Engineering topics

Daud, T.

Publications and source records attributed to Daud, T..

At least 55 records · Page 3

NASA NTRS1987

Front surface passivation of silicon solar cells with antireflection coating

It is demonstrated that the deposition and postdeposition sintering of an antireflection (AR) coating in hydrogen acts to passivate silicon solar cells. Cells with and without an SiO2 passivating layer, coated with a TiO(x)/Al2O3 AR coating, showed comparable enhancements in short-wavelength spectral response and in open-circuit voltage Voc after sintering at 400 C for 5 min in a hydrogen ambient. The improvement in Voc of cells without SiO2 is attributed to front-surface passivation by the AR coating during processing.

Crotty, G.↗

NASA NTRS1987

A Surface-Controlled Solar Cell

Open-circuit voltage and cell efficiency increased. Proposed technique for controlling recombination velocity on solar-cell surfaces provides cells of increased efficiency and open-circuit voltage. In present cells, uncontrolled surface recombination velocity degrades opencircuit voltage and efficiency. In cell using proposed technique, transparent conducting layer, insulated from cell contacts, biased to enable variable control of surface recombination velocity.

Daud, T.↗

NASA NTRS1987

Solar Cels With Reduced Contact Areas

Efficiency of silicon solar cells increased about 20 percent using smaller metal-contact area on silicon at front and back of each cell. Reduction in contact area reduces surface recombination velocity under contact and thus reduces reverse saturation current and increases opencircuit voltage..

Daud, T.↗

NASA NTRS1987

Neural network based feed-forward high density associative memory

A novel thin film approach to neural-network-based high-density associative memory is described. The information is stored locally in a memory matrix of passive, nonvolatile, binary connection elements with a potential to achieve a storage density of 10 to the 9th bits/sq cm. Microswitches based on memory switching in thin film hydrogenated amorphous silicon, and alternatively in manganese oxide, have been used as programmable read-only memory elements. Low-energy switching has been ascertained in both these materials. Fabrication and testing of memory matrix is described. High-speed associative recall approaching 10 to the 7th bits/sec and high storage capacity in such a connection matrix memory system is also described.

Daud, T.↗

NASA NTRS1986

Flat-plate solar array project. Volume 4: High-efficiency solar cells

The High Efficiency Solar Cell Task was assigned the objective of understanding and developing high efficiency solar cell devices that would meet the cost and performance goals of the Flat Plate Solar Array (FSA) Project. The need for research dealing with high efficiency devices was considered important because of the role efficiency plays in reducing price per watt of generated energy. The R&D efforts conducted during the 1982 to 1986 period are summarized to provide understanding and control of energy conversion losses associated with crystalline silicon solar cells. New levels of conversion efficiency were demonstrated. Major contributions were made both to the understanding and reduction of bulk and surface losses in solar cells. For example, oxides, nitrides, and polysilicon were all shown to be potentially useful surface passivants. Improvements in measurement techniques were made and Auger coefficients and spectral absorption data were obtained for unique types of silicon sheets. New modelling software was developed including a program to optimize a device design based on input characteristics of a cell.

Leipold, M.↗

NASA NTRS1986

Submicron Silicon MOSFET

Process for making metal-oxide/semiconductor field-effect transistors (MOSFET's) results in gate-channel lengths of only few hundred angstroms about 100 times as small as state-of-the-art devices. Gates must be shortened to develop faster MOSFET's; proposed fabrication process used to study effects of size reduction in MOS devices and eventually to build practical threedimensional structures.

Daud, T.↗

NASA NTRS1986

Improved Solar-Cell Tunnel Junction

Efficiency of multiple-junction silicon solar cells increased by inclusion of p+/n+ tunnel junctions of highly doped GaP between component cells. Relatively low recombination velocity at GaP junction principal reason for recommending this material. Relatively wide band gap also helps increase efficiency by reducing optical losses.

Daud, T.↗

NASA NTRS1986

Dot junction solar cells

A design of solar cells with reduced junction area on the cell surface is investigated for reduction of saturation current and increase in open-circuit voltage. Equidiameter dot junctions distributed across the surface of the cell offer an efficient alternative, with variations in dot diameter and in the spacing between dots giving the required variations in the ratio of junction area to total surface area. A simplified analysis for short-circuit current and other cell parameters, which enables cell design optimization, is presented. Experimental solar-cell performance results, as functions of different area ratios, are presented and compared with the model. It is shown that saturation current reduction is possible for achieving efficiencies as high as 18 percent in flat-plate terrestrial applications.

Daud, T.↗

NASA NTRS1986

Direct experimental determination of voltage across high-low junctions

High-low (HL) junctions form a part of many semiconductor devices, including back surface field solar cells. A first experimental determination and interpretation of the voltage across the HL junction under low- and high-injection conditions is presented as a function of the voltage across a nearby p/n junction. Theoretical analysis from first principles is shown to bear well on the experimental results. In addition, a test structure is proposed for measurement of the effective surface recombination velocity at the HL junctions.

Daud, T.↗

NASA NTRS1985

Low defect, high purity crystalline layers grown by selective deposition

The purity and perfection of a semiconductor is improved by depositing a patterned mask of a material impervious to impurities of the semiconductor on a surface of a blank. When a layer of semiconductor is grown on the mask, the semiconductor will first grow from the surface portions exposed by the openings in the mask and will bridge the connecting portions of the mask to form a continuous layer having improved purity, since only the portions overlying the openings are exposed to defects and impurities.

Morrison, A. D.↗

NASA NTRS1985

Solar Cells With Multiple Small Junctions

Concept for improving efficiency of photovoltaic solar cells based on decreasing p/n junction area in relation to total surface area of cell. Because of reduced junction area, surface leakage drops and saturation current density decreases. Surface passivation helps to ensure short-circuit current remains at high value and response of cells to blue light increases.

Daud, T.↗

NASA NTRS1985

New high-efficiency silicon solar cells

A design for silicon solar cells was investigated as an approach to increasing the cell open-circuit voltage and efficiency for flat-plate terrestrial photovoltaic applications. This deviates from past designs, where either the entire front surface of the cell is covered by a planar junction or the surface is textured before junction formation, which results in an even greater (up to 70%) junction area. The heavily doped front region and the junction space charge region are potential areas of high recombination for generated and injected minority carriers. The design presented reduces junction area by spreading equidiameter dot junctions across the surface of the cell, spaced about a diffusion length or less from each other. Various dot diameters and spacings allowed variations in total junction area. A simplified analysis was done to obtain a first-order design optimization. Efficiencies of up to 19% can be obtained. Cell fabrication involved extra masking steps for selective junction diffusion, and made surface passivation a key element in obtaining good collection. It also involved photolithography, with line widths down to microns. A method is demonstrated for achieving potentially high open-circuit voltages and solar-cell efficiencies.

Daud, T.↗

NASA NTRS1985

Backside charging of the CCD

Until recently, the usefulness of the charge coupled device (CCD) as an imaging sensor was thought to be restricted to within rather narrow boundaries of the visible and near IR spectrum. However, since the discovery of backside charging the full potential of CCD performance is now realized. Indeed, the technique of backside charging not only allows the CCD to be used directly in the UV, EUV, and soft X-ray regimes, it has opened up new opportunities in optimizing charge collection processes as well. The technique of backside charging is discussed, and its properties, use, and potential in the future as it applies to the CCD are described.

Janesick, J.↗

NASA NTRS1985

Fabrication and analysis of dot junction silicon solar cells

A design of solar cells with reduced junction area on the cell surface is investigated for reduction of saturation current and increase in open-circuit voltage. Equidiameter dot junctions distributed across the surface of the cell offer an efficient alternative, with variations in dot diameter and in the spacing between dots giving the required variations in the ratio of junctions area to total surface area. A simplified analysis for short-circuit current and other cell parameters, which enables cell design optimization, is presented. Efficiencies beyond 18 percent are obtainable in flat-plate terrestrial applications. Experimental solar-cell performance results, as functions of different area ratios, and bulk doping are presented. It is shown that saturation current reduction and open-circuit voltage increase is obtained by reduced junction area.

Crotty, G. T.↗

NASA NTRS1985

Simulation analysis of a novel high efficiency silicon solar cell

It is recognized that crystalline silicon photovoltaic module efficiency of 15 percent or more is required for cost-effective photovoltaic energy utilization. This level of module efficiency requires large-area encapsulated production cell efficiencies in the range of 18 to 20 percent. Though the theoretical maximum of silicon solar cell efficiency for an idealized case is estimated to be around 30 percent, practical performance of cells to-date are considerably below this limit. This is understood to be largely a consequence of minority carrier losses in the bulk as well as at all surfaces including those under the metal contacts. In this paper a novel device design with special features to reduce bulk and surface recombination losses is evaluated using numerical analysis technique. Details of the numerical model, cell design, and analysis results are presented.

Mokashi, Anant R.↗

NASA NTRS1984

High-efficiency silicon solar cell research

Progress reports on research in high-efficiency silicon solar cells were presented by eight contractors and JPL. The presentations covered the issues of Bulk and Surface Loss, Modeling, Measurements, and Proof of Concept.

Daud, T.↗

NASA NTRS1984

Oscillating-Crucible Technique for Silicon Growth

Technique yields better mixing of impurities and superior qualiity crystals. Accellerated motion stirs melt which reduces temperature gradients and decreases boundary layer for diffusion of impurities near growing surface. Results better mixing of impurities into melt, decrease in tendency for dendritic growth or cellular growth and crystals with low dislocation density. Applied with success to solution growth and Czochralski growth, resulting in large crystals of superior quality.

Daud, T.↗

NASA NTRS1984

Power system applications of fiber optics

Power system applications of optical systems, primarily using fiber optics, are reviewed. The first section reviews fibers as components of communication systems. The second section deals with fiber sensors for power systems, reviewing the many ways light sources and fibers can be combined to make measurements. Methods of measuring electric field gradient are discussed. Optical data processing is the subject of the third section, which begins by reviewing some widely different examples and concludes by outlining some potential applications in power systems: fault location in transformers, optical switching for light fired thyristors and fault detection based on the inherent symmetry of most power apparatus. The fourth and final section is concerned with using optical fibers to transmit power to electric equipment in a high voltage situation, potentially replacing expensive high voltage low power transformers. JPL has designed small photodiodes specifically for this purpose, and fabricated and tested several samples. This work is described.

Kirkham, H.↗