3,716 research outputs found
X-ray polarimetry with an active-matrix pixel proportional counter
We report the first results from an X-ray polarimeter with a micropattern gas
proportional counter using an amorphous silicon active matrix readout. With
100% polarized X-rays at 4.5 keV, we obtain a modulation factor of 0.33 +/-
0.03, confirming previous reports of the high polarization sensitivity of a
finely segmented pixel proportional counter. The detector described here has a
geometry suitable for the focal plane of an astronomical X-ray telescope.
Amorphous silicon readout technology will enable additional extensions and
improvements.Comment: 4 pages, 4 figures, 1 tabl
Recombination in polymer-fullerene bulk heterojunction solar cells
Recombination of photogenerated charge carriers in polymer bulk
heterojunction (BHJ) solar cells reduces the short circuit current (Jsc) and
the fill factor (FF). Identifying the mechanism of recombination is, therefore,
fundamentally important for increasing the power conversion efficiency. Light
intensity and temperature dependent current-voltage measurements on polymer BHJ
cells made from a variety of different semiconducting polymers and fullerenes
show that the recombination kinetics are voltage dependent and evolve from
first order recombination at short circuit to bimolecular recombination at open
circuit as a result of increasing the voltage-dependent charge carrier density
in the cell. The "missing 0.3V" inferred from comparison of the band gaps of
the bulk heterojunction materials and the measured open circuit voltage at room
temperature results from the temperature dependence of the quasi-Fermi-levels
in the polymer and fullerene domains - a conclusion based upon the fundamental
statistics of Fermions.Comment: Accepted for publication in Physical Review B.
http://prb.aps.org/accepted/B/6b07cO3aHe71bd1b149e1425e58bf2868cda2384d?ajax=1&height=500&width=50
Intrinsic hole mobility and trapping in a regio-regular poly(thiophene)
The transport properties of high-performance thin-film transistors (TFT) made
with a regio-regular poly(thiophene) semiconductor (PQT-12) are reported. The
room-temperature field-effect mobility of the devices varied between 0.004
cm2/V s and 0.1 cm2/V s and was controlled through thermal processing of the
material, which modified the structural order. The transport properties of TFTs
were studied as a function of temperature. The field-effect mobility is
thermally activated in all films at T<200 K and the activation energy depends
on the charge density in the channel. The experimental data is compared to
theoretical models for transport, and we argue that a model based on the
existence of a mobility edge and an exponential distribution of traps provides
the best interpretation of the data. The differences in room-temperature
mobility are attributed to different widths of the shallow localized state
distribution at the edge of the valence band due to structural disorder in the
film. The free carrier mobility of the mobile states in the ordered regions of
the film is the same in all structural modifications and is estimated to be
between 1 and 4 cm2/V s.Comment: 20 pages, 8 figure
Hydrogen dynamics and light-induced structural changes in hydrogenated amorphous silicon
We use accurate first principles methods to study the network dynamics of
hydrogenated amorphous silicon, including the motion of hydrogen. In addition
to studies of atomic dynamics in the electronic ground state, we also adopt a
simple procedure to track the H dynamics in light-excited states. Consistent
with recent experiments and computer simulations, we find that dihydride
structures are formed for dynamics in the light-excited states, and we give
explicit examples of pathways to these states. Our simulations appear to be
consistent with aspects of the Staebler-Wronski effect, such as the
light-induced creation of well separated dangling bonds.Comment: 9 pages, 8 figures, submitted to PR
Higher Descent Data as a Homotopy Limit
We define the 2-groupoid of descent data assigned to a cosimplicial
2-groupoid and present it as the homotopy limit of the cosimplicial space
gotten after applying the 2-nerve in each cosimplicial degree. This can be
applied also to the case of -groupoids thus providing an analogous
presentation of "descent data" in higher dimensions.Comment: Appeared in JHR
Thermally stimulated H emission and diffusion in hydrogenated amorphous silicon
We report first principles ab initio density functional calculations of
hydrogen dynam- ics in hydrogenated amorphous silicon. Thermal motion of the
host Si atoms drives H diffusion, as we demonstrate by direct simulation and
explain with simple models. Si-Si bond centers and Si ring centers are local
energy minima as expected. We also describe a new mechanism for break- ing Si-H
bonds to release free atomic H into the network: a fluctuation bond center
detachment (FBCD) assisted diffusion. H dynamics in a-Si:H is dominated by
structural fluctuations intrinsic to the amorphous phase not present in the
crystal.Comment: 4 pages, 5 figures, In press EPL (Jun. 2007
Approximate ab initio calculation of vibrational properties of hydrogenated amorphous silicon with inner voids
We have performed an approximate ab initio calculation of vibrational
properties of hydrogenated amorphous silicon (a-Si:H) using a molecular
dynamics method. A 216 atom model for pure amorphous silicon (a-Si) has been
employed as a starting point for our a-Si:H models with voids that were made by
removing a cluster of silicon atoms out of the bulk and terminating the
resulting dangling bonds with hydrogens.
Our calculation shows that the presence of voids leads to localized low
energy (30-50 cm^{-1}) states in the vibrational spectrum of the system. The
nature and localization properties of these states are analyzed by various
visualization techniques.Comment: 15 pages with 6 PS figures, to appear in PRB in December 199
SuperWASP: Wide Angle Search for Planets
SuperWASP is a fully robotic, ultra-wide angle survey for planetary transits.
Currently under construction, it will consist of 5 cameras, each monitoring a
9.5 x 9.5 deg field of view. The Torus mount and enclosure will be fully
automated and linked to a built-in weather station. We aim to begin
observations at the beginning of 2003.Comment: 4 pages, 1 figure, to be published in proceedings of "Scientific
Frontiers in Research on Extrasolar Planets
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