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Results for “diffusion length”

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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Giant spin Seebeck effect through an interface organic semiconductor

Interfacing an organic semiconductor C 60 with a non-magnetic metallic thin film (Cu or Pt) has created a novel heterostructure that is ferromagnetic at ambient temperature, while its interface with a magnetic metal (Fe or Co) can tune the anisotropic magnetic surface property of the material. Here, we demonstrate that sandwiching C60 in between a magnetic insulator (Y 3 Fe 5 O 12 :YIG) and a non-magnetic, strong spin–orbit metal (Pt) promotes highly efficient spin current transport via the thermally driven spin Seebeck effect (SSE). Experiments and first principles calculations consistently show that the presence of C 60 reduces significantly the conductivity mismatch between YIG and Pt and the surface perpendicular magnetic anisotropy of YIG, giving rise to enhanced spin mixing conductance across YIG/C 60 /Pt interfaces. As a result, a 600% increase in the SSE voltage (V LSSE ) has been realized in YIG/C 60 /Pt relative to YIG/Pt. Temperature-dependent SSE voltage measurements on YIG/C 60 /Pt with varying C 60 layer thicknesses also show an exponential increase in V LSSE at low temperatures below 200 K, resembling the temperature evolution of spin diffusion length of C 60 . Our report emphasizes the important roles of the magnetic anisotropy and the spin diffusion length of the intermediate layer in the SSE in YIG/C 60 /Pt structures, providing a new pathway for developing novel spin-caloric materials.

36 MATERIALS SCIENCE↗
NASA NTRS1982

Silicon solar cell process development, fabrication and analysis

For UCP Si, randomly selected wafers and wafers cut from two specific ingots were studied. For the randomly selected wafers, a moderate gettering diffusion had little effect. Moreover, an efficiency up to 14% AMI was achieved with advanced processes. For the two specific UCP ingots, ingot #5848-13C displayed severe impurity effects as shown by lower 3sc in the middle of the ingot and low CFF in the top of the ingot. Also the middle portions of this ingot responded to a series of progressively more severe gettering diffusion. Unexplained was the fact that severely gettered samples of this ingot displayed a negative light biased effect on the minority carrier diffusion length while the nongettered or moderately gettered ones had the more conventional positive light biased effect on diffusion length. On the other hand, ingot C-4-21A did not have the problem of ingot 5848-13C and behaved like to the randomly selected wafers. The top half of the ingot was shown to be slightly superior to the bottom half, but moderate gettering helped to narrow the gap.

Iles, P. A.↗
NASA NTRSDate not supplied

Electron Irradiation Study of Metamorphic 1.7 eV GaAsP Solar Cells

We investigated the effects of 1 MeV electron irradiation on metamorphic ~1.7 eV GaAsP solar cells on GaP and on GaP/Si. Effects of junction polarity, base thickness, and threading dislocation density on radiation hardness were investigated using the AM0 solar simulator at NASA Glenn Research Center. Degradation of solar cell efficiency after irradiation was dominated by reduced minority carrier diffusion length in the base, leading to loss of long-wavelength carrier collection. Designs with higher base diffusion length or thinner base were favored, and accordingly, devices with n+/p junction polarity were more radiation-hard than those with p+/n junction polarity.

Ryan D Hool↗
NASA NTRS1993

Investigation of ZnSe-coated silicon substrates for GaAs solar cells

Studies are being carried out to determine the feasibility of using ZnSe as a buffer layer for GaAs solar cells grown on silicon. This study was motivated by reports in the literature indicating ZnSe films had been grown by metallorganic chemical vapor deposition (MOCVD) onto silicon with EPD values of 2 x 10(exp 5) cm(sup -2), even though the lattice mismatch between silicon and ZnSe is 4.16 percent. These results combined with the fact that ZnSe and GaAs are lattice matched to within 0.24 percent suggest that the prospects for growing high efficiency GaAs solar cells onto ZnSe-coated silicon are very good. Work to date has emphasized development of procedures for MOCVD growth of (100) ZnSe onto (100) silicon wafers, and subsequent growth of GaAs films on ZnSe/Si substrates. In order to grow high quality single crystal GaAs with a (100) orientation, which is desirable for solar cells, one must grow single crystal (100) ZnSe onto silicon substrates. A process for growth of (100) ZnSe was developed involving a two-step growth procedure at 450 C. Single crystal, (100) GaAs films were grown onto the (100) ZnSe/Si substrates at 610 C that are adherent and specular. Minority carrier diffusion lengths for the GaAs films grown on ZnSe/Si substrates were determined from photoresponse properties of Al/GaAs Schottky barriers. Diffusion lengths for n-type GaAs films are currently on the order of 0.3 microns compared to 2.0 microns for films grown simultaneously by homoepitaxy.

Huber, Daniel A.↗
NASA NTRS1980

The influence of a voltage ramp on the measurement of I-V characteristics of a solar cell

For efficiency and convenience the voltage applied to a Si solar cell is often fairly rapidly driven from zero to the open circuit value typically at a common rate of 1 V per millisecond. During this time the values of current are determined as a function of the instantaneous voltage thus producing an I-V characteristic. The present paper shows that the customary expressions for the current as a function of cell parameters still remain valid provided that the diffusion length in the expression for the dark current is changed from its steady state value L to the effective diffusion length L1 given by L1 = L(1 + qV/kT.tau) to the -1/2, where V is the ramp rate considered constant and tau is the lifetime of minority carriers. This result is true to a very good approximation provided that low level injection prevails.

Von Roos, O.↗
NASA NTRS1977

Application of the SEM to the measurement of solar cell parameters

Techniques are described which make use of the SEM to measure the minority carrier diffusion length and the metallurgical junction depth in silicon solar cells. The former technique permits the measurement of the true bulk diffusion length through the application of highly doped field layers to the back surfaces of the cells being investigated. It is shown that the secondary emission contrast observed in the SEM on a reverse-biased diode can depict the location of the metallurgical junction if the diode has been prepared with the proper beveled geometry. The SEM provides the required contrast and the option of high magnification, permitting the measurement of extremely shallow junction depths.

Weizer, V. G.↗
NASA NTRS1986

Effect of low-energy hydrogen ion implantation on dendritic web silicon solar cells

The effect of a low-energy (0.4 keV), short-time (2-min), heavy-dose (10 to the 18th/sq cm) hydrogen ion implant on dendritic web silicon solar cells and material was investigated. Such an implant was observed to improve the cell open-circuit voltage and short-circuit current appreciably for a number of cells. In spite of the low implant energy, measurements of internal quantum efficiency indicate that it is the base of the cell, rather than the emitter, which benefits from the hydrogen implant. This is supported by the observation that the measured minority-carrier diffusion length in the base did not change when the emitter was removed. In some cases, a threefold increase of the base diffusion length was observed after implantation. The effects of the hydrogen implantation were not changed by a thermal stress test at 250 C for 111 h in nitrogen. It is speculated that hydrogen enters the bulk by traveling along dislocations, as proposed recently for edge-defined film-fed growth silicon ribbon.

Rohatgi, A.↗
DOE OSTI2024

Designing Particle Morphologies for Materials with Solid Transport Limitations: A Case Study of Lithium and Manganese Rich Cathode Oxides

A lithium and manganese rich nickel-manganese-cobalt oxide (LMR-NMC) cathode is a promising candidate for next-generation batteries due to its high specific capacity, low cost, and low cobalt content. However, the material suffers from poor rate capability due to the diffusion limitations of lithium in the cathode particles. Understanding the material performance requires careful control of the morphology of the cathode particles, taking into account the primary and agglomerated diffusion pathways and the presence of pores, some of which could be closed from electrolyte infiltration. Here, in this study, we use a microstructure-based mathematical model combined with experimental data to understand the role of the complex cathode particle morphology in the rate performance of the material. Scanning electron microscopy images of cathodes made under different synthesis conditions, which results in different agglomerate morphologies, serve as the input into the mathematical model. The model is then compared to rate data to understand the controlling parameters. The presence of intra-agglomerate closed pores results in a large agglomerate diffusion length in comparison to the ideal condition, where the primary particles are agglomerated in an open and dispersed manner such that the entire interfacial area is available for electrochemical reaction. Smaller primary and agglomerate diffusion lengths result in better electrochemical performance. This points us toward designing the morphology of the cathode particles to compensate for the diffusion limitation of LMR-NMC while maximizing the density.

Tewari, Deepti↗
DOE OSTI2025

Low-Field Regime of Magnon Transport in PLD-Grown YIG Films

The diffusive propagation of magnons in the archetypal magnetic insulator yttrium iron garnet (YIG) is being actively explored for low-power and low-loss data communication. However, operation under external magnetic fields reduces the magnon diffusion length and attenuates the voltage amplitude at measurement terminals of magnonic devices. Here, we explore the low-field and field-free regime of diffusive magnon transport in YIG films, demonstrating that the field-induced suppression of magnon diffusion length can be fully inhibited only at the zero-field limit. Even a modest field of 10 mT attenuates the nonlocal spin voltage by ~20% in an ~1 μm long transport channel. We further identify the often overlooked in-plane uniaxial magnetic anisotropy as the dominant factor governing magnon transport in the low-field regime. Using Stoner-Wohlfarth macrospin simulations, we quantify the anisotropy parameters and reveal a 10-fold enhancement at low temperatures, a key finding for field-free operation of magnonic devices under cryogenic conditions.

36 MATERIALS SCIENCE↗
NASA NTRS1978

Mechanisms of photon-induced changes in silicon solar cell parameters

Reversible changes in solar cell performance were induced by AM1 illumination and evaluated by measuring short-circuit current, weak light spectral response, and minority carrier diffusion length. In n(+)p cells the change manifests itself as a reduced short-circuit current, a loss of the red response, and a reduction in minority carrier diffusion length. In p(+)n cells there are two-photon induced effects: (1) enhancement of the blue spectral response, presumably occurring in the p(+) layer and (2) degradation in open-circuit voltage at low light levels due to increased leakage occurring only after the junction edge is exposed to bright light. Most of these photon effects are attributed to defects in the silicon.

Cheng, L. J.↗
NASA NTRS1980

Evaluation and optimization of silicon sheet solar cells

This paper describes the results and procedures to evaluate and improve the efficiency of solar cells made from various unconventional silicon sheets. The performance parameters included photovoltaic characteristics, spectral response, dark I-V characteristics, and diffusion length. The evaluation techniques used provided accurate and reliable information on sheet performance, and self-consistent results were obtained from the various measurement techniques used. Minority carrier diffusion length (L) was shown to be the ultimate limiting factor for the sheet cell performance (efficiency) and other back-up measurements confirmed this L-dependence. Limited efforts were made to identify defects which influence cell performance, and to use some improved process methods to increase cell efficiency.

Yoo, H.↗
NASA NTRS2022

Progress in InAs/InAsSb Superlattice Barrier Infrared Detectors

The InAs/InAsSb (Gallium-free) type-II strained-layer superlattice (T2SLS) has emerged in recent years as a viable infrared detector material. In particular, mid-wavelength InAs/InAsSb T2SLS focal plane array (FPA) based on the nBn detector architecture exhibits good operating characteristics at 160 K, demonstrating a considerably operating temperature advantage over InSb FPAs. In the long and very long cutoff wavelengths, InAs/InAsSb T2SLS detectors with n-type absorbers are limited in their attainable quantum efficiency (QE) due to shorter hole diffusion lengths and relatively modest absorption coefficients. We have explored long and very long wavelength InAs/InAsSb T2SLS complementary barrier infrared detectors (CBIRDs) that contain p-type absorber layers in order to take advantage of the longer electron diffusion length for QE enhancement. We also discuss recent development of the InAs/InAsSb T2SLS infrared detectors at the Jet Propulsion Laboratory for NASA Earth Science applications, including long-wavelength infrared FPAs for land imaging, and mid-wavelength infrared FPAs for CubeSat hyperspectral imaging.

Pagano, Thomas S.↗
NASA NTRS1980

Radiation damage annealing mechanisms and possible low temperature annealing in silicon solar cells

The defect responsible for reverse annealing in 2 ohm/cm n(+)/p silicon solar cells was identified. This defect, with energy level at e sub v + 0.30 eV was tentatively identified as a boron oxygen-vacancy complex. Results indicate that its removal could result in significant annealing for 2 ohm/cm and lower resistivity cells at temperatures as low as 200 C. These results were obtained by use of an expression derived from the Shockley-Read-Hall recombination theory which relates measured diffusion length ratios to relative defect concentrations and electron capture cross sections. The relative defect concentrations and one of the required capture cross sections are obtained from Deep Level Transient Spectroscopy. Four additional capture cross sections are obtained using diffusion length data and data from temperature dependent lifetime studied. These calculated results are in reasonable agreement with experimental data.

Weinberg, I.↗
DOE OSTI2022

Enhancing exciton diffusion by reducing energy disorder in organic solar cells

A highly crystalline, highly emissive, and wide-bandgap polymer AC174 with an extremely small Stokes shift is designed and synthesized in water, and is used to reduce system energetic disorder and increase the exciton diffusion length of the classical PM6:Y6 blend system. AC174 is incompatible with PM6 and Y6, improves molecular packing, and reduces system energetic disorder. The long-range Förster resonance energy transfer between the donor and acceptor is enhanced, and the exciton diffusion constant and exciton lifetime are increased, leading to a longer exciton diffusion length and more efficient exciton dissociation and charge generation. The addition of AC174 simultaneously improves the open-circuit voltage, short-circuit current density (J SC ) and fill factor of PM6:Y6 devices; especially the highest internal quantum efficiency approaches 100%, and the highest J SC is 28.4 mA cm –2 . Ternary devices with 5% AC174 in the donors achieve a power conversion efficiency of 17.2%, higher than those of the parent binary devices based on PM6:Y6 (15.9%) and AC174:Y6 (3.24%).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Mechanistic understanding of catalytic conversion of ethanol to 1-butene over 2D-pillared MFI zeolite

Ethanol is an important C2 platform molecule for producing value-added chemicals and distillate hydrocarbon fuels (e.g., jet and diesel). Among these, catalytic upgrading of ethanol to butenes can generate valuable commodity chemicals (e.g., 1-butene) and provide C4 olefin intermediates that can be further upgraded to jet/diesel fuels. 2D zeolites offer hierarchical mesoporous structures, leading to improved mass transport and reduced diffusion length, which can help to address the coking challenges faced by ethanol conversion to hydrocarbons over 3D zeolites. In this study, we investigate the acid-catalyzed conversion of ethanol to 1-butene over the Brønsted acid sites (BAS) in 2D-pillared MFI zeolite (2D-PMFI) using ab initio molecular dynamics (AIMD) simulations, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and calorimetric measurements. A detailed thermodynamic analysis, using quasi-harmonic approximation (QHA), on the Gibbs free energy pathway of ethanol conversion shows that the consideration of entropy is critical to accurately capture the detailed thermodynamic profiles. The formation of bulky ethoxy-ethene complex is found to be a potential rate-determining step (RDS), proceeding via stepwise mechanism. The reactivity of 2D-PMFI can be further tuned by manipulating RDS through a careful control of number of BAS and operating temperatures. The accuracy of computational model is confirmed through the comparison of calculated vibrational density of states (VDOS) against the experimental DRIFTS measurements. Overall, our study provides mechanistic insights into ethanol upgrading over the 2D-PMFI and shows the importance of evaluating entropic effects in such confined system.

Ethanol conversion, 2D-pillared zeolite, MFI, Anha↗
NASA NTRS1979

Analysis of the interaction of an electron beam with a solar cell. III - The effect of spacial variations of the number density of recombination centers on SEM measurements

By means of an exactly soluble model the short circuit current generated by a scanning electron microscope in a P-N junction has been determined in cases where the trap density is inhomogeneous. The diffusion length for minority carriers becomes then dependent on the spacial coordinates. It is shown that in this case the dependence of the Isc on characteristic parameters as cell thickness, distance of the beam excitation spot from ohmic contacts, etc., becomes very intricate. This fact precludes the determination of the local diffusion length in the usual manner. Although the model is somewhat simplified in order to make it amenable to exact solutions, it is nevertheless realistic enough to lead to the conclusion that SEM measurements of bulk transport parameters in inhomogeneous semiconductor material are impractical since they may lead to serious errors in the interpretation of the data by customary means.

Von Roos, O.↗
NASA NTRS1978

Forward-bias capacitance and current measurements for determining lifetimes and band narrowing in p-n junction solar cells

A new method is described and illustrated for determining the minority-carrier diffusion length and lifetime in the base region of p-n junction solar cells. The method requires only capacitance measurements at the device terminals and its accuracy is estimated to be + or - 5%. It is applied to a set of silicon p-n junction devices and the values of the diffusion lengths agree with those obtained using the current response to X-ray excitation but disagree with those obtained by the OCVD method. The reasons for the relative inaccuracy of OCVD applied to silicon devices are discussed. The capacitance method includes corrections for a two-dimensional fringing effects which occur in small area devices. For a device having highly-doped base region and surface (emitter) layer, the method can be extended to enable the determination of material properties of the degenerately doped surface layer. These material properties include the phenomenological emitter lifetime and a measure of the energy band-gap narrowing in the emitter. An alternate method for determining the energy band-gap narrowing from temperature dependence of emitter current is discussed and demonstrated.

Neugroschel, A.↗
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.↗