D.M. Wilt

Rochester Institute of Technology, Rochester, NY, United States

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Publications (128)66.3 Total impact

  • 38th IEEE Photovoltaic Specialists Conference (PVSC), Austin, TX; 06/2012
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    ABSTRACT: We present a thorough evaluation of a first-of-its-kind quad-source AM 0 solar simulator developed by TS-Space Systems and installed at the AFRL Space Vehicles Directorate in Kirtland AFB, NM. With an underlying need to evaluate more complex and advanced space photovoltaics, the TS-Space Unisim 100 was chosen as replacement to the Spectrolab X-25. Several parameters of the Unisim were characterized including spectral irradiance, lamp stability (long-term and short-term), spatial uniformity, and repeatability. In addition, the impact of test plane translation due to variable cell holders was studied. The potential impact of AC input voltage to lamp ballast was also investigated. Further, comparisons are made to the X-25. Finally, operational considerations for the use of a multi-source simulator are discussed.
    Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE; 01/2012
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    ABSTRACT: The 2nd Forward Technology Solar Cell Experiment (FTSCE II) that flew as part of the 7th Materials on the International Space Station Experiments (MISSE 7) successfully flew on orbit for 18 months from 23 November 2009 to 20 May 2011. The Air Force Research Laboratory Space Vehicles Directorate in collaboration with the Naval Research Laboratory flew a myriad of experiments to evaluate advanced photovoltaic technologies. Such data is critical for technology development and future transition to operational use. Applications of the data include validating ground test protocols and assessing LEO environmental effects (atomic oxygen, ultraviolet radiation, thermal cycling, etc.). In addition to reducing risk to future spacecraft, research and development risks are reduced through early technology assessment for space operation. These experiments were comprised of triple-junction production as well as advanced inverted metamorphic (IMM) and other thin film III-V cells from multiple vendors. In addition to the III-V based devices an experiment was included to evaluate advanced amorphous silicon concepts. The methodology and analysis of the on-orbit data collected during the mission is presented here.
    Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE; 01/2012
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    ABSTRACT: Solar cell manufacturers have indicated that new, high efficiency solar cells will require coverglass with higher UV transmittance than currently existing materials. To date, fused silica is the only known solution but has several significant technical and cost challenges. In addition, new solar cells demonstrating record efficiency and extreme flexibility have been developed. Integrating these advanced devices with traditional coverglasses limits the ability to take advantage of novel packaging options, such as rolled blankets. A new coverglass replacement technology has been developed, termed PseudoMorphic Glass (PMG), which has the potential to meet these needs. PMG has the potential to provide the high UV spectral transmissivity required for next generation solar cells in a robust, flexible and fully encapsulating format.
    Conference Record of the IEEE Photovoltaic Specialists Conference 01/2011;
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    ABSTRACT: A space flight opportunity was seized to fly emerging solar cell, coating, and coverglass technologies in a Low Earth Orbit (LEO) environment while attached to the exterior of the International Space Station (ISS). Included in the flight manifest were nano-crystalline optimized amorphous silicon (a-Si) thin-film, Indium Phosphide (InP), and Inverted Metamorphic (IMM) Photovoltaic (PV) technologies. An array of experimental coatings and coverglass technologies completed the sample set, which formed a part of the 2<sup>nd</sup> Forward Technology Solar Cell Experiment (FTSCE II), which is part of the larger 7<sup>th</sup> Materials on the International Space Station Experiment (MISSE-7). The design and method of flight article preparation is presented herein, as well as detail of preflight environmental tests.
    Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
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    ABSTRACT: Results on the development of polycrystalline III–V based devices grown by OMVPE on thin metallic foil substrates are presented. It has previously been demonstrated that device quality polycrystalline Ge suitable for OMVPE growth can be produced on metallic foils using a recrystallization process. This work reports on the development of textured metal foil substrates with low misfit grain boundary orientations designed to improve the semiconducting device parameters of the “epitaxially” deposited Ge films, the use of innovative device structures, and grain boundary passivation approaches for the polycrystalline GaAs films that are all designed to address performance issues associated with these types of solar cells. The Ge which serves as the III-V growth template could be activated and serve as the bottom junction of a conventional triple junction III-V cell design using this approach. The crystallographic, morphological, and electro-optical properties associated with these substrates and related epitaxial films will be presented. In addition, the thermal and radiation behavior, that is critical for the potential use of these devices in space, was investigated. The potential for these devices for future space development and exploration will be discussed.
    Photovoltaic Specialists Conference, 2008. PVSC '08. 33rd IEEE; 06/2008
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    ABSTRACT: GaAs on Si (GaAs/Si) solar cells with AM0 efficiencies in excess of 17% have been demonstrated using Si substrates coated with a step-graded buffer of SixGe1-x alloys graded to 100% Ge. A year of LEO testing of this technology aboard Materials International Space Station Experiment number 5 (MISSE5) was recently competed. Electrical performance data, sun angle and thermal conditions measured on-orbit, were telemetered to ground stations daily. Ground based measurements following flight were performed on both 1cm<sup>2</sup> and 4 cm<sup>2</sup> GaAs/GaAs and GaAs/Si devices. The smaller area GaAs/Si cells showed low degradation rates for Isc, while all other cell parameters were comparable to control cells. However, the larger area GaAs/Si devices, while demonstrating similarly low Voc and FF degradation, demonstrated a larger than expected decrease in Isc. Comparison of pre and post flight QE data suggests the decrease in Isc for the large area cell may result from reduced cell active area rather than a degradation in material properties. Ground based thermal cycle testing did not replicate these results, thus differences in mounting techniques and materials may have contributed to the degradation observed on orbit for the large area device in this initial on-orbit test. Crack free GaAs/Si based devices have been demonstrated and offer a mitigation strategy for microcrack degradation.
    Photovoltaic Specialists Conference, 2008. PVSC '08. 33rd IEEE; 06/2008
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    ABSTRACT: Tensile strain compensation (SC) layers were introduced into GaAs p-i-n solar cells grown with a five-stack of InAs quantum dots (QDs) within the i-region. The effects of strain within stacked layers of InAs quantum dots (QDs) were investigated using high resolution x-ray diffraction (HRXRD). Analysis of the HRXRD data shows that the average lattice strain is minimized for the optimal SC thickness. One sun air mass zero illuminated current-voltage curves show that SC results in improved conversion efficiency and reduced dark current when compared to uncompensated devices. The strain compensated 5-layer QD solar cell shows a 0.9 mA/cm<sup>2</sup> increase in short circuit current compared to a baseline GaAs cell. Quantum efficiency measurements show this additional current results from photo-generated carriers within the quantum confined material.
    Photovoltaic Specialists Conference, 2008. PVSC '08. 33rd IEEE; 06/2008
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    ABSTRACT: The effects of alpha-particle irradiation on the current-voltage characteristics and spectral responsivity of GaAs-based p-type / intrinsic / n-type solar cell devices containing 5-layers of InAs quantum dots (QD) grown with strain-compensation layers were investigated. The devices were subjected to ∼4.2 MeV alpha-particle irradiation and the variation in the air mass zero short circuit current, open circuit voltage, fill factor, efficiency, and spectral responsivity were monitored as function of fluence and displacement damage dose. The measured spectral responsivity values of the quantum dot solar cell at wavelengths above and below the GaAs bandgap were used to investigate the rate of degradation in the InAs QDs in comparison to that of bulk GaAs. A computational model was developed to study the effects of strain on the energy threshold for atomic displacement (knock-out energy) of indium and arsenic within an InAs QD. Using the many-body Tersoff potentials, the energy of the primary knock-on atom occupying various sites within the lattice was calculated as a function of strain. The observed increases in minimum knock-out energy and interstitial-site energy with strain suggest a potential mechanism for the increased radiation tolerance observed in Stranski-Krastanow grown QDs.
    Photovoltaic Specialists Conference, 2008. PVSC '08. 33rd IEEE; 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; · 3.79 Impact Factor
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    ABSTRACT: The effects of alpha-particle irradiation on an InAs quantum dot QD array and GaAs-based InAs QD solar cells were investigated. Using photoluminescence PL mapping, the PL intensity at 872 and 1120 nm, corresponding to bulk GaAs and InAs QD emissions, respectively, were measured for a five-layer InAs QD array which had a spatially varying total alpha-particle dose. The spectral response and normalized current-voltage parameters of the solar cells, measured as a function of alpha-particle fluence, were used to investigate the change in device performance between GaAs solar cells with and without InAs QDs. © 2007 American Institute of Physics. Evaluation of the radiation tolerance of InAs quantum dot QD/GaAs solar cells SCs is a crucial requirement for their future applicability in space applications. QD SCs con-taining InAs QD Refs. 1–3 or GaSb QD Ref. 4 in a GaAs host have recently demonstrated sub-GaAs band gap absorp-tion. These results demonstrate significant progress toward the realization of an intermediate band solar cell wherein an isolated intermediate band is formed within a wider band gap host material. 2 An alternative use of QD arrays is to incorporate them within the middle junction typically In 0.01 Ga 0.99 As of a III-V triple junction solar cell, resulting in a reduced dimensionality version of the multiple quantum well MQW solar cell. 5–7 The use of nanostructured materi-als enables one to tune the absorption properties of the device leading to a better match to the solar spectrum and subsequently an increased current throughout the current-matched triple junction stack. The total sub-GaAs photocur-rent production is expected to be greater in QD SC devices since the three-dimensional confinement of the QDs should allow for more isotropic absorption. An additional benefit resulting from the QD SC approach may be an enhanced tolerance in ionizing-radiation, which has been previously demonstrated in QD arrays and QD lasers. 8–12 In this study, the PL intensity was measured as a function of alpha-particle fluence for a vertically stacked InAs QD/GaAs structure grown on semi-insulating 001 GaAs substrates by metal organic vapor phase epitaxy MOVPE. The PL structure consisted of five layers of InAs QDs separated by 10 nm GaAs layers. The hemispheri-cal QDs had a mean size of 7 40 nm 2 and areal density of 5 ±0.5 10 10 cm −2 measured by atomic force micros-copy see Fig. 1c. Additionally, the current-voltage I-V response and spectral response SR for three single junction GaAs p-type/intrinsic/n-type pin solar cells were measured with respect to alpha-particle fluence. The three cells con-sisted of a reference pin device, a pin device with one-layer of InAs QDs embedded in the i-region referred to as QD1, and a five-layer InAs QD pin device QD5. Details of the InAs QD array growth, solar cell fabrication, and character-ization of the 1.0 cm 2 devices prior to cleaving can be found in Ref. 1. The InAs QD array PL structure was irradiated by plac-ing it 2 mm from a 1 mCi 210 Po isotropic alpha-particle source with a circular area of 2.5 cm 2 . A copper foil shield was placed over the PL structure reducing the portion of the surface exposed to the radiation. The position of the shield was shifted across the structure with 2 mm increments at exponentially spaced time intervals thereby creating a flu-ence gradient over the surface of the structure. An Accent RPM2000 PL mapper with pump wavelength of 532 nm was
    Applied Physics Letters 10/2007; 91(18):183108. · 3.79 Impact Factor
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    ABSTRACT: Thermophotovoltaic (TPV) energy conversion has long been considered a potential replacement for thermoelectrics in radioisotope powered deep space power systems. In this application, TPV offers significant potential improvements in both efficiency and mass specific power (W/kg), performance which is considered mission enabling for a variety of mission concepts. TPV systems powered by concentrated solar energy have also been proposed for inner planetary solar system missions. This concept takes advantage of TPV’s ability to store energy for shadow periods in the form of heat energy rather than as electrical energy (batteries), as is commonly done for photovoltaic power systems. The simplicity and large number of power cycles offered by the thermal energy storage offers potential system benefits compared to a photovoltaic / battery system. Recent efforts in the development of radioisotope TPV (RTPV) at Creare have resulted in the demonstration of converter efficiencies in excess of 19%. Several independent system mass analyses have been performed for the Creare RTPV system and they predict specific powers above 10W/kg at the system level. Trades have suggested increasing the rejection temperature can result in a 50% reduction in radiator area while only suffering a 15% reduction in mass specific power. © 2007 American Institute of Physics
    AIP Conference Proceedings. 02/2007; 890(1):335-345.
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    ABSTRACT: The effects of strain within stacked layers of InAs quantum dots (QDs) were investigated. InAs QD test structures with and without strain compensation (SC) were analyzed using atomic force microscopy, transmission electron microscopy, and X-ray diffraction. The affects of strain compensation on test structure morphology and on GaAs-based QD solar cell performance was studied as a function of the thickness of the SC layer. X-ray diffraction analysis of the QD embedded test structures reveals a relationship between the SC thickness and the observed crystalline quality. Air mass zero illuminated current vs. voltage data and spectral responsivity measurements were used for the solar cell comparison. When SC is employed, QD insertion shows a lower open circuit voltage, in reference to a baseline device without QDs, but leads to an enhancement in the short circuit current of the device.
    MRS Proceedings. 12/2006; 1031.
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    ABSTRACT: The growth of InAs quantum dots (QDs) by organometallic vapor phase epitaxy (OMVPE) for use in GaAs based photovoltaics devices was investigated. Growth of InAs quantum dots was optimized according to their morphology and photoluminescence using growth temperature and V/III ratio. The optimized InAs QDs had sizes near 7×40 nm with a dot density of 5(±0.5)×1010 cm-2. These optimized QDs were incorporated into GaAs based p-i-n solar cell structures. Cells with single and multiple (5x) layers of QDs were embedded in the i-region of the GaAs p-i-n cell structure. An array of 1 cm2 solar cells was fabricated on these wafers, IV curves collected under 1 sun AM0 conditions, and the spectral response measured from 300-1100 nm. The quantum efficiency for each QD cell clearly shows sub-bandgap conversion, indicating a contribution due to the QDs. Unfortunately, the overarching result of the addition of quantum dots to the baseline p-i-n GaAs cells was a decrease in efficiency. However, the addition of thin GaP strain compensating layers between the QD layers, was found to reduce this efficiency degradation and significantly enhance the subgap conversion in comparison to the un-compensated quantum dot cells.
    MRS Proceedings. 12/2006; 1017.
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    ABSTRACT: The viability of InGaP diodes coupled with α-particle sources as radioisotope power supplies is investigated both theoretically and experimentally. The electrical power output of epitaxially grown InGaP p-type∕n-type (p/n) junction diodes coupled with and α-particle sources was measured. A theoretical model was developed that determines the α-particle energy deposition profile within an InGaP diode when irradiated by an omnidirectional α-particle source. The results of the model illustrate the dramatic influence the radiation source∕diode configuration has on the α-particle energy deposition profile within a device. Progress has been shown towards increasing the radiation tolerance of the InGaP devices, which included utilizing an intrinsic region and reducing the junction thickness. Introduction of the intrinsic region within a conventional n/p diode to form a n-type∕intrinsic∕p-type diode enabled the device to withstand a ten times greater fluence of 4.2 MeV α particles before decreasing to 50% of its original power output under simulated air mass zero illumination, when compared to an abrupt junction device with the same active region thickness.
    Journal of Applied Physics 12/2006; 100(11):114519-114519-5. · 2.21 Impact Factor
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    ABSTRACT: First on-orbit data from the forward technology solar cell experiment (FTSCE) are presented. FTSCE is housed within the 5<sup>th </sup> Materials on the International Space Station Experiment (MISSE-5), and currently resides on the exterior of the ISS. A range of solar cell technologies are included in the experiment including triple junction (3J) InGaP/GaAs/Ge solar cells from several vendors, thin film amorphous Si and CuIn(Ga)Se<sub>2</sub> cells, and next-generation technologies like single-junction GaAs cells grown on Si wafers and metamorphic InGaP/InGaAs/Ge triple-junction cells. Measured current vs. voltage (IV) curves from on- board experiments are analyzed. All of the solar cell technologies are showing nominal performance with no obvious signs of degradation
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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    ABSTRACT: We have theoretically analyzed the potential efficiency improvement to multi-junction solar cell efficiencies which are available through the incorporation of quantum dot using detailed balance calculations. We have also experimentally investigated the Stranski-Krastanov growth of self-organized InAs quantum dots and quantum dot arrays on lattice-matched GaAs by metallorganic vapor phase epitaxy (MOVPE). The morphology of the quantum dots were investigated as a function of their growth parameters by atomic force microscopy (AFM). Photoluminescence and optical absorption measurements have demonstrated that the incorporation of InAs quantum dots (QD) into a GaAs structure can provide sub-GaAs bandgap electronic states
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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    ABSTRACT: High efficiency III-V multijunction solar cells deposited on metal foil or even polymer substrates can provide tremendous advantages in mass and stowage, particularly for planetary missions. As a first step towards that goal, polycrystalline p/i/n gallium arsenide (GaAs) solar cells are under development on bulk polycrystalline germanium (Ge) substrates. Single junction p/i/n GaAs photovoltaic devices, incorporating indium gallium phosphide (InGaP) front and back window layers, have been grown and processed. Device performance has shown a dependence upon the thickness of a GaAs buffer layer. An unintentional Ge subjunction was formed in the initial devices and limited the efficiency to ~13% (estimated with an antireflection coating) due to the current mismatch and lack of tunnel junction interconnect. Work is underway to control the Ge junction formation and extend the development to polycrystalline InGaP devices
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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    ABSTRACT: we have grown InAs QDs on two types of vicinal GaAs substrates and under various growth conditions. QDs grown on 2deg offcut substrates show superior optical characteristics compared to QDs on 6deg offcut substrates, which showed no QD luminescence. The InAs growth temperature was shown to have an impact on QD nucleation, with higher growth temperature leading to both improved dot densities and coherence. Finally, the V/III ratio during InAs growth dramatically effects the uniformity of the QD luminescence. Our best QDs were grown at 495degC using a V/III ratio of 12. These QDs were 7times40 nm in size with a density of 5(plusmn0.5)times10<sup>10</sup> cm<sup>-2</sup>. The spatial PL, peaking at 1220 nm, had a wavelength and intensity deviation across the 2" wafer of 0.3% and 16%, respectively
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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    ABSTRACT: GaAs on Si (GaAs/Si) solar cells grown on Si substrates coated with a step graded buffer of Si<sub>x</sub>Ge<sub>1-</sub> alloys graded to 100% Ge have demonstrated AM0 efficiencies in excess of 17%. This technology offers the potential for significant increases in mass specific power (kW/kg) due to the low density and robust nature of the silicon substrate. Concerns exist that this III-V on Si technology may degrade when used in an application with significant thermal cycles due to the ~60% difference in thermal expansion coefficients. To examine this potential problem, devices were developed and launched to the International Space Station for on-orbit testing aboard Materials International Space Station Experiment number 5 (MISSES). In low Earth orbit, the devices are experiencing up to 100degC thermal cycles every 90 minutes. Data telemetered from MISSES indicates that the devices have successfully completed <6 months of on-orbit testing with no degradation observed
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006

Publication Stats

443 Citations
66.30 Total Impact Points

Institutions

  • 2000–2008
    • Rochester Institute of Technology
      • School of Physics and Astronomy
      Rochester, NY, United States
  • 2002–2005
    • The Ohio State University
      • Department of Electrical and Computer Engineering
      Columbus, OH, United States