James R. Woodyard

MASS SPECTROMETRIC STUDY OF NEUTRAL PARTICLES SPUTTERED FROM Cu BY 0- TO 100-ev Ar IONS

A low‐pressure magnetically confined argon arc discharge was used in a mass spectrometer ion source to study low‐energy (0–100 eV) sputtering of polycrystalline copper. Target bombarding ion current densities ranged from 60–200 μA/cm2. Neutral particles were studied. Cu atoms and Cu2 molecules were detected. The mass ratio of analyzed Cu2 molecules to Cu atoms increased with bombarding ion energies to about 5½% at ion energies of 100 eV. Target voltages for appearance of Cu atoms and Cu2 molecules were − 19 and − 50 V, respectively. No Cu3 molecules were detected; if they were present, it was estimated that the ratio 189Cu3 to 63Cu is less than 0.09%. The method has been found to be promising for the study of neutral particles in low‐energy sputtering. Yield curves agree well with results of other observers; sensitivities of 7×10−4 atoms/ion were attained, and this figure can be improved.

Radiation resistance of thin-film solar cells for space photovoltaic power

Copper indium diselenide, cadmium telluride, and amorphous silicon alloy solar cells have achieved noteworthy performance and are currently being studied for space power applications. Cadmium sulfide cells had been the subject of much effort but are no longer considered for space applications. A review is presented of what is known about the radiation degradation of thin film solar cells in space. Experimental cadmium telluride and amorphous silicon alloy cells are reviewed. Damage mechanisms and radiation induced defect generation and passivation in the amorphous silicon alloy cell are discussed in detail due to the greater amount of experimental data available.

Low energy sputtering of neutral Cu2 molecules

Abstract The ratios of relative yields of neutral sputtered Cu 2 molecules to neutral sputtered Cu atoms were found to be linearly proportional to the sputtering yield of Cu, from a Cu target under bombardment by Ar + ions (energy 50–90 eV), as determined by secondary neutral mass spectrometry.

Investigation of Shunt Resistances in Single-Junction a-Si:H Alloy Solar Cells

Dark current-voltage characteristics of one hundred twenty single-junction a-Si:H alloy solar cells were studied. Parametric Modelling of dark I-V Measurements was used to determine some of the cell parameters. Average shunt resistances were 31 to 1200 kΩ for intrinsic layer thicknesses of 200 to 800 nm, respectively. Current switching was observed during dark I-V Measurements; the current in the reverse-bias region varies approximately as the square of the voltage following the onset of switching. The I-V characteristics in the switched mode are not understood. Voltage history and scan rate play a role in dark I-V characteristics.

Dehydrogenation studies of amorphous silicon

The dehydrogenation of highly photoconductive amorphous silicon films has been studied using 15N,p nuclear reaction hydrogen depth profiling and isochronal anneals at temperatures between 300 and 500 °C. The studies shown that neither diffusion nor a first‐order reaction are the rate limiting dehydrogenation mechanism for hydrogen concentrations between 5 and 40 at. %. Dehydrogenation is not rate limited by processes at the surface. The studies suggest that for concentrations above 5 at. % the dehydrogenation may be explained by second‐order reaction kinetics involving the reordering of bonds. Below about 5 at. % a first‐order process leading to dangling silicon bonds appears to dominate.

Analysis of LIPS III satellite a-Si:H alloy solar cell data

The data collected from single-junction p-i-n a-Si:H alloy solar cells during a 2309 day flight of the LIPS III satellite were analyzed. While there are problems associated with the interpretation of the data, the data are useful for characterizing the effect of the space environment on a-Si:H solar cells. Marked decreases in the cell parameters were observed. Evidence is presented to show that photodegradation is the primary cell degradation mechanism.

Laboratory instrumentation and techniques for characterizing multi-junction solar cells for space applications

An integrated system is described which consists of a spectral radiometer and dual-source solar simulator, and personal computer-based current-voltage and quantum efficiency equipment. The apparatus was used to measure the dependence of the absolute external quantum efficiency of triple-junction a-Si:H alloy solar cells. The cells were measured under AM0 light bias at various forward-bias voltages including the maximum-power point. The quantum efficiencies of the cells were measured with both spectral-light bias and AM0 light bias.

Investigation of light and dark I-V characteristics of a-Si:H alloy solar cells, irradiated with 1.0 MeV protons

Light and dark I-V characteristics of both virgin and irradiated solar cells with the same history differ; the objective of this work is to elucidate the mechanisms responsible for the observations. Thirty-seven triple-junction and 120 single-junction hydrogenated amorphous silicon alloy cells were investigated. Triple and single-junction cells degrade similarly with 1.0 MeV proton irradiation; the power density degrades for fluences above 1E12 cm/sup -2/. The fill-factor degrades initially; above 5E13 cm/sup -2/ the short-circuit current density dominates the degradation. Dark I-V measurements show changes in the shunt resistance and injection current. High fluences tend to decrease both the shunt and injection currents. Shunt resistances of virgin cells are unstable under reverse bias. Annealing at 200/spl deg/C for two hours increases and stabilizes shunt resistances. A parametric model was used to fit light and dark I-V measurements. The effect of irradiation and light bias on the parameters is reported.<<ETX>>

Radiation resistance studies of amorphous silicon films

A study of hydrogenated amorphous silicon thin films irradiated with 2.00 MeV helium ions using fluences ranging from 1E11 to 1E15 cm/sup -2/ is presented. The films were characterized using photothermal deflection spectroscopy, transmission and reflection spectroscopy, and photoconductivity and annealing measurements. Large changes were observed in the subband-gap optical absorption for energies between 0.9 and 1.7 eV. The steady-state photoconductivity showed decreases of almost five orders of magnitude for a fluence of 1E15 cm/sup -2/, but the slope of the intensity dependence of the photoconductivity remained almost constant for all fluences. Substantial annealing occurs even at room temperature, and for temperatures greater than 448 K the damage is completely annealed. The data are analyzed to describe the defects and the density of states function.<<ETX>>

1.00 MeV proton radiation resistance studies of single-junction and single dual-junction amorphous-silicon alloy solar cells

A comparative study of the radiation resistance of a-Si:H and a-SiGe:H single-junction and a-Si:H dual-junction solar cells was conducted. The cells were irradiated with 1.00 MeV protons with fluences of 1.0*10/sup 14/, 5.0*10/sup 14/ and 1.0*10/sup 15/ cm/sup -2/ and characterized using I-V and quantum efficiency measurements. The radiation resistance of single-junction cells cannot be used to explain the behavior of dual-junction cells at a fluence of 1.0*10/sup 15/ cm/sup -2/. The a-Si H single-junction cells degraded the least of the three cells; a-SiGe:H single-junction cells showed the largest reduction in short-circuit current. while a-Si:H dual-junction cells exhibited the largest degradation in the open-circuit voltage. The quantum efficiency of the cells degraded more in the red part of the spectrum: the bottom junction degrades first in dual-junction cells.<<ETX>>