Engineering topics

Brandhorst, H. W., Jr.

Publications and source records attributed to Brandhorst, H. W., Jr..

At least 19 records

NASA NTRS1986

Lithium counterdoped silicon solar cell

The resistance to radiation damage of an n(+)p boron doped silicon solar cell is improved by lithium counterdoping. Even though lithium is an n-dopant in silicon, the lithium is introduced in small enough quantities so that the cell base remains p-type. The lithium is introduced into the solar cell wafer by implantation of lithium ions whose energy is about 50 keV. After this lithium implantation, the wafer is annealed in a nitrogen atmosphere at 375 C for two hours.

Weinberg, I.↗

NASA NTRS1986

Spacecraft 2000 - The challenge of the future

The need for spacecraft bus technology advances in order to develop the spacecraft for the 21st century is discussed. Consideration is given to the power and electric propulsion systems for mass-limited satellites such as LEO and GEO. The goal of spacecraft bus technology programs is to design a cost-effective spacecraft which operates well in the satellite environment. The possibility of collaboration between government and industry is examined.

Brandhorst, H. W., Jr.↗

NASA NTRS1984

Solar-array-materials passive LDEF experiment (A0171)

The objective of this experiment is to evaluate the synergistic effects of the space environment on various solar-array materials, including solar cells, cover slips with various antireflectance coatings, adhesive, encapsulants, reflector materials, substrate strength materials, mast and harness materials, structural composites, and thermal control treatments. The experiment is passive and consists of an arrangement of material specimens mounted in a 3-in.-deep peripheral tray. The effects of the space environment on the specimens will be determined by comparison of preflight and postflight measurements of mechanical, electrical, and optical properties.

Whitaker, A. F.↗

NASA NTRS1984

The effects of lithium counterdoping on radiation damage and annealing in n(+)p silicon solar cells

Boron-doped silicon n(+)p solar cells were counterdoped with lithium by ion implantation and the resultant n(+)p cells irradiated by 1 MeV electrons. Performance parameters were determined as a function of fluence and a deep level transient spectroscopy (DLTS) study was conducted. The lithium counterdoped cells exhibited significantly increased radiation resistance when compared to boron doped control cells. Isochronal annealing studies of cell performance indicate that significant annealing occurs at 100 C. Isochronal annealing of the deep level defects showed a correlation between a single defect at E sub v + 0.43 eV and the annealing behavior of short circuit current in the counterdoped cells. The annealing behavior was controlled by dissociation and recombination of this defect. The DLTS studies showed that counterdoping with lithium eliminated three deep level defects and resulted in three new defects. The increased radiation resistance of the counterdoped cells is due to the interaction of lithium with oxygen, single vacancies and divacancies. The lithium-oxygen interaction is the most effective in contributing to the increased radiation resistance.

Weinberg, I.↗

NASA NTRS1984

Photovoltaics: The endless spring

An overview of the developments in the photovoltaic field over the past decade or two is presented. Accomplishments in the terrestrial field are reviewed along with projections and challenges toward meeting cost goals. The contrasts and commonality of space and terrestrial photovoltaics are presented. Finally, a strategic philosophy of photovoltaics research highlighting critical factors, appropriate directions, emerging opportunities, and challenges of the future is given.

Brandhorst, H. W., Jr.↗

NASA NTRS1984

The NASA photovoltaic technology program

The NASA Office of Aeronautical and Space Technology OAST Program in space photovoltaics is reviewed. From the perspective of national landmark mission requirements and five year and 25-year long range plans, the texture of the program is revealed. Planar silicon and concentrator GaAs array technology advances are discussed. Advances in lightweight (50 micro cell) arrays and radiation tolerance research are presented. Recent progress in cascade cells and ultralightweight GaAs planar cells is noted. Progress in raising silicon cell voltage to its theoretical maximum is detailed. Advanced concepts such as plasmon converters and the Long Duration Exposure Facility LDEF flight experiments pertaining to solar cell and array technology are also shown.

Mullin, J. P.↗

NASA NTRS1984

Photovoltaics - The endless spring

An overview of the developments in the photovoltaic field over the past decade or two is presened. Accomplishments in the terrestrial field are reviewed along with projections and challenges toward meeting cost goals. The contrasts and commonality of space and terrestrial photovoltaics are presented. Finally, a strategic philosophy of photovoltaics research highlighting critical factors, appropriate directions, emerging opportunities, and challenges of the future is given.

Brandhorst, H. W., Jr.↗

NASA NTRS1984

The NASA photovoltaic technology program

Mullin, J. P.↗

NASA NTRS1982

On the cause of the flat-spot phenomenon observed in silicon solar cells at low temperatures and low intensities

A model that explains the flat-spot power loss phenomenon is presented. Evidence suggests that the effect is due to localized metallurgical interactions between the silicon substrate and the contact metallization. These reactions are shown to result in localized regions in which the P-N junction is destroyed and replaced with a metal semiconductor-like interface. The effects of thermal treatment, crystallographic orientation, junction depth, and metallization are shown along with a method of preventing the effect through the suppression of vacancy formation at the free surface of the contact metallization. Data indicating the effectiveness of a TiN diffusion barrier in preventing the effect are also given.

Weizer, V. G.↗

NASA NTRS1982

Effects of processing and dopant on radiation damage removal in silicon solar cells

Gallium and boron doped silicon solar cells, processed by ion implantation followed by either laser or furnace anneal were irradiated by 1 MeV electrons and their postirradiation recovery by thermal annealing was determined. During the postirradiation anneal, gallium doped cells prepared by both processes recovered more rapidly and exhibited none of the severe reverse annealing observed for similarly processed 2 ohm-cm boron doped cells. Ion implanted furnace annealed 0.1 ohm-cm boron doped cells exhibited the lowest post-irradiation annealing temperatures after irradiation. The drastically lowered recovery temperature is attributed to the reduced oxygen and carbon content of the 0.1 ohm-cm cells. Analysis based on defect properties and annealing kinetics indicates that further reduction in annealing temperature is attainable with further reduction in the silicon's carbon and/or divacancy content after irradiation.

Weinberg, I.↗

NASA NTRS1982

Effects of processing and dopant on radiation damage removal in silicon solar cells

Gallium and boron doped silicon solar cells, processed by ion-implantation followed by either laser or furnace anneal were irradiated by 1 MeV electrons and their post-irradiation recovery by thermal annealing determined. During the post-irradiation anneal, gallium-doped cells prepared by both processes recovered more rapidly and exhibited none of the severe reverse annealing observed for similarly processed 2 ohm-cm boron doped cells. Ion-implanted furnace annealed 0.1 ohm-cm boron doped cells exhibited the lowest post-irradiation annealing temperatures (200 C) after irradiation to 5 x 10 to the 13th e(-)/sq cm. The drastically lowered recovery temperature is attributed to the reduced oxygen and carbon content of the 0.1 ohm-cm cells. Analysis based on defect properties and annealing kinetics indicates that further reduction in annealing temperature should be attainable with further reduction in the silicon's carbon and/or divacancy content after irradiation.

Weinberg, I.↗

NASA NTRS1982

On the cause of the flat-spot phenomenon observed in silicon solar cells at low temperatures and low intensities

A model is presented that explains the "flat-spot" (FS) power loss phenomenon observed in silicon solar cells operating deep space (low temperature, low intensity) conditions. Evidence is presented suggesting that the effect is due to localized metallurgical interactions between the silicon substrate and the contact metallization. These reactions are shown to result in localized regions in which the PN junction is destroyed and replaced with a metal-semiconductor-like interface. The effects of thermal treatment, crystallographic orientation, junction depth, and metallurization are presented along with a method of preventing the effect through the suppression of vacancy formation at the free surface of the contact metallization. Preliminary data indicating the effectiveness of a TiN diffusion barrier in preventing the effect are also given.

Weizer, V. G.↗

NASA NTRS1982

Radiation damage in front and back illuminated high resistivity silicon solar cells

Radiation induced degradation, in front and back illuminated 84 and 1250 ohm-cm n+pp+ silicon solar cells, was determined and cell performance interpreted using calculated optically injected charge distributions and cell voltage components. The 84 ohm-cm cell degraded less when illuminated from the front or n+ side compared to that when illuminated from the back or p+ side. On the other hand, the 1250 ohm-cm cell degraded less when back illuminated. It is concluded that, in addition to the usual mechanisms leading to decreased collection efficiencies, loss of conductivity modulation is a major cause of radiation damage in high resistivity silicon solar cells. These results suggest that radiation damage to high resistivity n+pp+ cells can be decreased by increasing cell collection efficiency and illuminating the cells from the p+ side.

Weinberg, I.↗

NASA NTRS1981

Material and processing needs for silicon solar cells in space

The technical concerns of NASA in the area of space grade solar cells are summarized. Solar power needs are projected through 1987. The degradation of solar cell performance due to the effects of radiation on impurities and crystal defects and the improved performance of float zone silicon are illustrated. The reduction of oxygen and carbon in float zone silicon allows for much faster low temperature annealing of the defects. The effects of improved crystal purity on cell performance are summarized.

Brandhorst, H. W., Jr.↗

NASA NTRS1980

Radiation damage in high voltage silicon solar cells

Three high open-circuit voltage cell designs based on 0.1 ohm-cm p-type silicon were irradiated with 1 MeV electrons and their performance determined to fluences as high as 10 to the 15th/sq cm. Of the three cell designs, radiation induced degradation was greatest in the high-low emitter (HLE) cell. The diffused and ion implanted cells degraded approximately equally but less than the HLE cell. Degradation was greatest in an HLE cell exposed to X-rays before electron irradiation. The cell regions controlling both short-circuit current and open-circuit voltage degradation were defined in all three cell types. An increase in front surface recombination velocity accompanied time dependent degradation of a HLE cell after X-irradiation. It was speculated that this was indirectly due to a decrease in positive charge at the silicon-oxide interface. Modifications aimed at reducing radiation induced degradation are proposed for all three cell types.

Weinberg, I.↗

NASA NTRS1980

Radiation damage in lithium-counterdoped n/p silicon solar cells

Lithium counterdoped n+/p silicon solar cells were irradiated with 1 MV electrons and their post irradiation performance and low temperature annealing properties were compared to that of the 0.35 ohm cm control cells. Cells fabricated from float zone and Czochralski grown silicon were investigated. It was found that the float zone cells exhibited superior radiation resistance compared to the control cells, while no improvement was noted for the Czochralski grown cells. Room temperature and 60 C annealing studies were conducted. The annealing was found to be a combination of first and second order kinetics for short times. It was suggested that the principal annealing mechanism was migration of lithium to a radiation induced defect with subsequent neutralization of the defect by combination with lithium. The effects of base lithium gradient were investigated. It was found that cells with negative base lithium gradients exhibited poor radiation resistance and performance compared to those with positive or no lithium gradients; the latter being preferred for overall performance and radiation resistance.

Hermann, A. M.↗

NASA NTRS1980

Space solar cells - High efficiency and radiation damage

The proceedings of the Third Solar Cell High Efficiency and Radiation Damage Meeting are outlined. The topics covered included high efficiency silicon solar cells, silicon solar cell radiation damage, GaAs solar cell performance, and 30 percent conversion devices. The study of radiation damage from a fundamental defect-centered basis is discussed and evaluated as a focus of future work. 18% AM0 efficiency and 0.7 V open-circuit voltages are designated as achievable goals for silicon solar cells, and the potential for 30% AM0 efficiencies from monolithic tandem cell designs without sunlight concentration is noted. In addition to its potential for 20% AM0 efficiencies, the GaAs cell offers the possibility of a radiation-insensitive power supply when operated at temperatures near 200 C.

Brandhorst, H. W., Jr.↗

NASA NTRS1980

Radiation damage in lithium-counterdoped N/P silicon solar cells

The radiation resistance and low-temperature annealing properties of lithium-counterdoped n(+)-p silicon solar cells are investigated. Cells fabricated from float zone and Czochralski grown silicon were irradiated with 1 MeV electrons and their performance compared to that of 0.35 ohm-cm control cells. The float zone cells demonstrated superior radiation resistance compared to the control cells, while no improvement was noted for the Czochralski grown cells. Annealing kinetics were found to lie between first and second order for relatively short times, and the most likely annealing mechanism was found to be the diffusion of lithium to defects with the subsequent neutralization of defects by combination with lithium. Cells with zero lithium gradients exhibited the best radiation resistance.

Hermann, A. M.↗