14,259 research outputs found
Sunrise to sunset optimization of thin film antireflective coatings for encapsulated, planar silicon solar cells
We present an approach for the optimization of thin film antireflective coatings for encapsulated planar silicon solar cells in which the variations in the incident spectra and angle of incidence (AOI) over a typical day are fully considered. Both the angular and wavelength dependences of the reflectance from the surface, absorptance within the coating, and transmittance into the device are calculated for both single- and doublelayer antireflection coatings with and without thin silicon oxide passivation layers. These data are then combined with spectral data as a function of time of day and internal quantum efficiency to estimate the average short-circuit current produced by a fixed solar cell during a day. This is then used as a figure of merit for the optimization of antireflective layer thicknesses for modules placed horizontally at the equator and on a roof in the UK. Our results indicate that only modest gains in average short-circuit current could be obtained by optimizing structures for sunrise to sunset irradiance rather than AM15 at normal incidence, and fabrication tolerances and uniformities are likely to be more significant. However, we believe that this overall approach to optimization will be of increasing significance for new, potentially asymmetric, antireflection schemes such as those based on subwavelength texturing or other photonic or plasmonic technologies currently under development especially when considered in combination with modules fixed at locations and directions that result in asymmetric spectral conditions
Interface Engineering to Create a Strong Spin Filter Contact to Silicon
Integrating epitaxial and ferromagnetic Europium Oxide (EuO) directly on
silicon is a perfect route to enrich silicon nanotechnology with spin filter
functionality.
To date, the inherent chemical reactivity between EuO and Si has prevented a
heteroepitaxial integration without significant contaminations of the interface
with Eu silicides and Si oxides.
We present a solution to this long-standing problem by applying two
complementary passivation techniques for the reactive EuO/Si interface:
() an hydrogen-Si passivation and () the
application of oxygen-protective Eu monolayers --- without using any additional
buffer layers.
By careful chemical depth profiling of the oxide-semiconductor interface via
hard x-ray photoemission spectroscopy, we show how to systematically minimize
both Eu silicide and Si oxide formation to the sub-monolayer regime --- and how
to ultimately interface-engineer chemically clean, heteroepitaxial and
ferromagnetic EuO/Si in order to create a strong spin filter contact to
silicon.Comment: 11 pages of scientific paper, 10 high-resolution color figures.
Supplemental information on the thermodynamic problem available (PDF).
High-resolution abstract graphic available (PNG). Original research (2016
Parameter Identification of Pressure Sensors by Static and Dynamic Measurements
Fast identification methods of pressure sensors are investigated. With regard
to a complete accurate sensor parameter identification two different
measurement methods are combined. The approach consists on one hand in
performing static measurements - an applied pressure results in a membrane
deformation measured interferometrically and the corresponding output voltage.
On the other hand optical measurements of the modal responses of the sensor
membranes are performed. This information is used in an inverse identification
algorithm to identify geometrical and material parameters based on a FE model.
The number of parameters to be identified is thereby generally limited only by
the number of measurable modal frequencies. A quantitative evaluation of the
identification results permits furthermore the classification of processing
errors like etching errors. Algorithms and identification results for membrane
thickness, intrinsic stress and output voltage will be discussed in this
contribution on the basis of the parameter identification of relative pressure
sensors.Comment: Submitted on behalf of EDA Publishing Association
(http://irevues.inist.fr/EDA-Publishing
Density Functional Theory screening of gas-treatment strategies for stabilization of high energy-density lithium metal anodes
To explore the potential of molecular gas treatment of freshly cut lithium
foils in non-electrolyte based passivation of high energy-density Li anodes,
density functional theory (DFT) has been used to study the decomposition of
molecular gases on metallic lithium surfaces. By combining DFT geometry
optimization and Molecular Dynamics, the effects of atmospheric (N2, O2, CO2)
and hazardous (F2, SO2) gas decomposition on Li(bcc) (100), (110), and (111)
surfaces on relative surface energies, work functions, and emerging electronic
and elastic properties are investigated. The simulations suggest that exposure
to different molecular gases can be used to induce and control reconstructions
of the metal Li surface and substantial changes (up to over 1 eV) in the work
function of the passivated system. Contrary to the other considered gases,
which form metallic adlayers, SO2 treatment emerges as the most effective in
creating an insulating passivation layer for dosages <= 1 mono-layer. The
substantial Li->adsorbate charge transfer and adlayer relaxation produce marked
elastic stiffening of the interface, with the smallest change shown by
nitrogen-treated adlayers
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Studies on Pitting Corrosion of Al-Cu-Li Alloys Part III: Passivation Kinetics of AA2098-T851 Based on the Point Defect Model.
In this paper, the passivation kinetics of AA2098-T851 was investigated by a fundamental theoretical interpretation of experimental results based on the mixed potential model (MPM). The steady state passive layer formed on the AA2098-T851 in NaHCO3 solution in a CO2 atmosphere upon potentiostatic stepping in the anodic direction followed by stepping in the opposite direction was explored. Potentials were selected in a way that both anodic passive dissolution of the metal and hydrogen evolution reaction (HER) occur, thereby requiring the MPM for interpretation. Optimization of the MPM on the experimental electrochemical impedance spectroscopy (EIS) data measured after each potentiostatic step revealed the important role of the migration of Al interstitials in determining the kinetics of passive layer formation and dissolution. More importantly, it is shown that the inequalities of the kinetics of formation and dissolution of the passive layer as observed in opposite potential stepping directions lead to the irreversibility of the passivation process. Finally, by considering the Butler-Volmer (B-V) equation for the cathodic reaction (HER) in the MPM, and assuming the quantum mechanical tunneling of the charge carriers across the barrier layer of the passive film, it was shown that the HER was primarily controlled by the slow electrochemical discharge of protons at the barrier layer/solution (outer layer) interface
Strategies for controlled electronic doping of colloidal quantum dots
Over the last several years tremendous progressed progress has been made in incorporating
Colloidal Quantum Dots (CQDs) as photoactive components in optoelectronic devices. A
significant part of that progress is associated with significant advancements made in
achievingon controlled electronic doping of the CQDs and thus improving the electronic
properties of CQDs solids. Today, a variety of strategies exists towards that purpose and this
minireview aims at surveying major published works in this subject. Additional attention is
given to the many challenges associated with the task of doping CQDs as well as to the
optoelectronic functionalities and applications being realized when successfully achieving light
and heavy electronic doping of CQDs.Peer ReviewedPostprint (author's final draft
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Understanding macroscale functionality of metal halide perovskites in terms of nanoscale heterogeneities
Hybrid metal halide perovskites have shown an unprecedented rise as semiconductor building blocks for solar energy conversion and light-emitting applications. Currently, the field moves empirically towards more and more complex chemical compositions, including mixed halide quadruple cation compounds that allow optical properties to be tuned and show promise for better stability. Despite tremendous progress in the field, there is a need for better understanding of mechanisms of efficiency loss and instabilities to facilitate rational optimization of composition. Starting from the device level and then diving into nanoscale properties, we highlight how structural and compositional heterogeneities affect macroscopic optoelectronic characteristics. Furthermore, we provide an overview of some of the advanced spectroscopy and imaging methods that are used to probe disorder and non-uniformities. A unique feature of hybrid halide perovskite compounds is the propensity for these heterogeneities to evolve in space and time under relatively mild illumination and applied electric fields, such as those found within active devices. This introduces an additional challenge for characterization and calls for application of complimentary probes that can aid in correlating the properties of local disorder with macroscopic function, with the ultimate goal of rationally tailoring synthesis towards optimal structures and compositions
Extended Huckel theory for bandstructure, chemistry, and transport. II. Silicon
In this second paper, we develop transferable semi-empirical parameters for
the technologically important material, silicon, using Extended Huckel Theory
(EHT) to calculate its electronic structure. The EHT-parameters areoptimized to
experimental target values of the band dispersion of bulk-silicon. We obtain a
very good quantitative match to the bandstructure characteristics such as
bandedges and effective masses, which are competitive with the values obtained
within an orthogonal-tight binding model for silicon. The
transferability of the parameters is investigated applying them to different
physical and chemical environments by calculating the bandstructure of two
reconstructed surfaces with different orientations: Si(100) (2x1) and Si(111)
(2x1). The reproduced - and -surface bands agree in part
quantitatively with DFT-GW calculations and PES/IPES experiments demonstrating
their robustness to environmental changes. We further apply the silicon
parameters to describe the 1D band dispersion of a unrelaxed rectangular
silicon nanowire (SiNW) and demonstrate the EHT-approach of surface passivation
using hydrogen. Our EHT-parameters thus provide a quantitative model of
bulk-silicon and silicon-based materials such as contacts and surfaces, which
are essential ingredients towards a quantitative quantum transport simulation
through silicon-based heterostructures.Comment: 9 pages, 9 figure
Overview of CMOS process and design options for image sensor dedicated to space applications
With the growth of huge volume markets (mobile phones, digital cameras…) CMOS technologies for image sensor improve significantly. New process flows appear in order to optimize some parameters such as quantum efficiency, dark current, and conversion gain. Space applications can of course benefit from these improvements. To illustrate this evolution, this paper reports results from three technologies that have been evaluated with test vehicles composed of several sub arrays designed with some space applications as target. These three technologies are CMOS standard, improved and sensor optimized process in 0.35µm generation. Measurements are focussed on quantum efficiency, dark current, conversion gain and noise. Other measurements such as Modulation Transfer Function (MTF) and crosstalk are depicted in [1]. A comparison between results has been done and three categories of CMOS process for image sensors have been listed. Radiation tolerance has been also studied for the CMOS improved process in the way of hardening the imager by design. Results at 4, 15, 25 and 50 krad prove a good ionizing dose radiation tolerance applying specific techniques
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