273 research outputs found

    Lessons learned from device modeling of organic & perovskite solar cells

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    Our rising need for energy combined with the realization that we need to drastically reduce our consumption of fossil energy to mitigate climate change, forces us to find better and cleaner ways to produce energy. While the price of solar energy has significantly decreased over the past two decades there is still a long way to go for solar to be a truly important part of our energy production.To increase the relative proportion of our energy produced with solar we not only need to make more efficient solar panels but also diversify the applications. Organic and perovskite solar cells present advantages for both objectives, they can be used in combination with classical technology in tandem or multi-junction structures to improve their efficiency. They can also be used in a broader range of applications as both technology can be solution-processed, made flexible, semi-transparent and perform well under low light intensity conditions. These characteristics open many doors for the future, where solar energy production will not only be confined to roof-top and solar farms applications but also in a more building-integrated fashion such as semi-transparent solar windows or even used to power internet of things products.In the work presented in this thesis, we used simulation to gain more insight into the limitations of organic and perovskite solar cells, whether those are due to a poor extraction or significant recombination. We provide guidelines on how to optimize the solar cells and also refined the analysis of known characterization methods to make them adequate for this class of materials studied

    Spin nematic order in antiferromagnetic spinor condensates

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    Large spin systems can exhibit unconventional types of magnetic ordering different from the ferromagnetic or N\'eel-like antiferromagnetic order commonly found in spin 1/2 systems. Spin-nematic phases, for instance, do not break time-reversal invariance and their magnetic order parameter is characterized by a second rank tensor with the symmetry of an ellipsoid. Here we show direct experimental evidence for spin-nematic ordering in a spin-1 Bose-Einstein condensate of sodium atoms with antiferromagnetic interactions. In a mean field description this order is enforced by locking the relative phase between spin components. We reveal this mechanism by studying the spin noise after a spin rotation, which is shown to contain information hidden when looking only at averages. The method should be applicable to high spin systems in order to reveal complex magnetic phases.Comment: published versio

    Price Transmission in Food Chains: The Case of the Dairy Industry

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    International audienceThis chapter outlines the state of the art of empirical analyses of cost pass-through in food chains with reference to the use of structural econometric models using scanner data. Statistical analyses show that cost pass-through is imperfect, due in part to market power of firms in food markets where food supply chains are frequently composed of oligopolies and where the nature of contracts linking food retailers and food manufacturers could have an important influence on price transmission. Employing a structural model permits estimation of product-specific elasticities which, together with cost data, can allow estimates of markups and how they change in the face of cost changes. The scanner data are used to estimate structural models of the French fluid milk and yoghurt sectors. The results show considerable heterogeneity in price transmission, where evidence of both under- and over-shifting can be found

    Spin fragmentation of Bose-Einstein condensates with antiferromagnetic interactions

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    We study spin fragmentation of an antiferromagnetic spin 1 condensate in the presence of a quadratic Zeeman (QZ) effect breaking spin rotational symmetry. We describe how the QZ effect turns a fragmented spin state, with large fluctuations of the Zeemans populations, into a regular polar condensate, where atoms all condense in the m=0m=0 state along the field direction. We calculate the average value and variance of the Zeeman state m=0m=0 to illustrate clearly the crossover from a fragmented to an unfragmented state. The typical width of this crossover is q‚ąľkBT/Nq \sim k_B T/N, where qq is the QZ energy, TT the spin temperature and NN the atom number. This shows that spin fluctuations are a mesoscopic effect that will not survive in the thermodynamic limit N‚Üí‚ąěN\rightarrow \infty, but are observable for sufficiently small atom number.Comment: submitted to NJ

    Detecting changes in the annual movements of terrestrial migratory species : using the first-passage time to document the spring migration of caribou

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    Background : Migratory species face numerous threats related to human encroachment and climate change. Several migratory populations are declining and individuals are losing their migratory behaviour. To understand how habitat loss or changes in the phenology of natural processes affect migrations, it is crucial to clearly identify the timing and the patterns of migration. We propose an objective method, based on the detection of changes in movement patterns, to identify departure and arrival dates of the migration. We tested the efficiency of our approach using simulated paths before applying it to spring migration of migratory caribou from the Rivière-George and Rivière-aux-Feuilles herds in northern Québec and Labrador. We applied the First-Passage Time analysis (FPT) to locations of 402 females collected between 1986 and 2012 to characterize their movements throughout the year. We then applied a signal segmentation process in order to segment the path of FPT values into homogeneous bouts to discriminate migration from seasonal range use. This segmentation process was used to detect the winter break and the calving ground use because spring migration is defined by the departure from the winter range and the arrival on the calving ground. Results : Segmentation of the simulated paths was successful in 96% of the cases, and had a high precision (96.4% of the locations assigned to the appropriate segment). Among the 813 winter breaks and 669 calving ground use expected to be detected on the FPT profiles, and assuming that individuals always reduced movements for each of the two periods, we detected 100% of the expected winter breaks and 89% of the expected calving ground use, and identified 648 complete spring migrations. Failures to segment winter breaks or calving ground use were related to individuals only slowing down or performing less pronounced pauses resulting in low mean FPT. Conclusion : We show that our approach, which relies only on the analysis of movement patterns, provides a suitable and easy-to-use tool to study species exhibiting variations in their migration patterns and seasonal range use

    Impact of Electrodes on Recombination in Bulk Heterojunction Organic Solar Cells

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    In recent years, the efficiency of organic solar cells (OSCs) has increased to more than 13%, although different barriers are on the way for reaching higher efficiencies. One crucial barrier is the recombination of charge carriers, which can either occur as the bulk recombination of photogenerated charges or the recombination of photogenerated charges and electrodic induced charges (EICs). This work studies the impact of EICs on the recombination lifetime in OSCs. To this end, the net recombination lifetime of photogenerated charge carriers in the presence of EICs is measured by means of conventional and newly developed transient photovoltage techniques. Moreover, a new approach has been introduced to exclusively measure the bulk recombination lifetime, i.e., in the absence of EICs; this approach was conducted by depositing transparent insulating layers on both sides of the OSC active layer. An examination of these approaches on OSCs with different active layer materials, thicknesses, and varying light intensities determined that the EICs can only reduce the recombination lifetime of the photogenerated charges in OSCs with very weak recombination strength. This work supports that for OSCs with highly reduced recombination strength, eliminating the recombination of photogenerated charges and EICs is critical for achieving better performance. Therefore, the use of a proper blocking layer suppresses EIC recombination in systems with very weak recombination.</p

    Influence du climat, de la disponibilité des ressources et de la taille des populations sur la phénologie et les patrons de migration du caribou migrateur, "Rangifer tarandus"

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    De nombreuses populations migratrices sont actuellement en d√©clin. Les changements climatiques entrainent des modifications dans les habitats des esp√®ces migratrices et la ph√©nologie des processus naturels, lesquels se r√©percutent sur la migration, une p√©riode critique pour ces esp√®ces. Comprendre comment les variables environnementales et climatiques affectent la ph√©nologie et les patrons de migration est donc crucial. Ma th√®se s'int√©resse √† l'impact du climat, des ressources alimentaires et de la comp√©tition sur les migrations printani√®res et automnales des caribous migrateurs, Rangifer tarandus, des troupeaux Rivi√®re-George (TRG) et Rivi√®re-aux-Feuilles (TRF) du Nord-du-Qu√©bec et du Labrador. Le premier volet de ma th√®se propose une approche objective, bas√©e sur la d√©tection des changements dans la structure des d√©placements saisonniers, pour identifier les dates de d√©part et arriv√©e en migration. Valid√©e √† l'aide de trajets simul√©s, elle a √©t√© appliqu√©e aux migrations printani√®res et automnales de femelles caribous. Le second volet porte sur l'impact des conditions environnementales sur la ph√©nologie des migrations de printemps et d'automne. Il montre que la ph√©nologie de la migration est principalement affect√©e par les conditions climatiques rencontr√©es lors de la migration, les conditions d'enneigement affectant notamment les co√Ľts des d√©placements. Au printemps, les caribous subissent des conditions d√©favorables lorsque la fonte des neiges est pr√©coce. √Ä l'automne, ils semblent ajuster leurs d√©placements et migrent plus vite quand la neige d√©bute t√īt pour limiter les co√Ľts de d√©placement dans une neige profonde. Le troisi√®me volet porte sur les patrons de migration √† l'automne et montre que ceux-ci sont affect√©s essentiellement par une comp√©tition intra- et inter-troupeaux pour les aires d'hivernages. Les caribous du TRG r√©pondent √† une augmentation de la comp√©tition sur les aires les plus proches de l'aire de mise bas, li√©e √† une taille de population √©lev√©e, en migrant pr√©f√©rentiellement vers les aires les plus √©loign√©es. L'utilisation des aires hivernales par les caribous du TRF est, quant √† elle, contrainte par la pr√©sence et l‚Äôabondance du TRG, cette contrainte diminuant √† mesure que le TRG d√©cline et abandonne les migrations vers les aires d'hivernages communes aux deux troupeaux. Cette th√®se am√©liore notre compr√©hension de l'influence des facteurs environnementaux sur la ph√©nologie et les patrons de migration du caribou migrateur. Ces connaissances sont tr√®s utiles pour comprendre l'impact des changements climatiques et √©tablir les plans de conservation pour les esp√®ces migratrices.Several populations of migratory species are actually declining. Climate changes affect the habitat of migratory species and the phenology of natural processes, and impact the migration, a critical period for migratory species. Thus, it is crucial to understand how environmental and climatic variables affect the timing and the patterns of migration. This thesis assesses the impact of climate, food resources and competition on the spring and fall migrations of migratory caribou, Rangifer tarandus, from the Rivi√®re-George (RGH) and Rivi√®re-aux-Feuilles (RFH) herds, in Northern Qu√©bec and Labrador. The first part of my thesis presents an objective approach, based on the detection of changes in the structure of seasonal movements, to assess the departure and the arrival dates of the migrations. The approach was validated on simulated paths, and was then applied on the spring and fall migrations of female caribou. The second part focuses on the impact of environmental conditions on the phenology of the spring and fall migrations. It revealed that migration is mainly affected by the climatic conditions encountered during migration, snow conditions partly determining the cost of movements. In the spring, caribou suffer from adverse conditions when the snowmelt is early. In the fall, caribou adjust their movements and migrate faster when snowfall occurs early to limit the cost of moving through deep snow. The third part of my thesis focuses on fall migration patterns and revealed that migration patterns are mainly affected by intra- and inter-herds competition for the winter ranges. Caribou from RGH migrate preferentially toward the furthest winter ranges in response to increased competition, linked with a high population size, limiting the competition on the closest winter ranges. The use of the winter ranges by caribou from RFH is constrained by the abundance of RGH. This constraint decreased as RGH declined and abandoned the migrations toward the winter ranges commonly used by both herds. My thesis increases our knowledge of the environmental factors that affect the phenology and patterns of caribou migrations. This knowledge is useful to understand the impact of climate changes and establish conservation plans for migratory species

    Ocelotl: Large Trace Overviews Based on Multidimensional Data Aggregation

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    International audiencePerformance analysis of parallel applications is commonly based on execution traces that might be investigated through visualization techniques. The weak scalability of such techniques appears when traces get larger both in time (many events registered) and space (many processing elements), a very common situation for current large-scale HPC applications. In this paper we present an approach to tackle such scenarios in order to give a correct overview of the behavior registered in very large traces. Two configurable and controlled aggregation-based techniques are presented: one based exclusively on the temporal aggregation, and another that consists in a spatiotemporal aggregation algorithm. The paper also details the implementation and evaluation of these techniques in Ocelotl, a performance analysis and visualization tool that overcomes the current graphical and interpretation limitations by providing a concise overview registered on traces. The experimental results show that Ocelotl helps in detecting quickly and accurately anomalies in 8 GB traces containing up to two hundred million of events

    Overproduction and identification of butyrolactones SCB1-8 in the antibiotic production superhost Streptomyces M1152

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    Gamma-butyrolactones (GBLs) are signalling molecules that control antibiotic production in Streptomyces bacteria. The genetically engineered strain S. coelicolor M1152 was found to overproduce GBLs SCB1-3 as well as five novel GBLs named SCB4-8. Incorporation experiments using isotopically-labelled precursors confirmed the chemical structures of SCB1-3 and established those of SCB4-8
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