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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

Optical damage and internal fields in pyroelectrics.

The physical conditions under which the models of optical damage in pyroelectrics suggested by Johnston (1970) and by Amodei (1971) might be valid are described. Whether the zero-internal-field final state described by Johnston or the no-current-flow steady state of Amodei can be reached depends on the pattern of excitation and the dynamics of the damage process. In view of the small carrier diffusion lengths, the one-dimensional models generally used may not be entirely satisfactory for investigation of dynamics.

Wood, V. E.↗
NASA NTRS1973

Performance of epitaxial back surface field cells

Epitaxial back surface field structures were formed by depositing a 10 micron thick 10 Omega-cm epitaxial silicon layer onto substrates with resistivities of 0.01, 0.1, 1.0 and 10 Omega-cm. A correlation between cell open-circuit voltage and substrate resistivity was observed and was compared to theory. The cells were also irradiated with 1 MeV electrons to a fluence of 5 X 10 to the 15th power e/cm2. The decrease of cell open-circuit voltage was in excellent agreement with theoretical predictions and the measured short circuit currents were within 2% of the prediction. Calculations are presented of optimum cell performance as functions of epitaxial layer thickness, radiation fluence and substrate diffusion length.

Brandhorst, H. W., Jr.↗
NASA NTRS1973

The drift field model applied to the lithium-containing silicon solar cell

The drift field model used by Wolf to calculate the short-circuit current was extended to permit calculations of the open-circuit voltage and the maximum power under conditions of illumination of either tungsten (2800 C) source or AMO sunlight. Voltages were calculated using an expression for the drift field diode saturation current derived here. The model, applied to the oxygen rich (C-13 group) lithium solar cells, was used to calculate the pre-and post-electron bombardment trends for lithium gradients in the range of 10 to the 18th power to 10 to the 19th power Li/cm to the 4th power. Published experimental data characterizing these cells were used to tailor the model. The calculated trends are in reasonable agreement with the empirical data of Faith. Diffusion length degradation and carrier removal effects were sufficient to predict the cell performance up to 3 x 10 to the 14th power e/sq cm. Beyond this fluence it was necessary to include drift field removal effects.

Godlewski, M. P.↗
NASA NTRS1973

Low-high junction theory applied to solar cells

Recent use of alloying techniques for rear contact formation has yielded a new kind of silicon solar cell, the back surface field (BSF) cell, with abnormally high open circuit voltage and improved radiation resistance. Several analytical models for open circuit voltage based on the reverse saturation current are formulated to explain these observations. The zero SRV case of the conventional cell model, the drift field model, and the low-high junction (LHJ) model can predict the experimental trends. The LHJ model applies the theory of the low-high junction and is considered to reflect a more realistic view of cell fabrication. This model can predict the experimental trends observed for BSF cells. Detailed descriptions and derivations for the models are included. The correspondences between them are discussed. This modeling suggests that the meaning of minority carrier diffusion length measured in BSF cells be reexamined.

Godlewski, M. P.↗
NASA NTRS1974

Methods of improving the efficiency of photovoltaic cells

Work on aluminum-alloyed silicon grating cells is continued. Optimization of the geometry (grating line width and spacing) confirms the analysis of such cells. A 1 sq cm grating cell was fabricated and its i-V characteristic was measured under an AMO solar simulator. It is found that the efficiency of this cell would be about 7.9%, if it were covered by the usual antireflection coating. The surface of the cell is not covered by a diffused junction. The response is blue shifted; the current is somewhat higher than that produced by a commercial Si cell. However, the open circuit voltage is low, and attempts to optimize the open circuit voltage of the aluminum-alloy junctions are described. A preliminary X-ray topographic examination of GaAs specimens of the type commonly used to make solar cells is studied. The X-ray study shows that the wafers are filled with regions having strain gradients, possibly caused by precipitates. It is possible that a correlation exists between the presence of low mechanical perfection and minority carrier diffusion lengths of GaAs crystals.

Loferski, J. J.↗
NASA NTRS1974

Performance of epitaxial back surface field cells

Epitaxial back surface field structures were formed by depositing a 10 micron thick 10 ohm-cm epitaxial silicon layer onto substrates with resistivities of 0.01, 0.1, 1.0 and 10 ohm-cm. A correlation between cell open-circuit voltage and substrate resistivity was observed and was compared to theory. The cells were also irradiated with 1-MeV electrons to a fluence of 5 times 10 to the 15th electrons per sq cm. The decrease of cell open-circuit voltage was in excellent agreement with theoretical predictions and the measured short-circuit currents were within 2% of the prediction. Calculations are presented for optimum cell performance as functions of epitaxial layer thickness, radiation fluence, and substrate diffusion length.

Brandhorst, H. W., Jr.↗
NASA NTRS1974

The drift field model applied to the lithium-containing silicon solar cell

The drift field model used by Wolf (1963) to calculate the short-circuit current was extended to permit calculations of the open-circuit voltage and the maximum power under conditions of illumination of either tungsten (2800 C) source or air mass zero sunlight. Voltages were calculated using an expression for the drift field diode saturation current. The model, applied to the oxygen-rich (C-13 group) lithium solar cells, was used to calculate the pre- and post-electron bombardment trends of the open-circuit voltage, maximum power, and short-circuit current for lithium gradients in the range from 10 to the 18th power to 10 to the 19th power Li/cm to the 4th power. Diffusion length degradation and carrier removal effects were sufficient to predict the cell performance up to 3 x 10 to the 14th electrons per sq cm. Beyond this fluence it was necessary to include drift field removal effects.

Godlewski, M. P.↗
NASA NTRS1975

Aeroacoustic performance of scale model sonic inlets

A low speed wind tunnel was used to evaluate the inflight aeroacoustic performance of several single- and multiple-passage sonic inlets. Takeoff and approach geometries were tested, and the effects of inlet lip and diffuser design were determined. Results indicate that the single passage geometries, in particular a cylindrical centerbody takeoff geometry and a bulb shaped centerbody approach geometry, provide the highest level of aeroacoustic performance. Increasing inlet lip contraction ratio extends the maximum incidence angle for attached lip flow, while increasing inlet diffuser length results in higher total pressure recovery for a given amount of noise suppression.

Abbott, J. M.↗
NASA NTRS1975

Damage coefficients in low resistivity silicon

Electron and proton damage coefficients are determined for low resistivity silicon based on minority-carrier lifetime measurements on bulk material and diffusion length measurements on solar cells. Irradiations were performed on bulk samples and cells fabricated from four types of boron-doped 0.1 ohm-cm silicon ingots, including the four possible combinations of high and low oxygen content and high and low dislocation density. Measurements were also made on higher resistivity boron-doped bulk samples and solar cells. Major observations and conclusions from the investigation are discussed.

Srour, J. R.↗
NASA NTRS1975

High-efficiency graded band-gap Al/x/Ga/1-x/As-GaAs solar cell

A detailed theoretical analysis of an n-on-p graded band-gap Al(x)Ga(1-x)As-GaAs solar cell yields a maximum air mass zero power conversion efficiency of 17% compared to 9% for a similar GaAs cell. The analysis includes surface and bulk minority carrier recombination, junction recombination current, spectrally varying surface reflection, and series resistance loss. The maximum efficiency is determined for a surface recombination velocity of 10,000 cm/sec and hole and electron diffusion lengths of 2.1 and 7.6 microns, respectively. The improved efficiency is primarily due to a built-in electric field, caused by the band-gap gradation, accelerating photogenerated holes toward the p-n junction. This field reduces the surface and bulk recombination of the holes, and thereby enhances their collection.

Hutchby, J. A.↗
NASA NTRS1975

Effect of impurity doping concentration on solar cell output

Experimental measurements were made of solar cell and related photovoltaic parameters for silicon with high concentrations of dopant impurities. The cell output peaked for doping levels around 10 to the 17th power per cu cm. Independent measurements of diffusion length and open circuit voltage at high doping levels showed severe reductions at concentrations above 10 to the 18th power per cu cm. Theoretical reasons are given to explain these reductions. Indication is given of the problems requiring solution before increased cell output can be achieved at high doping levels.

Iles, P. A.↗
NASA NTRS1975

Technique for producing 'good' GaAs solar cells using poor-quality substrates

Relatively good GaAs solar cells can be made from poor-quality substrates by making the junction deep (greater than 1 micron) instead of shallow and by 'leaching' both the pGaAs and nGaAs regions during the growth process. AM0 efficiencies of 14.7% (19% AM1) have been obtained from substrates with starting substrate diffusion lengths of 0.6 micron.

Hovel, H. J.↗
NASA NTRS1975

Studies of heteroface solar cell performance

The development, fabrication, and failure modes of AlxGa(1-x)As-GaAs heteroface solar cells are described. Crystal growth, the diffusion of Zn into the GaAs layer to form the p-n junction, SEM studies of the diffusion length of GaAs, and procedures for making ohmic contacts are discussed.

Feucht, D. L.↗
NASA NTRS1975

Solar cells for terrestrial applications

The power efficiency curves of photovoltaic solar cells were investigated as a function of the forbidden energy gap (E sub g) and the current-voltage characteristic of the diode. Minority carrier injection, depletion layer recombination, and interface recombination terms were considered in models for the I-V characteristic. The collection efficiency for photons with energy between (E sub g) and an upper energy cutoff (E sub w) was assumed to be 100% and zero otherwise. Results are presented in terms of a single parameter related to the ratio of depletion layer width and minority carrier diffusion length. It was found that increasing depletion layer recombination shifts the efficiency curves to larger values of the energy without changing the shape of the efficiency curve appreciably. It is believed that similar results would be obtained whenever the quality factors in the exponential energy gap and forward bias terms are equal.

Chernow, F.↗
NASA NTRS1976

The optimization of Ga (1-x)Al (x)As-GaAs solar cells for air mass zero operation and a study of Ga (1-x)Al (x)As-GaAs solar cells at high temperatures, phase 1

The three types of solar cells investigated were: (1) one consisting of a nGaAs substrate, a Zn doped pGaAs region, and a Zn doped Ga(1-x)Al(x)As layer, (2) one consisting of an nGaAs substrate, a Ge doped pGaAs region, and a pGa(1-x)Al(x)As upper layer, and (3) one consisting of an n+GaAs substrate, an nGa(1-x)Al(X)As region, a pGa(1-x)Bl(X) As region, and a pGa(1-y)Al(y)As upper layer. In all three cases, the upper alloy layer is thin and of high Al composition in order to obtain high spectral response over the widest possible range of photon energies. Spectral response, capacitance-voltage, current-voltage, diffusion length, sunlight (or the equivalent)-efficiency, and efficiency-temperature measurements were made as a function of device parameters in order to analyze and optimize the solar cell behavior.

Hovel, H. J.↗
NASA NTRS1975

High efficiency graded band-gap Al/x/Ga/1-x/As-GaAs p-on-n solar cell

A theoretical analysis of p-on-n (p/n) graded band-gap Al/x/Ga/1-x/As-GaAs solar cells including all practical energy loss mechanisms predicts air mass zero efficiencies of 17.3%. The energy losses include those due to spectral reflection, surface, bulk and junction recombination currents, and series resistance. The device consists of a layer of p-type Al/x/Ga/1-x/As 3.5-micron thick on top of an n-type GaAs substrate. The graded band-gap is achieved by decreasing x from 0.35 at the surface to zero at the junction. The same structure without the graded Al concentration, no longer an optimum device, has an efficiency of 10.4%. The primary function of the graded band-gap material is the reduction of surface and bulk recombination losses in the surface layer by a built-in electric field. A comparison of the p/n structure with an optimized n/p structure indicates that the latter has a slightly higher efficiency (17.7%) for assumed minority carrier diffusion lengths and surface recombination velocity.

Hutchby, J. A.↗
NASA NTRS1976

Theoretical optimization and parametric study of n-on-p Al/x/Ga/1-x/As-GaAs graded band-gap solar cell

A comprehensive theoretical model of the graded band-gap Al(x)Ga(1-x)As-GaAs solar cell is used to optimize the n-on-p cell. The model includes power losses due to surface, bulk, and junction minority-carrier recombination, series resistance, and photon reflection from an SiO antireflection coating of optimum thickness. The optimized cell has a junction depth/graded band-gap layer thickness of 1.0 micron, respective donor and acceptor concentrations of 4 x 10 to the 17th power and 2 x 10 to the 17th power per cu cm, and a surface AlAs mode fraction of x = 0.35. The optimized graded band-gap cell has an air-mass-zero efficiency of 17.7% (not corrected for a 13% front surface contact area) and is shown to be less sensitive than a similar n-on-p GaAs cell to material degradation in the form of decreased minority-carrier diffusion lengths and increased surface-recombination velocity

Hutchby, J. A.↗
NASA NTRS1976

Analysis of epitaxial drift field N on P silicon solar cells

The performance of epitaxial drift field silicon solar cell structures having a variety of impurity profiles was calculated. These structures consist of a uniformly doped P-type substrate layer, and a P-type epitaxial drift field layer with a variety of field strengths. Several N-layer structures were modeled. A four layer solar cell model was used to calculate efficiency, open circuit voltage and short circuit current. The effect on performance of layer thickness, doping level, and diffusion length was determined. The results show that peak initial efficiency of 18.1% occurs for a drift field thickness of about 30 micron with the doping rising from 10 to the 17th power atoms/cu cm at the edge of the depletion region to 10 to the 18th power atoms/cu cm in the substrate. Stronger drift fields (narrow field regions) allowed very high performance (17% efficiency) even after irradiation to 3x10 to the 14th power 1 MeV electrons/sq cm.

Baraona, C. R.↗