178 research outputs found
An electronic ratchet is required in nanostructured intermediate band solar cells
We investigate in this letter the intrinsic properties that have limited the
efficiency of nanostructured intermediate band solar cells. Those devices take
advantage of intra-band transitions, which occur on narrow energy width, and
present low radiative recombination efficiency. We derive the minimum
requirements in terms of those two characteristics to achieve efficiencies in
excess of the Shockley-Queisser limit, and show that compatible nanostructures
are challenging to obtain. Especially, we evidence that currently
experimentally considered materials cannot overcome the best single junction
cells. In order to solve those issues, we consider devices including an
electronic ratchet mechanism. Firstly, such devices are shown to be much less
sensitive on the limitations of the nanostructures characteristics, so that
requirements for high efficiencies can be met. Secondly, we show that quantum
well devices present advantages over their quantum dots counterparts, although
they have attracted much less interest so far
Chemo-enzymatic hybrid process for production of monatin, a high intensity sweetener
Monatin, 4-hydroxy-4-(3-indolylmethyl)-glutamic acid, is a naturally occurring sweet amino acid isolated from the plant Sclerochiton ilicifolius, found in South Africa [1]. Monatin has two asymmetric centers at C2 and C4, the (2R,4R)-monatin isomer has been found to be the sweetest among its four stereoisomers. It is 2700 times sweeter than sugar and has a clean taste like sugar. Because of these properties, (2R,4R)-monatin has been expected as an new high-intensity sweetener [2]. However, industrial production process of (2R,4R)-monatin using inexpensive raw materials has not been established owing to the difficulty for optically specific synthesis. Here, we report a chemo-enzymatic hybrid process for production of (2R,4R)-monatin from l-tryptophan. In the steps of enzymatic reaction from l-tryptophan, l-amino acid deaminase and aldolase were used for production of 4-(Indole-3-ylmethyl)-4-hydroxy-2-oxoglutaric acid (IHOG) with pyruvic acid as co-substrate. The keto-form of (2R,4R)-monatin, (R)-IHOG, was specifically synthesized by using R-specific aldolase from Shingomonas sp. in the second reaction. In the next chemical reaction steps, (R)-IHOG was converted to the oxime form, reduced to (2R,4R) and (2S,4R)-monatin, and (2R,4R)-monatin salt was obtained from optical resolution by crystallization. By the combination of epimerization and crystallization, (2R,4R)-monatin was obtained specifically from the mixture of diastereomers. In this study, we established an efficient production process for (2R,4R)-monatin using both chemical and enzymatic reactions, and a large amount of (2R,4R)-monatin was prepared by the bench-scale production.
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Generalized Reciprocity Relations in Solar Cells with Voltage-Dependent Carrier Collection: Application to p-i-n Junction Devices
Two reciprocity theorems are important for fundamental understanding of the
solar cell operation and applications to device evaluation: (1) the
carrier-transport reciprocity connecting the dark-carrier injection with the
short-circuit photocarrier collection and (2) the optoelectronic reciprocity
connecting the electroluminescence with the photovoltaic quantum efficiency at
short circuit. These theorems, however, fail in devices with thick depletion
regions such as p-i-n junction solar cells. By properly linearizing the
carrier-transport equation in such devices, we report that the dark-carrier
injection is related to the photocarrier collection efficiency at the operating
voltage, not at short circuit as suggested in the original theorem. This leads
to the general form of the optoelectronic reciprocity relation connecting the
electroluminescence with the voltage-dependent quantum efficiency, providing a
correct interpretation of the optoelectronic properties of p-i-n junction
devices. We also discuss the validity of the well-known relation between the
open-circuit voltage and the external luminescence efficiency. The impact of
illumination intensity and device parameters on the validity of the reciprocity
theorems is quantitatively investigated
A New Experimental Approach to Evaluate Plasma-induced Damage in Microcantilever
Plasma etching, during micro-fabrication processing is indispensable for fabricating MEMS structures. During the plasma processes, two major matters, charged ions and vacuum–ultraviolet (VUV) irradiation damage, take charge of reliability degradation. The charged ions induce unwanted sidewall etching, generally called as “notching”, which causes degradation in brittle strength. Furthermore, the VUV irradiation gives rise to crystal defects on the etching surface. To overcome the problem, neutral beam etching (NBE), which use neutral particles without the VUV irradiation, has been developed. In order to evaluate the effect of the NBE quantitatively, we measured the resonance property of a micro-cantilever before and after NBE treatment. The thickness of damage layer (δ) times the imaginary part of the complex Young's modulus (Eds) were then compared, which is a parameter of surface damage. Although plasma processes make the initial surface of cantilevers damaged during their fabrication, the removal of that damage by NBE was confirmed as the reduction in δEds. NBE will realize a damage-free surface for microstructures
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