1,064 research outputs found
Graded-bandgap AlGaAs solar cells for AlGaAs/Ge cascade cells
Some p/n graded-bandgap Al(x)Ga(1-x)As solar cells were fabricated and show AMO conversion efficiencies in excess of 15 percent without antireflection (AR) coatings. The emitters of these cells are graded between 0.008 is less than or equal to x is less than or equal to 0.02 during growth of 0.25 to 0.30 micron thick layers. The keys to achieving this performance were careful selection of organometallic sources and scrubbing oxygen and water vapor from the AsH3 source. Source selection and growth were optimized using time-resolved photoluminescence. Preliminary radiation-resistance measurements show AlGaAs cells degraded less than GaAs cells at high 1 MeV electron fluences, and AlGaAs cells grown on GaAs and Ge substrates degrade comparably
HMM-Based Characterization of Channel Behavior for Networked Control Systems
We study the problem of characterizing the behavior of lossy and data corrupting communication channels in a networked control setting, where the channel\u27s behavior exhibits temporal correlation. We propose a behavior characterization mechanism based on a hidden Markov model (HMM). The use of a HMM in this regard presents multiple challenges including dealing with incomplete observation sequences (due to data losses and corruptions) and the lack of a priori information about the model complexity (number of states in the model). We address the first challenges by using the plant state information and history of received/applied control inputs to fill in the gaps in the observation sequences, and by enhancing the HMM learning algorithm to deal with missing observations . Further, we adopt two model quality criteria for determining behavior model complexity. The contributions of this paper include: (1) an enhanced learning algorithm for refining the HMM model parameters to handle missing observations, and (2) simultaneous use of two well-defined model quality criteria to determine the model complexity. Simulation results demonstrate over 90\% accuracy in predicting the output of a channel at a given time step, when compared to a traditional HMM based model that requires complete knowledge of the model complexity and observation sequence
High-efficiency, deep-junction, epitaxial InP solar cells on (100) and (111)B InP substrates
We report on the development and performance of deep-junction (approximately 0.25 micron), graded-emitter-doped, n(sup +)-p InP solar cells grown by metallorganic chemical vapor deposition (MOCVD). A novel, diffusion-transport process for obtaining lightly-doped p-type base regions of the solar cell is described. The I-V data and external quantum-efficiency response of these cells are presented. The best active-area AMO efficiency for these deep-junction cells on (100)-oriented InP substrates is 16.8 percent, with a J(sub SC) of 31.8 mA/sq cm, a V(sub OC) of 0.843 V, and a fill-factor of 0.85. By comparison, the best cell efficiency on the (111)B-oriented InP substrates was 15.0 percent. These efficiency values for deep-junction cells are encouraging and compare favorably with performance of thin-emitter (0.03 micron) epitaxial cells as well as that of deep-emitter diffused cells. The cell performance and breakdown voltage characteristics of a batch of 20 cells on each of the orientations are presented, indicating the superior breakdown voltage properties and other characteristics of InP cells on the (111)B orientation. Spectral response, dark I-V data, and photoluminescence (PL) measurements on the InP cells are presented with an analysis on the variation in J(sub SC) and V(sub OC) of the cells. It is observed, under open-circuit conditions, that lower-V(sub OC) cells exhibit higher band-edge PL intensity for both the (100) and (111)B orientations. This anomalous behavior suggests that radiative recombination in the heavily-doped n(sup +)-InP emitter may be detrimental to achieving higher V(sub OC) in n(sup +)-p InP solar cells
Lorenz function of BiTe/SbTe superlattices
Combining first principles density functional theory and semi-classical
Boltzmann transport, the anisotropic Lorenz function was studied for
thermoelectric BiTe/SbTe superlattices and their bulk
constituents. It was found that already for the bulk materials BiTe
and SbTe, the Lorenz function is not a pellucid function on charge
carrier concentration and temperature. For electron-doped
BiTe/SbTe superlattices large oscillatory deviations
for the Lorenz function from the metallic limit were found even at high charge
carrier concentrations. The latter can be referred to quantum well effects,
which occur at distinct superlattice periods
Behavioural activation written self-help to improve mood, wellbeing and quality of life in people with dementia supported by informal carers (PROMOTE): study protocol for a single-arm feasibility study.
Background: Increases in life expectancy have resulted in a global rise in dementia
prevalence. Dementia is associated with poor wellbeing, low quality of life and
increased incidence of mental health difficulties such as, low mood or depression.
However, currently there is limited access to evidence-based psychological
interventions for people with dementia experiencing low mood and poor wellbeing.
Behavioural activation-based self-help, supported by informal carers and guided by
mental health professionals, may represent an effective and acceptable solution.
Methods/design: The present study is a Phase II (feasibility) single-arm trial informed
by the MRC Complex Interventions Research Methods Framework. Up to fifty
dementia participant/informal carer dyads will be recruited from a variety of settings
including primary care, dementia-specific health settings, and community outreach.
People living with dementia will receive behavioural activation based self-help and be
supported by their informal carer who has received training in the skills required to
support the self-help approach. In turn, during the use of the intervention the informal
carer will be guided by mental health professionals to help them work through the
materials and problem solve any difficulties. Consistent with the objectives of feasibility
studies, outcomes relating to recruitment from different settings, employment of
different recruitment methods, attrition, data collection procedures, clinical delivery and
acceptability of the intervention will be examined. Clinical outcomes for people with
dementia (symptoms of depression and quality of life) and informal carers (symptoms
of depression and anxiety, carer burden and quality of life) will be measured pretreatment
and at 3 months post-treatment allocation.
Discussion: This study will examine the feasibility and acceptability of a novel
behavioural activation-based self-help intervention designed to promote wellbeing and
improve low mood in people living with dementia, alongside methodological and
procedural uncertainties associated with research-related procedures. As determined
by pre-specified progression criteria, if research procedures and the new intervention
demonstrate feasibility and acceptability, results will then be used to inform the design
of a pilot randomised controlled trial (RCT) to specifically examine remaining
methodological uncertainties associated with recruitment into a randomised controlled
design.This study is collaboratively funded by Cornwall Foundation Partnership Trust, South West
Peninsula Academic Health Sciences Network and the University of Exeter
Annealing of irradiated n+p InP buried homojunctions
At the last SPRAT conference, the Naval Research Laboratory (NRL) presented results from two experiments. One studied n+p diffused junction (DJ) InP solar cells, and the other studied n+p shallow homojunction (SHJ) InP mesa diodes grown by metalorganic chemical vapor deposition (MOCVD). The former work showed that a DJ solar cell in which the maximum power P(sub max) had been degraded by nearly 80 percent under irradiation recovered completely under short circuit illumination at 450K. The recovery was accompanied by the removal of all but one of the radiation-induced defect levels. The latter work, on the other hand, showed that the radiation-induced defects in the SHJ diodes did not anneal until the temperature reached 650K. These results suggest that an irradiated DJ solar cell, under illumination, will anneal at a temperature 200K lower than an irradiated SHJ cell. This is an unexpected result considering the similarity of the devices. The goal of the present research is to explain this different behavior. This paper investigates two points which arose from the previous studies. The first point is that the DJ cells were annealed under illumination while the SHJ diodes were annealed without bias. The second point investigated here is that the emitters of the DJ and SHJ devices were significantly different
An efficient algorithm to calculate intrinsic thermoelectric parameters based on Landauer approach
The Landauer approach provides a conceptually simple way to calculate the
intrinsic thermoelectric (TE) parameters of materials from the ballistic to the
diffusive transport regime. This method relies on the calculation of the number
of propagating modes and the scattering rate for each mode. The modes are
calculated from the energy dispersion (E(k)) of the materials which require
heavy computation and often supply energy relation on sparse momentum (k)
grids. Here an efficient method to calculate the distribution of modes (DOM)
from a given E(k) relationship is presented. The main features of this
algorithm are, (i) its ability to work on sparse dispersion data, and (ii)
creation of an energy grid for the DOM that is almost independent of the
dispersion data therefore allowing for efficient and fast calculation of TE
parameters. The inclusion of scattering effects is also straight forward. The
effect of k-grid sparsity on the compute time for DOM and on the sensitivity of
the calculated TE results are provided. The algorithm calculates the TE
parameters within 5% accuracy when the K-grid sparsity is increased up to 60%
for all the dimensions (3D, 2D and 1D). The time taken for the DOM calculation
is strongly influenced by the transverse K density (K perpendicular to
transport direction) but is almost independent of the transport K density
(along the transport direction). The DOM and TE results from the algorithm are
bench-marked with, (i) analytical calculations for parabolic bands, and (ii)
realistic electronic and phonon results for .Comment: 16 Figures, 3 Tables, submitted to Journal of Computational
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