743 research outputs found
Cadmium Telluride Solar Cells on Ultrathin Glass for Space Applications
This paper details the preliminary findings of a study to achieve a durable thin film CdTe photovoltaic device structure onto ultra-thin space qualified cover glass. An aluminium doped zinc oxide (AZO) transparent conducting oxide (TCO) was deposited directly onto cover glass using metal organic chemical vapour deposition (MOCVD). The AZO demonstrated a low sheet resistance of 10 Ω/⥠and high optical transparency of 85% as well as an excellent adherence and environmental stability. Preliminary deposition of the photovoltaic layers onto the AZO on cover glass, by MOCVD, showed the possibility of such a structure yielding a device conversion efficiency of 7.2 %. High series resistance (10 Ω.cm2) and low Voc (586 mV) were identified as the limiting factors when compared to the authors platform process on indium tin oxide (ITO) coated aluminosilicate. The coverage of the Cd1-xZnxS window layer along with the front contacting of the device was shown to be the major cause of the low efficiency. Further deposition of the AZO/CdTe employing an oxygen plasma cleaning step to the cover glass and evaporated gold front contacts significantly improved the device performance. A best conversion efficiency of 10.2 % with series resistance improved to 4.4 Ω.cm2 and open circuit voltage (Voc) up to 667 mV and good adhesion has demonstrated for the first time direct deposition of CdTe solar cells onto 100 ÎŒm thick space qualified cover glass
CdCl2 treatment related diffusion phenomena in Cd1xZnxS/CdTe solar cells
Utilisation of wide bandgap Cd1_xZnxS alloys as an alternative to the CdS window layer is an attractive route to enhance the performance of CdTe thin film solar cells. For successful implementation, however, it is vital to control the composition and properties of Cd1_xZnxS through device fabrication processes involving the relatively high-temperature CdTe deposition and CdCl2 activation steps. In this study, cross-sectional scanning transmission electron microscopy and depth profiling methods were employed to investigate chemical and structural changes in CdTe/Cd1_xZnxS/CdS superstrate device structures deposited on an ITO/boro-aluminosilicate substrate. Comparison of three devices in different states of completionâfully processed (CdCl2 activated), annealed only (without CdCl2 activation), and a control (without CdCl2 activation or anneal)ârevealed cation diffusion phenomena within the window layer, their effects closely coupled to the CdCl2 treatment. As a result, the initial Cd1_xZnxS/CdS bilayer structure was observed to unify into a single Cd1_xZnxS layer with an increased Cd/Zn atomic ratio; these changes defining the properties and performance of the Cd1_xZnxS/CdTe device
Marked colour divergence in gliding membranes of a tropical lizard mirror population differences in the colour of falling leaves
Populations of the Bornean gliding lizard, Draco cornutus, differ markedly in the colour of their gliding membranes. They also differ in local vegetation type (mangrove forest versus lowland rainforest) and consequently, the colour of falling leaves (red and brown/black in mangrove versus green, brown and black in rainforest). We show that the gliding membranes of these lizards closely match the colours of freshly fallen leaves in the local habitat as they appear to the visual system of birds (their probable predators). Furthermore, gliding membranes more closely resembled colours of local fallen leaves than standing foliage or fallen leaves in the other populationâs habitat. This suggests that the two populations have diverged in gliding membrane coloration to match the colours of their local falling leaves, and that mimicking falling leaves is an adaptation that functions to
reduce predation by birds
Asymptotic properties of Born-improved amplitudes with gauge bosons in the final state
For processes with gauge bosons in the final state we show how to
continuously connect with a single Born-improved amplitude the resonant region,
where resummation effects are important, with the asymptotic region far away
from the resonance, where the amplitude must reduce to its tree-level form.
While doing so all known field-theoretical constraints are respected, most
notably gauge-invariance, unitarity and the equivalence theorem. The
calculations presented are based on the process , mediated by a
possibly resonant Higgs boson; this process captures all the essential
features, and can serve as a prototype for a variety of similar calculations.
By virtue of massive cancellations the resulting closed expressions for the
differential and total cross-sections are particularly compact.Comment: 23 pages, Latex, 4 Figures, uses axodra
From Predicting Solar Activity to Forecasting Space Weather: Practical Examples of Research-to-Operations and Operations-to-Research
The successful transition of research to operations (R2O) and operations to
research (O2R) requires, above all, interaction between the two communities. We
explore the role that close interaction and ongoing communication played in the
successful fielding of three separate developments: an observation platform, a
numerical model, and a visualization and specification tool. Additionally, we
will examine how these three pieces came together to revolutionize
interplanetary coronal mass ejection (ICME) arrival forecasts. A discussion of
the importance of education and training in ensuring a positive outcome from
R2O activity follows. We describe efforts by the meteorological community to
make research results more accessible to forecasters and the applicability of
these efforts to the transfer of space-weather research.We end with a
forecaster "wish list" for R2O transitions. Ongoing, two-way communication
between the research and operations communities is the thread connecting it
all.Comment: 18 pages, 3 figures, Solar Physics in pres
Two-Loop Corrections to the Fermionic Decay Rates of the Standard-Model Higgs Boson
Low- and intermediate mass Higgs bosons decay preferably into fermion pairs.
The one-loop electroweak corrections to the respective decay rates are
dominated by a flavour-independent term of . We calculate
the two-loop gluon correction to this term. It turns out that this correction
screens the leading high- behaviour of the one-loop result by roughly
10\%. We also present the two-loop QCD correction to the contribution induced
by a pair of fourth-generation quarks with arbitrary masses. As expected, the
inclusion of the QCD correction considerably reduces the renormalization-scheme
dependence of the prediction.Comment: 14 pages, latex, figures 2-5 appended, DESY 94-08
Mapping crustal shear wave velocity structure and radial anisotropy beneath West Antarctica using seismic ambient noise
Using 8â25s period Rayleigh and Love wave phase velocity dispersion data extracted from seismic ambient noise, we (i) model the 3D shear wave velocity structure of the West Antarctic crust and (ii) map variations in crustal radial anisotropy. Enhanced regional resolution is offered by the UK Antarctic Seismic Network. In the West Antarctic Rift System (WARS), a ridge of crust ~26â30km thick extending south from Marie Byrd Land separates domains of more extended crust (~22km thick) in the Ross and Amundsen Sea Embayments, suggesting alongâstrike variability in the Cenozoic evolution of the WARS. The southern margin of the WARS is defined along the southern Transantarctic Mountains (TAM) and Haag NunataksâEllsworth Whitmore Mountains (HEW) block by a sharp crustal thickness gradient. Crust ~35â40km is modelled beneath the Haag NunataksâEllsworth Mountains, decreasing to ~30â32km km thick beneath the Whitmore Mountains, reflecting distinct structural domains within the composite HEW block. Our analysis suggests that the lower crust and potentially the mid crust is positively radially anisotropic (VSH > VSV) across West Antarctica. The strongest anisotropic signature is observed in the HEW block, emphasising its unique provenance amongst West Antarctica's crustal units, and conceivably reflects a ~13km thick metasedimentary succession atop Precambrian metamorphic basement. Positive radial anisotropy in the WARS crust is consistent with observations in extensional settings, and likely reflects the latticeâpreferred orientation of minerals such as mica and amphibole by extensional deformation. Our observations support a contention that anisotropy may be ubiquitous in continental crust
Thinking about growth : a cognitive mapping approach to understanding small business development
School of Managemen
Towards Machine Wald
The past century has seen a steady increase in the need of estimating and
predicting complex systems and making (possibly critical) decisions with
limited information. Although computers have made possible the numerical
evaluation of sophisticated statistical models, these models are still designed
\emph{by humans} because there is currently no known recipe or algorithm for
dividing the design of a statistical model into a sequence of arithmetic
operations. Indeed enabling computers to \emph{think} as \emph{humans} have the
ability to do when faced with uncertainty is challenging in several major ways:
(1) Finding optimal statistical models remains to be formulated as a well posed
problem when information on the system of interest is incomplete and comes in
the form of a complex combination of sample data, partial knowledge of
constitutive relations and a limited description of the distribution of input
random variables. (2) The space of admissible scenarios along with the space of
relevant information, assumptions, and/or beliefs, tend to be infinite
dimensional, whereas calculus on a computer is necessarily discrete and finite.
With this purpose, this paper explores the foundations of a rigorous framework
for the scientific computation of optimal statistical estimators/models and
reviews their connections with Decision Theory, Machine Learning, Bayesian
Inference, Stochastic Optimization, Robust Optimization, Optimal Uncertainty
Quantification and Information Based Complexity.Comment: 37 page
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