1,329 research outputs found
Conformal GaP layers on Si wire arrays for solar energy applications
We report conformal, epitaxial growth of GaP layers on arrays of Si microwires. Silicon wires grown using chlorosilane chemical vapor deposition were coated with GaP grown by metal-organic chemical vapor deposition. The crystalline quality of conformal, epitaxial GaP/Si wire arrays was assessed by transmission electron microscopy and x-ray diffraction. Hall measurements and photoluminescence show p- and n-type doping with high electron mobility and bright optical emission. GaP pn homojunction diodes on planar reference samples show photovoltaic response with an open circuit voltage of 660 mV
10 µm minority-carrier diffusion lengths in Si wires synthesized by Cu-catalyzed vapor-liquid-solid growth
The effective electron minority-carrier diffusion length, L_(n,eff), for 2.0 µm diameter Si wires that were synthesized by Cu-catalyzed vapor-liquid-solid growth was measured by scanning photocurrent microscopy. In dark, ambient conditions, L_(n,eff) was limited by surface recombination to a value of ≤ 0.7 µm. However, a value of L_(n,eff) = 10.5±1 µm was measured under broad-area illumination in low-level injection. The relatively long minority-carrier diffusion length observed under illumination is consistent with an increased surface passivation resulting from filling of the surface states of the Si wires by photogenerated carriers. These relatively large L_(n,eff) values have important implications for the design of high-efficiency, radial-junction photovoltaic cells from arrays of Si wires synthesized by metal-catalyzed growth processes
High-performance Si microwire photovoltaics
Crystalline Si wires, grown by the vapor–liquid–solid (VLS)
process, have emerged as promising candidate materials for lowcost, thin-film photovoltaics. Here, we demonstrate VLS-grown Si microwires that have suitable electrical properties for high-performance photovoltaic applications, including long minority-carrier diffusion lengths (L_n » 30 µm) and low surface recombination velocities (S « 70 cm·s^(-1)). Single-wire radial p–n junction solar cells were fabricated with amorphous silicon and silicon nitride
surface coatings, achieving up to 9.0% apparent photovoltaic efficiency, and exhibiting up to ~600 mV open-circuit voltage with over 80% fill factor. Projective single-wire measurements and optoelectronic simulations suggest that large-area Si wire-array solar cells have the potential to exceed 17% energy-conversion efficiency, offering a promising route toward cost-effective crystalline Si photovoltaics
GaP/Si wire array solar cells
Si wire arrays have recently demonstrated their potential
as photovoltaic devices [1-3]. Using these arrays as a
base, we consider a next generation, multijunction wire
array architecture consisting of Si wire arrays with a
conformal GaN_xP_(1-x-y)As_y coating. Optical absorption and
device physics simulations provide insight into the design
of such devices. In particular, the simulations show that
much of the solar spectrum can be absorbed as the angle
of illumination is varied and that an appropriate choice of
coating thickness and composition will lead to current
matching conditions and hence provide a realistic path to
high efficiencies. We have previously demonstrated high
fidelity, high aspect ratio Si wire arrays grown by vapor-liquid-solid techniques, and we have now successfully
grown conformal GaP coatings on these wires as a
precursor to considering quaternary compound growth.
Structural, optical, and electrical characterization of these GaP/Si wire array heterostructures, including x-ray
diffraction, Hall measurements, and optical absorption of
polymer-embedded wire arrays using an integrating
sphere were performed. The GaP epilayers have high
structural and electrical quality and the ability to absorb a significant amount of the solar spectrum, making them
promising for future multijunction wire array solar cells
Optoelectronic analysis of multijunction wire array solar cells
Wire arrays have demonstrated promising photovoltaic performance as single junction solar cells and are well suited to defect mitigation in heteroepitaxy. These attributes can combine in tandem wire array solar cells, potentially leading to high efficiencies. Here, we demonstrate initial growths of GaAs on Si_(0.9)Ge_(0.1) structures and investigate III-V on Si_(1-x)Ge_x device design with an analytical model and optoelectronic simulations. We consider Si_(0.1)Ge_(0.9) wires coated with a GaAs_(0.9)P_(0.1) shell in three different geometries: conformal, hemispherical, and spherical. The analytical model indicates that efficiencies approaching 34% are achievable with high quality materials. Full field electromagnetic simulations serve to elucidate the optical loss mechanisms and demonstrate light guiding into the wire core. Simulated current-voltage curves under solar illumination reveal the impact of a varying GaAs_(0.9)P_(0.1) minority carrier lifetime. Finally, defective regions at the hetero-interface are shown to have a negligible effect on device performance if highly doped so as to serve as a back surface field. Overall, the growths and the model demonstrate the feasibility of the proposed geometries and can be used to guide tandem wire array solar cell designs
Si microwire-array solar cells
Si microwire-array solar cells with Air Mass 1.5 Global conversion efficiencies of up to 7.9% have been fabricated using an active volume of Si equivalent to a 4 μm thick Si wafer. These solar cells exhibited open-circuit voltages of 500 mV, short-circuit current densities (J_(sc)) of up to 24 mA cm^(-2), and fill factors >65% and employed Al_2O_3 dielectric particles that scattered light incident in the space between the wires, a Ag back reflector that prevented the escape of incident illumination from the back surface of the solar cell, and an a-SiN_x:H passivation/anti-reflection layer. Wire-array solar cells without some or all of these design features were also fabricated to demonstrate the importance of the light-trapping elements in achieving a high J_(sc). Scanning photocurrent microscopy images of the microwire-array solar cells revealed that the higher J_(sc) of the most advanced cell design resulted from an increased absorption of light incident in the space between the wires. Spectral response measurements further revealed that solar cells with light-trapping elements exhibited improved red and infrared response, as compared to solar cells without light-trapping elements
Photoelectrochemical Hydrogen Evolution Using Si Microwire Arrays
Arrays of B-doped p-Si microwires, diffusion-doped with P to form a radial n+ emitter and subsequently coated with a 1.5-nm-thick discontinuous film of evaporated Pt, were used as photocathodes for H_2 evolution from water. These electrodes yielded thermodynamically based energy-conversion efficiencies >5% under 1 sun solar simulation, despite absorbing less than 50% of the above-band-gap incident photons. Analogous p-Si wire-array electrodes yielded efficiencies <0.2%, largely limited by the low photovoltage generated at the p-Si/H_2O junction
High resolution X-ray spectroscopy and imaging of Mrk573
We present a detailed analysis of the XMM-Newton RGS high resolution X-ray
spectra of the Seyfert 2 galaxy, Mrk573. This analysis is complemented by the
study of the Chandra image, and its comparison to optical (HST) and radio (VLA)
data. The soft X-ray emission is mainly due to gas photoionised by the central
AGN, as indicated by the detection of radiative recombination continua from
OVII and OVIII, as well as by the prominence of the OVII forbidden line. This
result is confirmed by the best fit obtained with a self-consistent CLOUDY
photoionisation model. However, a collisionally excited component is also
required, in order to reproduce the FeXVII lines, accounting for about 1/3 of
the total luminosity in the 15-26 A band. Once adopted the same model in the
Chandra ACIS data, another photoionised component, with higher ionisation
parameter, is needed to take into account emission from higher Z metals. The
broadband ACIS spectrum also confirms the Compton-thick nature of the source.
The imaging analysis shows the close morphological correspondence between the
soft X-ray and the [OIII] emission. The radio emission appears much more
compact, although clearly aligned with the narrow line region. The collisional
phase of the soft X-ray emission may be due to starburst, requiring a star
formation rate of M yr, but there is no clear
evidence of this kind of activity from other wavelengths. On the other hand, it
may be related to the radio ejecta, responsible for the heating of the plasma
interacting with the outflow, but the estimated pressure of the hot gas is much
larger than the pressure of the radio jets, assuming equipartition and under
reasonable physical parameters.Comment: 12 pages, 6 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society Main Journa
Defective cortex glia plasma membrane structure underlies light-induced epilepsy in cpes mutants
Seizures induced by visual stimulation (photosensitive epilepsy; PSE) represent a common type of epilepsy in humans, but the molecular mechanisms and genetic drivers underlying PSE remain unknown, and no good genetic animal models have been identified as yet. Here, we show an animal model of PSE, in Drosophila, owing to defective cortex glia. The cortex glial membranes are severely compromised in ceramide phosphoethanolamine synthase (cpes)-null mutants and fail to encapsulate the neuronal cell bodies in the Drosophila neuronal cortex. Expression of human sphingomyelin synthase 1, which synthesizes the closely related ceramide phosphocholine (sphingomyelin), rescues the cortex glial abnormalities and PSE, underscoring the evolutionarily conserved role of these lipids in glial membranes. Further, we show the compromise in plasma membrane structure that underlies the glial cell membrane collapse in cpes mutants and leads to the PSE phenotype
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