6,016 research outputs found
Reduced Coulomb interaction in organic solar cells by the introduction of inorganic high-k nanostructured materials
In this article a concept is introduced, which allows for reduced Coulomb
interaction in organic solar cells and as such for enhanced power conversion
efficiencies. The concept is based on the introduction of electrically
insulating, nanostructured high-k materials into the organic matrix, which do
not contribute to the charge transport, however, effectively enhance the
permittivity of the organic active layer and thereby reduce the Coulomb
interaction. Using an analytical model it is demonstrated that even at a
distance of 20 nm to the organic / inorganic interface of the nanostructure,
the Coulomb interaction can be reduced by more than 15 %. The concept is
implemented using P3HT:PCBM solar cells with integrated high-k nanoparticles
(strontium titanate). It could be demonstrated that in comparison to a
reference cell without integrated nanoparticles, the power conversion
efficiencies could be improved by ~20 %.Comment: 11 pages, 7 figure
Redshifts and Luminosities for 112 Gamma Ray Bursts
Two different luminosity indicators have recently been proposed for Gamma Ray
Bursts that use gamma-ray observations alone. They relate the burst luminosity
(L) with the time lag between peaks in hard and soft energies, and the
spikiness or variability of the burst's light curve (V). These relations are
currently justified and calibrated with only 6 or 7 bursts with known red
shifts. We have examined BATSE data for the lag and V for 112 bursts. (1) A
strong correlation between the lag and V exists, and it is exactly as predicted
from the two proposed relations. This is proof that both luminosity indicators
are reliable. (2) GRB830801 is the all-time brightest burst, yet with a small V
and a large lag, and hence is likely the closest known event being perhaps as
close as 3.2 Mpc. (3) We have combined the luminosities as derived from both
indicators as a means to improve the statistical and systematic accuracy when
compared with the accuracy from either method alone. The result is a list of
112 bursts with good luminosities and hence red shifts. (4) The burst averaged
hardness ratio rises strongly with the luminosity of the burst. (5) The burst
luminosity function is a broken power law, with the break at L = 2x10^{52}
erg/s. The luminosity function has power law indices of -2.8+-0.2 above the
break and -1.7+-0.1 below the break. (6) The number density of GRBs varies with
red shift roughly as (1+z)^(2.5+-0.3) between 0.2<z<5. Excitingly, this result
also provides a measure of the star formation rate out to z~5 with no effects
from reddening, and the rate is rising uniformly for red shifts above 2.Comment: 13 pages, 4 figures, submitted to ApJLet
Pursuing Parameters for Critical Density Dark Matter Models
We present an extensive comparison of models of structure formation with
observations, based on linear and quasi-linear theory. We assume a critical
matter density, and study both cold dark matter models and cold plus hot dark
matter models. We explore a wide range of parameters, by varying the fraction
of hot dark matter , the Hubble parameter and the spectral
index of density perturbations , and allowing for the possibility of
gravitational waves from inflation influencing large-angle microwave background
anisotropies. New calculations are made of the transfer functions describing
the linear power spectrum, with special emphasis on improving the accuracy on
short scales where there are strong constraints. For assessing early object
formation, the transfer functions are explicitly evaluated at the appropriate
redshift. The observations considered are the four-year {\it COBE} observations
of microwave background anisotropies, peculiar velocity flows, the galaxy
correlation function, and the abundances of galaxy clusters, quasars and damped
Lyman alpha systems. Each observation is interpreted in terms of the power
spectrum filtered by a top-hat window function. We find that there remains a
viable region of parameter space for critical-density models when all the dark
matter is cold, though must be less than 0.5 before any fit is found and
significantly below unity is preferred. Once a hot dark matter component is
invoked, a wide parameter space is acceptable, including . The
allowed region is characterized by \Omega_\nu \la 0.35 and 0.60 \la n \la
1.25, at 95 per cent confidence on at least one piece of data. There is no
useful lower bound on , and for curious combinations of the other parameters
it is possible to fit the data with as high as 0.65.Comment: 19 pages LaTeX file (uses mn.sty). Figures *not* included due to
length. We strongly recommend obtaining the full paper, either by WWW at
http://star-www.maps.susx.ac.uk/papers/lsstru_papers.html (UK) or
http://www.bartol.udel.edu/~bob/papers (US), or by e-mailing ARL. Final
version, to appear MNRAS. Main revision is update to four-year COBE data.
Miscellaneous other changes and reference updates. No significant changes to
principal conclusion
Twist and Turn: Weak Lensing Image Distortions to Second Order
We account for all the image distortions relevant to weak gravitational
lensing to second order. Besides the familiar shear, convergence, rotation and
flexions, we find a new image distortion with two distinct descriptions, the
twist and the turn. Like rotation, this distortion is not activated
gravitationally to first order, but will be activated by systematic effects. We
examine the rotational properties of twist and turn, and their effect on images
in real and shapelet space. We construct estimators for the new distortion,
taking into account the centroid shift which it generates. We then use these
estimators to make first constraints on twist using the STAGES HST survey; we
find that the mean twist estimator is consistent with zero. We measure
correlation functions for our twist estimator on the survey, again finding no
evidence of systematic effects.Comment: 9 pages, 9 figures. Accepted by MNRAS; expands discussion of
distortions and estimator
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