial quantities over large datasets. The next generation of survey telescopes will yield measurements of billions of galaxies. The scale of these datasets, and the nature of the calculations involved, make cosmological calculations ideal models for implementation on graphics processing units (GPUs). We consider two cosmological calculations, the two-point angular correlation function and the aperture mass statistic, and aim to improve the calculation time by constructing code for calculating them on the GPU. Using CUDA, we implement the two algorithms on the GPU and compare the calculation speeds to comparable code run on the CPU. We obtain a code speed-up of between 10 and 180 faster, compared to performing the same calculation on the CPU. The code has been made publicly available. GPUs are a useful tool for cosmological calculations, even for datasets the size of current surveys, allowing calculations to be made one or two orders of magnitude faster. © 2012 Elsevier B.V. Source

Density functional theory (DFT) calculations are used to predict the mechanism for the intramolecular carbon-hydrogen bond activation of an ortho methyl group on the RuII(SC6H3Me2-2,6-κ1S)2(PMe3)3 complex to form the cycloruthenated product cis-Ru[SC6H3-(2-CH2)(6-Me)-κ2S2C](PMe3)4 and HSC6H3Me2-2,6 in the presence of PMe3. The DFT calculations also show how changing the solvent from benzene to methanol prevents C-H activation and results in the unactivated six-coordinate product Ru(SC6H3Me2-2,6-κ1S)2(PMe3)4 in 100% yield. The reactant was determined to have two plausible σ-bond metathesis pathways in which to react, one for each of the two thiolate ligands. The steps in both mechanisms were influenced by the electronic interactions between the sulfur lone pairs and the Ru 4d orbitals and the steric repulsion between the methyl groups on the five ligands in such a way that the methyl group in the SAr (Ar = SC6H3Me2-2,6) ligand closest to the Ru pirouettes away to activate the other methyl group. The equatorial pathway was calculated to be the lower energy mechanism and, therefore, the dominant pathway for the overall reaction. The difference between reaction mediums was predicted, by both implicit and explicit solvation modeling, to be a result of the polarity and binding of methanol, which transforms the geometry of the reactant from a less polar distorted trigonal-bipyramidal geometry to a more polar distorted square-pyramidal geometry. This change in geometry favors the more rapid addition of a fourth PMe3 ligand to the more open coordination site, which prevents the C-H activation. © 2015 American Chemical Society. Source

Bauermeister M.R.,Foothill College
International Journal of Agricultural Sustainability

Social movement actors are seeking alternatives to the highly industrialized, global food system through alternative agri-food initiatives to counter the conventional practices of the industrial food system. Many of these initiatives have proven successful. Social movement researchers have documented the importance of the roles and services social movement organizations provide for movement constituents to realize their success, emphasizing human and financial capital as key components for mobilizing collective action. Researchers have also documented the value of interorganizational networks, and the benefits of collaboration to expand the share of resources, and perhaps more importantly design social movement frames to direct collective action for social change. However, what local food movement research has yet to address are some of the potential barriers that minimize collaboration among organizational leaders as it relates to social capital and collective identity. This paper takes a cross-sectional network analysis of social movement organizations working to increase the sustainability of various areas within the local food system in Marin County, California, a historically agricultural region serving a number of urban communities. Findings from the mixed-methods research reveal evidence of collective identity and social capital as enhancing collaboration among particular types of organizations while reducing potential collaboration among and between other social movement organizations. By analysing the collective identity and dichotomous nature of social capital among social movement organizations, this research contributes a clearer understanding of the existing gaps for increasing the sustainability of a local food system. © 2015 Taylor & Francis. Source

One of the fundamental structures of a cell is the membrane. Self-assembling lipid bilayer vesicles can form the membrane of an artificial cell and could also have plausibly assembled prebiotically for the origin of life. Such cell-like structures, that encapsulate some basic subset of the functions of living cells, are important for research to infer the minimum chemistry necessary for a cell, to help understand the origin of life, and to allow the production of useful species in microscopic containers. We show that the encapsulation of TiO2 particles has the potential to provide the basis for an energy transduction system inside vesicles which can be used to drive subsequent chemistry. TiO2 encapsulated in vesicles can be used to produce biochemical species such as NADH. The NADH is formed from NAD+ reduction and is produced in a form that is able to drive further enzymatic chemistry. This allows us to link a mineral-based, nonbiological photosystem to biochemical reactions. This is a fundamental step toward being able to use this mineral photosystem in a protocell/artificial cell. © 2015 American Chemical Society. Source

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