GaAs converters for high power densities of laser illumination

Photovoltaic power converters can be used to generate electricity directly from laser light. In this paper we report the development of GaAs PV power converters with improved conversion efficiency at high power densities. The incorporation of a lateral conduction layer (LCL) on top of the window layer resulted in a considerable gain in efficiency at high illumination levels. Additional performance improvements were obtained by using a metal electrode grid design and antireflection coating optimised for monochromatic and inhomogeneous laser light. Maximum monochromatic (810 nm) optical‐to‐electrical conversion efficiency of 54·9% at 36·5 W/cm² has been achieved. The characteristics of laser power converters with p/n and n/p polarity are discussed in this paper. Moreover, different materials and doping levels were applied in the LCL. The performance of these different device structures at high laser intensity is presented and discussed. It is shown that the lateral series resistance of the cell has a major impact on the overall device performance. Copyright © 2008 John Wiley & Sons, Ltd.     

Development,characterisation and 1000 Suns outdoor tests of GaAs monolithic interconnected module (MIM) receivers

Monolithic interconnected modules (MIMs) are large area, high voltage PV devices which perform well at very high light intensities. They are therefore well suited for the assembly of dense array receivers. The latter can be employed in solar concentrator systems such as parabolic dishes at a concentration ratio of 1000 Suns or more. This paper reports on progress in the development and testing of GaAs MIMs and of water‐cooled dense array receivers assembled from MIMs. The MIMs are electrically protected by integrated bypass diodes and, under indoor laboratory tests, reach an efficiency of 20·0% at 1000 Suns and 22·9% at 200 Suns. Several dense array receivers have been assembled, one of which was tested outdoors at 1‐Sun and at concentration ratios of several hundred Suns and up to slightly above 1000 Suns using the PETAL solar dish facility in Sede Boqer, Israel. In addition to I–V curve measurements, the high‐concentration tests included measurements that quantified the light intensity distribution over the dense array. Deformations in some of the I–V plots were observed for intensity distributions that departed substantially from perfect uniformity. The shapes of these plots were successfully reproduced by an electronic network simulation of the inhomogeneously illuminated receiver. 1‐Sun I–V curve measurements and visual inspections performed before and after exposure of the module to concentrated sunlight revealed no indications of degradation. Copyright © 2007 John Wiley & Sons, Ltd.     

Order and membrane stability from magnetic resonance measurements: an application of statistical thermodynamics

Summary This paper compares different approaches, reported in the literature, for obtaining thermodynamic statistical information from ESR and NMR measurements on the conformations of lipid bilayers. A statistical approach recently proposed by the authors has been employed to reexamine several literature values of order parameters and to calculate Helmholtz free energy and entropy. This approach was applied to eggphosphatidylcholine, cerebroside and gangliosoyds, spin labelled at different acyl chain positions, and to bilayered membranes containing phosphatidylcholine and different gangliosides (GM1, GD1a and GT1b) at increasing glycolipid mole percent. The variation of the stability of the membrane with the peroxidation is also reexamined at different intramembrane locations by various probes. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Influence of the deposited-energy function on the sputtering yield

Summary An explicit spatial dependence (a Gaussian) of the depositedenergy function has been considered in order to compare the sputtering yield obtained by taking into account all the energy deposited inside the target with that obtained by assuming the deposited-energy function throughout constant, namely equal to its value at the surface. The results indicate that the two yields differ sensibly for high incident-ion atomic number and/or for small target/ion mass ratio. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Spectrophotometric study of the complexation reaction between uranium/VI/ and one semicarbazone /SAS/

Salicylaldehyde-semicarbazone /SAS/ forms a complex with uranyl ion at pH 3.8±0.1 which has been studied spectrophotometrically. The results of Job's and moral ratio methods indicate 14 composition for the complex which at 230 nm obeys Beer's law in the range of 1 to 2500 ppb of uranium/VI/.     

The effect of the correlation energy on the mechanism of the Diels—Alder reaction

The effect of the correlation energy on the mechanism of the butadiene + ethylene reaction has been studied by means of Móller-Plesset perturbation theory. It is shown that its introduction stabilizes preferentially the asynchronous structures, thus favoring the asynchronism of the process.     

Standing optical phonons in finite semiconductor superlattices studied by resonant Raman scattering in a double microcavity

We report optical double resonant enhancement of Raman scattering in a new double microcavity geometry. The design allows almost backscattering geometries, providing easy access to the excitations' in-plane dispersion. The cavity is used to study the phonon spectra of a finite GaAs/AlAs superlattice. A new type of "standing optical vibration" is demonstrated involving the GaAs confined phonons with a standing wave envelope determined by the superlattice thickness. A strong dispersion of the first order standing wave mode is observed, as well as its anticrossing with higher order confined modes of the same symmetry.     

A theoretical study of the reaction between cyclopentadiene and protonated imine derivatives: a shift from a concerted to a stepwise molecular mechanism

The reaction between cyclopentadiene and protonated pyridine-2-carboxaldehyde imine derivatives has been studied by using Hartree-Fock (HF) and B3LYP methods together with the 6-31G basis set. The molecular mechanism is stepwise along an inverted energy profile. This results from the protonation on both nitrogen atoms of the imine group and the pyridine framework. The first step corresponds to the nucleophilic attack of cyclopentadiene on the electron-poor carbon atom of the iminium cation group to give an acyclic cation intermediate, and the second step is associated with the ring closure of this intermediate via the formation of a C-N single bond yielding the final cycloadduct. Two reactive channels have been characterized corresponding to the endo and exo approach modes of the cyclopentadiene to the iminium cation. The role of the pyridium cation substituent and the nitrogen position (ortho, meta, and para) along the reaction pathway has been also considered. Solvent effects (dichloromethane) by means of a continuum model have been taken into account to model the experimental environment.