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Results for “surface recombination velocities”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Heavy doping effects in high efficiency silicon solar cells

The temperature dependence of the emitter saturation current for bipolar devices was studied by varying the surface recombination velocity at the emitter surface. From this dependence, the value was derived for bandgap narrowing that is in better agreement with other determinations that were obtained from the temperature dependence measure on devices with ohmic contacts. Results of the first direct measurement of the minority-carrier transit time in a transparent heavily doped emitter layer were reported. The value was obtained by a high-frequency conductance method recently developed and used for doped Si. Experimental evidence is presented for significantly greater charge storage in highly excited silicon near room temperature than conventional theory would predict. These data are compared with various data for delta E sub G in heavily doped silicon.

Lindholm, F. A.↗
NASA NTRS1997

ZnSe Window Layers for GaAs and GaInP2 Solar Cells

This report concerns studies of the use of n-type ZnSe as a window layer for n/p GaAs and GaInP2 solar cells. Emphasis was placed in this phase of the project on characterizing the interface between n-type ZnSe films grown on epi-GaAs films grown onto single crystal GaAs. Epi-GaAs and heteroepitaxial ZnSe films were grown by MOCVD with a Spire 50OXT Reactor. After growing epitaxial GaAs films on single crystal GaAs wafers, well-oriented crystalline ZnSe films were grown by MOCVD. ZnSe films were grown with substrate temperatures ranging from 250 C to 450 C. Photoluminescence studies carried out by researchers at NASA Lewis determined that the surface recombination velocity at a GaAs surface was significantly reduced after the deposition of a heteroepitaxial layer of ZnSe. The optimum temperature for ZnSe deposition appears to be on the order of 350 C.

Olsen, Larry C.↗
NASA NTRS1986

Identification of some key parameters limiting the performance of high-efficiency silicon solar cells

This paper presents, for the first time, a detailed sensitivity analysis of key cell parameters on silicon-cell efficiency by incorporating advanced solar cell physics in a sophisticated numerical simulation program. It delineates the true physical barriers to obtaining a high-efficiency silicon solar cell. Specific parameters presently limiting cell efficiency are identified to be the minority carrier lifetime and the recombination velocities at the front and back surfaces. Practical cell efficiencies in the vicinity of 22 percent are estimated to be attainable by using good quality silicon crystal and substantially reducing surface recombination velocities.

Mokashi, Anant R.↗
NASA NTRS1978

Interpretation of scanning electron microscope measurements of minority carrier diffusion lengths in semiconductors

In scanning electron microscope (SEM) injection measurements of minority carrier diffusion lengths some uncertainties of interpretation exist when the response current is nonlinear with distance. This is significant in epitaxial layers where the layer thickness is not large in relation to the diffusion length, and where there are large surface recombination velocities on the incident and contact surfaces. An image method of analysis is presented for such specimens. A method of using the results to correct the observed response in a simple convenient way is presented. The technique is illustrated with reference to measurements in epitaxial layers of GaAs. Average beam penetration depth may also be estimated from the curve shape.

Flat, A.↗
NASA NTRS1984

Silicon surface passivation by silicon nitride deposition

Silicon nitride deposition was studied as a method of passivation for silicon solar cell surfaces. The following three objectives were the thrust of the research: (1) the use of pecvd silicon nitride for passivation of silicon surfaces; (2) measurement techniques for surface recombination velocity; and (3) the importance of surface passivation to high efficiency solar cells.

Olsen, L. C.↗
NASA NTRS1983

Analysis of the interaction of an electron beam with back surface field solar cells

In this paper the short circuit current Isc induced by the electron beam of a scanning electron microscope in a back surface field solar cell will be determined theoretically. It will be shown that, in a configuration used previously for solar cells with an ohmic back surface, the Isc gives a convenient means for estimating the back surface recombination velocities and thus the quality of back surface field cells. Numerical data will be presented applicable to a point source model for the electron-hole pair generation.

Von Roos, O.↗
NASA NTRS1981

Determination of lifetimes and recombination currents in p-n junction solar cells, diodes, and transistors

New methods are presented and illustrated that enable the accurate determination of the diffusion length of minority carriers in the narrow regions of a solar cell or a diode. Other methods now available are inaccurate for the desired case in which the width of the region is less than the diffusion length. Once the diffusion length is determined by the new methods, this result can be combined with measured dark I-V characteristics and with small-signal admittance characteristics to enable determination of the recombination currents in each quasi-neutral region of the cell - for example, in the emitter, low-doped base, and high-doped base regions of the BSF (back-surface-field) cell. This approach leads to values for the effective surface recombination velocity of the high-low junction forming the back-surface field of BSF cells or the high-low emitter junction of HLE cells. These methods are also applicable for measuring the minority-carrier lifetime in thin epitaxial layers grown on substrates with opposite conductivity type.

Neugroschel, A.↗
NASA NTRS1985

Effect of recombination on the open-circuit voltage of a silicon solar cell

A theoretical study of the influence of band-band Auger, band-trap Auger, and the ordinary Shockley-Read-Hall mechanism for carrier recombination on the open-circuit voltage VOC of a solar cell is presented. Under reasonable assumptions for the magnitude of rate constants and realistic values for trap densities, surface recombination velocities and band-gap narrowing, the maximum VOC for typical back surface field solar cells is found to lie in the range between 0.61 and 0.72 V independent of base width.

Von Roos, O.↗
NASA NTRS1990

Interface properties of various passivations of HgCdTe

The effects of different surface treatments of the p-type Hg(x-1)Cd(x)Te (x about 0.2) alloy, including anodic-sulfide, anodic-oxide, and ZnS coatings, on the surface charge density and surface mobility were investigated. Measurements of surface recombination velocities in samples passivated by either a ZnS coating on a freshly etched surface or by anodic native sulfide combined with deposited ZnS showed essentially the same temperature dependence down to 50 K. For the same original material, both the surface state density and the activation energy were found to be similar. It is suggested that, for both passivations, the surface is controlled by traps that can be related to lattice defects, such as vacancies.

Schacham, S. E.↗
NASA NTRS1981

Moderate temperature detector development

P-side backside reflecting constant, photodiode characterization, and photodiode diffusion and G-R currents were investigated in an effort to develop an 8 m to 12 m infrared quantum detector using mercury cadmium telluride. Anodization, phosphorus implantation, and the graded band gap concept were approaches considered for backside formation. Variable thickness diodes were fabricated with a back surface anodic oxide to investigate the effect of this surface preparation on the diffusion limited zero bias impedance. A modeling technique was refined to thoroughly model diode characteristics. Values for the surface recombination velocity in the depletion region were obtained. These values were improved by implementing better surface damage removal techniques.

Marciniec, J. W.↗
NASA NTRS2005

Indium Phosphide Window Layers for Indium Gallium Arsenide Solar Cells

Window layers help in reducing the surface recombination at the emitter surface of the solar cells resulting in significant improvement in energy conversion efficiency. Indium gallium arsenide (In(x)Ga(1-x)As) and related materials based solar cells are quite promising for photovoltaic and thermophotovoltaic applications. The flexibility of the change in the bandgap energy and the growth of InGaAs on different substrates make this material very attractive for multi-bandgap energy, multi-junction solar cell approaches. The high efficiency and better radiation performance of the solar cell structures based on InGaAs make them suitable for space power applications. This work investigates the suitability of indium phosphide (InP) window layers for lattice-matched In(0.53)Ga(0.47)As (bandgap energy 0.74 eV) solar cells. We present the first data on the effects of the p-type InP window layer on p-on-n lattice-matched InGaAs solar cells. The modeled quantum efficiency results show a significant improvement in the blue region with the InP window. The bare InGaAs solar cell performance suffers due to high surface recombination velocity (10(exp 7) cm/s). The large band discontinuity at the InP/InGaAs heterojunction offers a great potential barrier to minority carriers. The calculated results demonstrate that the InP window layer effectively passivates the solar cell front surface, hence resulting in reduced surface recombination and therefore, significantly improving the performance of the InGaAs solar cell.

Jain, Raj K.↗
NASA NTRS1975

Summary of theoretical and experimental investigation of grating type, silicon photovoltaic cells

Theoretical and experimental aspects are summarized for single crystal, silicon photovoltaic devices made by forming a grating pattern of p/n junctions on the light receiving surface of the base crystal. Based on the general semiconductor equations, a mathematical description is presented for the photovoltaic properties of such grating-like structures in a two dimensional form. The resulting second order elliptical equation is solved by computer modeling to give solutions for various, reasonable, initial values of bulk resistivity, excess carrier concentration, and surface recombination velocity. The validity of the computer model is established by comparison with p/n devices produced by alloying an aluminum grating pattern into the surface of n-type silicon wafers. Current voltage characteristics and spectral response curves are presented for cells of this type constructed on wafers of different resistivities and orientations.

Chen, L. Y.↗
NASA NTRS1983

An EBIC equation for solar cells

When an electron beam of a scanning electron microscope (SEM) impinges on an N-P junction, the generation of electron-hole pairs by impact ionization causes a characteristic short circuit current I(sc) to flow. The I(sc), i.e., EBIC (electron beam induced current) depends strongly on the configuration used to investigate the cell's response. In this paper the case where the plane of the junction is perpendicular to the surface is considered. An EBIC equation amenable to numerical computations is derived as a function of cell thickness, source depth, surface recombination velocity, diffusion length, and distance of the junction to the beam-cell interaction point for a cell with an ohmic contact at its back surface. It is shown that the EBIC equation presented here is more general and easier to use than those previously reported. The effects of source depth, ohmic contact, and diffusion length on the normalized EBIC characteristic are discussed.

Luke, K. L.↗
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 NTRS1983

Developments toward an 18% efficient silicon solar cell

Limitations to increased open-circuit voltage were identified and experimentally verified for 0.1 ohm-cm solar cells with heavily doped emitters. After major reduction in the dark current contribution from the metal-silicon interface of the grid contacts, the surface recombination velocity of the oxide-silicon interface of shallow junction solar cells is the limiting factor. In deep junction solar cells, where the junction field does not aid surface collection, the emitter bulk is the limiting factor. Singly-diffused, shallow junction cells have been fabricated with open circuit voltages in excess of 645 mV. Double-diffusion shallow and deep junctions cells have displayed voltages above 650 mV. MIS solar cells formed on 0.1 ohm-cm substrates have exibited the lowest dark currents produced in the course of the contract work.

Meulenberg, A., Jr.↗
NASA NTRS1995

Design of a three-layer antireflection coating for high efficiency indium phosphide solar cells using a chemical oxide as first layer

It is well known that the behavior of III-V compound based solar cells is largely controlled by their surface, since the majority of light generated carriers (63% for GaAs and 79% for InP) are created within 0.2 mu m of the surface of the illuminated cell. Consequently, the always observed high surface recombination velocity (SRV) on these cells is a serious limiting factor for their high efficiency performance, especially for those with p-n junction made by either thermal diffusion or ion implantation. A good surface passivation layer, ideally a grown oxide as opposed to a deposited one, will cause a significant reduction in the SRV without adding interface problems, thus improving the performance of III-V compound based solar cells. Another significant benefit to the overall performance of the solar cells can be achieved by a substantial reduction of their large surface optical reflection by the use of a well designed antireflection (AR) coating. In this paper, we demonstrate the effectiveness of using a chemically grown thermally and chemically stable oxide, not only for surface passivation but also as an integral part of a 3-layer AR coating for thermally diffused p+n InP solar cells. A phosphorus-rich interfacial oxide, In(PO3)3, is grown at the surface of the p+ emitter using an etchant based on HNO3, o-H3PO4 and H2O2. This oxide has the unique properties of passivating the surface as well as serving as an efficient antireflective layer yielding a measured record high AMO open-circuit voltage of 890.3 mV on a thermally diffused InP(Cd,S) solar cell. Unlike conventional single layer AR coatings such as ZnS, Sb2O3, SiO or double layer AR coatings such as ZnS/MgF2 deposited by e-beam or resistive evaporation, this oxide preserves the stoichiometry of the InP surface. We show that it is possible to design a three-layer AR coating for a thermally diffused InP solar cell using the In(PO3)3 grown oxide as the first layer and Al2O3 and MgF2 as the second and third layers respectively, so as to yield an overall theoretical reflectance of less than 2%. Since chemical oxides are readily grown on III-V semiconductors materials, the technique of using the grown oxide layer to both passivate the surface as well as serve as the first of a multilayer AR coating should work well for all III-V compound-based solar cells.

Moulot, Jacques↗
NASA NTRS1995

Design of a Three-Layer Antireflection Coating for High Efficiency Indium Phosphide Solar Cells Using a Chemical Oxide as First Layer

It is well known that the behavior of III-V compound based solar cells is largely controlled by their surface, since the majority of light generated carriers (63% for GaAs and 79% for InP) are created within 0.2 microns of the illuminated surface of the cell. Consequently, the always observed high surface recombination velocity (SRV) on these cells is a serious limiting factor for their high efficiency performance, especially for those with the p-n junction made by either thermal diffusion or ion implantation. A good surface passivation layer, ideally, a grown oxide as opposed to a deposited one, will cause a significant reduction in the SRV without adding interface problems, thus improving the performance of III-V compound based solar cells. Another significant benefit to the overall performance of the solar cells can be achieved by a substantial reduction of their large surface optical reflection by the use of a well designed antireflection (AR) coating. In this paper, we demonstrate the effectiveness of using a chemically grown, thermally and chemically stable oxide, not only for surface passivation but also as an integral part of a 3- layer AR coating for thermally diffused p(+)n InP solar cells. A phosphorus-rich interfacial oxide, In(PO3)3, is grown at the surface of the p(+) emitter using an etchant based on HNO3, o-H3PO4 and H2O2. This oxide has the unique properties of passivating the surface as well as serving as a fairly efficient antireflective layer yielding a measured record high AM0, 25 C, open-circuit voltage of 890.3 mV on a thermally diffused InP(Cd,S) solar cell. Unlike conventional single layer AR coatings such as ZnS, Sb2O3, SiO or double layer AR coatings such as ZnS/MgF2 deposited by e-beam or resistive evaporation, this oxide preserves the stoichiometry of the InP surface. We show that it is possible to design a three-layer AR coating for a thermally diffused InP solar cell using the In(PO3)3 grown oxide as the first layer and Al2O3, MgF2 or ZnS, MgF2 as the second and third layers respectively, so as to yield an overall theoretical reflectance of less than 2%. Since chemical oxides are readily grown on III-V semiconductor materials, the technique of using the grown oxide layer to both passivate the surface as well as serve as the first of a multilayer AR coating, should work well for essentially all III-V compound-based solar cells.

Moulot, Jacques↗