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ABSTRACT: The addition of quantum dots (QDs) or quantum wells (QW) to a solar cell allows one to extend the absorption spectrum of the solar cell to match spectral conditions under concentration. In this paper the effect of bandgap tuning using quantum dots is investigated. P-i-n GaAs concentrator solar cells, both with and without InAs QD superlattice (SL) inserted into the i-region, were fabricated. In order to investigate effects of increased InAs QD layers, between 5 and 20 stacked InAs QDs were grown. The 20 layer stack QD tuned cell showed an 11% improvement in JSC compared to the baseline from concentrations of 2-450 suns. The increased Jsc of the QD devices was shown to be a direct result of photo-generated current contributed by the QDs. The 20X QD cell gave a 1% absolute efficiency enhancement, compared to a baseline solar cell, at 400 sun intensity, showing potential for QD spectral tuning under solar concentration.
Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
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ABSTRACT: A suite of characterization techniques including electron beam induced current and cross-sectional transmission electron microscopy in cooperation with current voltage and external quantum efficiency solar measurements are used to analyze the effects of incorporating strain-compensating layers in GaAs-based InAs quantum dot solar cells. The data indicate that strain compensation layers can reduce defect densities and increase device performance.
Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
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ABSTRACT: A framework for modeling the power generation of laterally-contacted n-type/intrinsic/p-type/intrinsic (nipi) diodes coupled with an alpha-particle radioisotope source is developed. The framework consists of two main parts, the alpha-particle energy deposition profile (ADEP) and a lumped parameter equivalent circuit model describing the nipi device operation. Experimental measurements are used to verify the ADEP modeling approach which determines the spatially varying energy deposited within the device. Using these results, nipi-diode radioisotope batteries are simulated and the effect of the number of junctions, the thickness of the junction, and the alpha-particle flux on output voltage and power are investigated. The modeling results indicate that a 1 cm<sup>2</sup> bi-layer device (consisting of one source and two adjacent nipi-diodes) with a source activity of 300 mCi can reach a power output of 2 mW, excluding radiation damage effects.
IEEE Transactions on Nuclear Science 07/2008; · 1.45 Impact Factor
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ABSTRACT: Production alternatives for single-walled carbon nanotubes (SWCNT) such as chemical vapor deposition, laser, arc and flame, vary widely in material and energy yields, catalyst requirements and product characteristics. The overall environmental profile must be assessed relative to performance in a specific end-use application, such as lithium ion batteries for electric or plug-in hybrid vehicles. Although in general SWCNT have several properties that make them attractive for transportation applications, production is a material- and energy-intensive process. High-yield synthesis pathways may be environmentally inefficient if extensive purification is required. Life cycle assessment (LCA) is an approach to quantifying the environmental trade-offs engendered by technology substitution. However, it is essential to recognize that the results of LCA for one type of SWCNT may not be applicable to SWCNT of different purity, length, diameter, chirality or conductivity. This paper discusses sources of variability and uncertainty in production of SWCNT and makes several recommendations with regard to LCA of nanomaterials.
Electronics and the Environment, 2008. ISEE 2008. IEEE International Symposium on; 06/2008
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ABSTRACT: GaP tensile strain compensation (SC) layers were introduced into GaAs solar cells enhanced with a five layer stack of InAs quantum dots (QDs). One sun air mass zero illuminated current-voltage curves show that SC results in improved conversion efficiency and reduced dark current. The strain compensated QD solar cell shows a slight increase in short circuit current compared to a baseline GaAs cell due to sub-GaAs bandgap absorption by the InAs QD. Quantum efficiency and electroluminescence were also measured and provide further insight to the improvements due to SC.
Applied Physics Letters 04/2008; · 3.84 Impact Factor
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ABSTRACT: We have been investigating the effect of screw dislocation and other structural defects on the electrical properties of SiC. SiC is a wide-bandgap semiconductor that is currently received much attention due to its favorable high temperature behavior and high electric field breakdown strength. Unfortunately, the current state-of-the-art crystal growth and device processing methods produce material with high defect densities, resulting in a limited commercial viability
06/2005;
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ABSTRACT: The enhancement of polymeric solar cells through the addition of nanostructured material complexes has been investigated. These novel materials are intended to facilitate exciton dissociation and carrier transport through the polymer matrix. The dispersion of single wall carbon nanotubes (SWNTs) into poly(3-octylthiophene)-(P3OT) has been shown to dramatically improve both the electrical conductivity and optical absorption of the polymer compared to the pure polymer. The photoresponse of solar cells using P3OT doped with SWNTs is significantly improved over the undoped version under simulated AM0 illumination. In addition, CdSe quantum dots (QDs) have been used by several groups to facilitate exciton dissociation and improve the efficiency of P3OT-based solar cells. Through the synthesis of QD-SWNT complexes we have attempted to produce a nanostructured additive for polymeric solar cells which exhibits both a high electron affinity and high electrical conductivity. The synthesis of CulnS<sub>2</sub>-SWNT complexes and an assessment of their viability as an additive in polymeric solar cells is presented.
Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005
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ABSTRACT: The majority of satellites and near-earth probes developed to date have used photovoltaic (PV) arrays for power generation. For future missions probing closer to the sun, solar cells need to be developed that can function at higher temperatures, light intensity, and radiation conditions. The theoretical and experimental performance of wide-bandgap materials for use as high-temperature solar cells has been studied. The dependence of solar cell parameters as a function of temperature and cell bandgap was investigated. Several wide-bandgap materials (i.e., GaP, SiC, and GaN) are compared to the conventional materials used for space solar cells.
Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005
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ABSTRACT: A systematic survey of the opportunities for incorporating quantum well structures into multi-junction space cells is presented. The quantum well solar cell provides a means for extending the absorption edge of a conventional p/n cell, but normally at the cost of a reduction in the open circuit voltage. However, in a current limited multi-junction solar cell, an increased efficiency can result from trading the sub-cell voltage for a higher overall device current. The suitability of a strain-balanced MQW junction as a higher current replacement for the GaAs junction is discussed, as well as an approach for attaining a high-quality 1 eV junction, suitable for a highly efficient 4-junction solar cell. The minimum performance that must be attained from the MQW junction is outlined and offered as a roadmap for the successful incorporation of quantum wells into the next generation of space solar cells.
Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005
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ABSTRACT: The metamorphic or lattice mis-matched triple junction cell under development by ERI and its partners has an InGaAs junction (bottom cell) of the three-cell stack. This junction is the current limiting, and therefore efficiency limiting, junction due to current matching which must be maintained through the device. This situation is further exacerbated when these devices are used in space, due to the bottom junction being the most affected by radiation degradation. This limitation may be addressed through the incorporation of InAs quantum dot array into the depletion region of an InGaAs cell. The InAs quantum dots in the InGaAs cell will provide sub-gap absorption and thus improve its short circuit current. This cell could then be integrated into the three-cell stack to achieve a space solar cell whose efficiency exceeds current state-of-the-art standards. A theoretical estimate predicts that a InGaAlP(1.95 eV)/InGaAsP(1.35 eV)/InGaAs(1.2 eV) triple junction cell with an improved bottom cell current could have an efficiency exceeding 40%. In addition, there is now a growing body of work that theoretically and experimentally indicates that the use of quantum dot structures may also hold ancillary benefits such as improved temperature coefficients and better radiation tolerance. These benefits are extremely important considering the intended space utilization of these devices. In this study, InAs quantum dots have been grown on lattice-mismatched InGaAs (1.2 eV bandgap) grown epitaxially on GaAs by metalorganic chemical vapor deposition (MOCVD) and characterized via photoluminescence (PL) and atomic force microscopy (AFM). Arrays of these InAs Quantum dots have been incorporated into prototype InGaAs devices. The photovoltaic efficiency under simulated 1 sun intensity and air mass zero (AM0) illumination was measured. The spectral response demonstrated that sub-gap photoconversion in InGaAs cells is possible through the incorporation of the InAs quantum dots.
Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005
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ABSTRACT: An alpha voltaic battery utilizes a radioactive substance, which emits energetic alpha particles, that is coupled to a semiconductor p/n junction diode. Alpha voltaics have not been technologically successful to date primarily because the alpha particles damage the semiconductor material, thus degrading the electrical output of the solar cell in just a matter of hours. The key to future development resides in the ability to limit this degradation. Several approaches to solving this problem have been investigated. One approach uses photovoltaic devices which have good radiation tolerance such as InGaP. Another involves the use of non-conventional cell designs, such as a lateral junction n-type/intrinsic/p-type/intrinsic cell, which minimizes the effect of radiation damage on the overall cell performance. A third approach uses an intermediate absorber which converts the alpha energy into light which can be converted by the photovoltaic junction. The intermediate absorbers used in this approach are inherently radiation-hard semiconducting quantum dots. The synthesis of both the quantum dots and the InGaP devices are presented. A summary of the various approaches and resulting performance of the alpha voltaic devices is given.
Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005
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ABSTRACT: Recent advances in dye-sensitized and organic polymer solar cells have lead NASA to investigate the potential of these devices for space power generation. Dye-sensitized solar cells were exposed to simulated low-earth orbit conditions and their performance evaluated. All cells were characterized under simulated air mass zero (AM0) illumination. Complete cells were exposed to pressures less than 1×10<sup>-7</sup> torr for over a month, with no sign of sealant failure or electrolyte leakage. Cells from Solaronix SA were rapid thermal cycled under simulated low-earth orbit conditions. The cells were cycled 100 times from -80 C to 80 C, which is equivalent to 6 days in orbit. The best cell had a 4.6% loss in efficiency as a result of the thermal cycling. In a separate project, novel -Bridge-Donor-Bridge-Acceptor- (-BDBA-) type conjugated block copolymer systems have been synthesized and characterized by photoluminescence (PL). In comparison to pristine donor or acceptor, the PL emissions of final -B-D-B-A- block copolymer films were quenched over 99%. Effective and efficient photo induced electron transfer and charge separation occurs due to the interfaces of micro phase separated donor and acceptor blocks. The system is very promising for a variety high efficiency light harvesting applications. Under an SBIR contract, fullerene-doped polymer-based photovoltaic devices were fabricated and characterized. The best devices showed overall power efficiencies of -0.14% under white light. Devices fabricated from 2% solids content solutions in chlorobenzene gave the best results. Presently, device lifetimes are too short to be practical for space applications.
Energy Conversion Engineering Conference, 2002. IECEC '02. 2002 37th Intersociety; 08/2004
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ABSTRACT: The development of thin-film solar cells on flexible, lightweight, space-qualified substrates provides an attractive cost solution to fabricating solar arrays with high specific power, (W/kg). The use of a polycrystalline chalcopyrite absorber layer for thin film solar cells is considered as the next generation photovoltaic devices. A key technical issues outlined in the 2001 US Photovoltaic Roadmap, is the need to develop low cost, high throughput manufacturing for high-efficiency thin film solar cells. At NASA GRC we have focused on the development of new single-source-precursors (SSP's) and their utility to deposit the chalcopyrite semiconducting layer (CIS) onto flexible substrates for solar cell fabrication. The syntheses and thermal modulation of SSP's via molecular engineering is described. Thin-film fabrication studies demonstrate the SSP's can be used in a spray CVD process, for depositing CIS at reduced temperatures, which display good electrical properties, suitable for PV devices.
Energy Conversion Engineering Conference, 2002. IECEC '02. 2002 37th Intersociety; 08/2004
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ABSTRACT: Nanocrystalline (or quantum dot) materials hold potential as components of next-generation photovoltaic (PV) devices. The inclusion of quantum dots in PV devices has been proposed as a means to improve the efficiency of photon conversion (quantum dot solar cell), enable low-cost deposition of thin-films, provide sites for exciton dissociation, and pathways for electron transport. Quantum dots are also expected to be more resistant to degradation from electron, proton, and alpha particle radiation than the corresponding bulk material, a requirement for use in space solar ~sells. Chalcopyrite nanocrystals can be produced by low-temperature thermal decomposition of single-source precursors such as (PR3)2CuIn(ER')4 (R = Ph, R' = Et, E = S; R = R' = Ph, E = Se). Single-source precursors are molecules which contain all the necessary elements for synthesis of a desired material. Thermal decomposition of the precursor results in the formation of material with the correct stoichiometry as a nanocrystalline powder or a thin film, often at significantly lower temperatures than those typically employed for thin-film deposition by multi-source evaporation techniques, typically less than 500 C. We show that CuInSz and CuInSe2 nanocrystals can be synthesized from the precursors at temperatures as low as 250 C. The nanocrystals are characterized by optical spectroscopy, X-ray diffraction, and electron microscopy.
08/2003;
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ABSTRACT: The use of both inorganic and organic nanostructured materials in producing high efficiency photovoltaics is discussed in this paper. Recent theoretical results indicate that dramatic improvements in device efficiency may be attainable through the use of semiconductor quantum dots in an ordinary p-i-n solar cell. In addition, it has also recently been demonstrated that quantum dots can also be used to improve conversion efficiencies in polymeric thin film solar cells. A similar improvement in these types of cells has also been observed by employing single wall carbon nanotubes. This relatively new carbon allotrope may assist both in the disassociation of excitons as well as carrier transport through the composite material. This paper reviews the efforts that are currently underway to produce and characterize these nanoscale materials and to exploit their unique properties.
Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on; 06/2003
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ABSTRACT: Nanometer sized particles of the chalcopyrite compounds CuInS/sub 2/ and CuInSe/sub 2/ were synthesized by thermal decomposition of the molecular single-source precursors (PPh/sub 3/)/sub 2/CuIn(SEt)/sub 4/ and (PPh/sub 3/)/sub 2/CuIn(SePh)/sub 4/, respectively, in a non-coordinating solvent at temperatures below 300/spl deg/C. The nanoparticles range in size from 3-30 nm and are aggregated to form roughly spherical clusters of about 500 nm in diameter and can be deaggregated by capping with thiolated ligands. The nanoparticles are characterized by powder X-ray diffraction, scanning and tunneling electron microscopies, and optical spectroscopy.
Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on; 06/2003
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ABSTRACT: Solar cells have been prepared using atmospheric pressure spray chemical
vapor deposited CuInS2 absorbers. The CuInS2 films were deposited at 390
C using the single source precursor (PPh3)2CuIn(SEt)4 in an argon
atmosphere. The absorber ranges in thickness from 0.75 - 1.0
micrometers, and exhibits a crystallographic gradient, with the leading
edge having a (220) preferred orientation and the trailing edge having a
(112) orientation. Schottky diodes prepared by thermal evaporation of
aluminum contacts on to the CuInS2 yielded diodes for films that were
annealed at 600 C. Solar cells were prepared using annealed films and
had the (top down) composition of Al/ZnO/CdS/CuInS2/Mo/Glass. The Jsc,
Voc, FF and (eta) were 6.46 mA per square centimeter, 307 mV, 24% and
0.35%, respectively for the best small area cells under simulated AM0
illumination.
09/2002; -1:84-90.
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ABSTRACT: The need for smaller, lightweight, autonomous power systems has increased with the current focus on micro- and nanosatellites. Small-area, high-efficiency, thin-film batteries and solar cells are an attractive choice for such applications. The NASA Glenn Research Center, Johns Hopkins Applied Physics Laboratory, Lithium Power Technologies, MicroSat Systems, and others, have been working on the development of autonomous monolithic packages combining these elements, or what are called integrated power supplies (IPS). These supplies can be combined with individual satellite components and are capable of providing continuous power, even under intermittent illumination associated with a spinning or earth-orbiting satellite. This paper discusses the space mission applicability, benefits, and current development efforts associated with IPS components and systems. The characteristics and several mission concepts for an IPS that combines thin-film photovoltaic power generation with thin-film lithium ion energy storage are described. Based on this preliminary assessment, it is concluded that the most likely and beneficial application of an IPS will be for small ‘nanosatellites’ or in specialized applications serving as a decentralized or as a distributed power source or uninterruptible power supply. Copyright © 2002 John Wiley & Sons, Ltd.
Progress in Photovoltaics Research and Applications 08/2002; 10(6):391 - 397. · 5.79 Impact Factor
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ABSTRACT: The ability to determine the in-situ optoelectronic behavior of semiconductor materials has become especially important as the size of device architectures is reduced and the development of complex microsystems has increased. Scanning Tunneling Optical Resonance Microscopy or STORM has the ability to interrogate the optical bandgap as a function of position within a semiconductor microstructure. This technique uses a tunable solid-state Ti sapphire laser whose output is “chopped” using a spatial light modulator and is coupled by a fiber optic to a scanning tunneling microscope in order to illuminate the tip-sample junction. The photoenhanced portion of the tunneling current is spectroscopically measured using a lock-in technique. The capabilities of this technique were verified using semiconductor microstructure calibration standards that were grown by organometallic vapor phase epitaxy (OMVPE) at the NASA Glenn Research Center. Bandgaps characterized by STORM measurements were found to be in good agreement with the bulk values determined by transmission spectroscopy, photoluminescence, and with the theoretical values that were based on x-ray diffraction results.
MRS Proceedings. 12/2001; 738.
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ABSTRACT: We show the first direct experimental correlation between the presence of closed core screw dislocations in 6H-SiC epilayers with recombination centers, as well as with some of the small growth pits on the epilayer surface in lightly-doped 6H-SiC Schottky diodes. At every Synchrotron White-Beam X-ray Topography (SWBXT)-identified closed core screw dislocation, an Electron Beam Induced Current (EBIC) image showed a dark spot indicating a recombination center, and Nomarski optical microscope and Atomic Force Microscope (AFM) images showed a corresponding small growth pit with a sharp apex on the surface of the epilayer.
03/2000;
