D.M. Wilt

Rochester Institute of Technology, Rochester, New York, United States

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Publications (143)74.15 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Specific coating processes and materials were investigated in the quest to develop multilayer coatings with greater tolerance to space radiation exposure. Ultraviolet reflection (UVR) and wide-band antireflection (AR) multilayer coatings were deposited on solar cell covers and test substrates and subsequently exposed to simulated space environments and also flown on the Materials International Space Station Experiment-7 (MISSE-7) to determine their space environment stability. Functional solar cells integrated with these coatings underwent simulated UV and MISSE-7 low earth orbit flight exposure. The effects of UV, proton, and atomic oxygen exposure on coatings and on assembled solar cells as related to the implemented deposition processes and material compositions were small. The UVR/AR coatings protected flexible polymer substrate materials that are intended for future flexible multijunction cell arrays to be deployed from rolls. Progress was made toward developing stable and protective coatings for extended space-mission applications. Test results are presented.
    Applied Optics 02/2014; 53(4):A339-50. DOI:10.1364/AO.53.00A339 · 1.78 Impact Factor
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    ABSTRACT: An approach to laying down, interconnecting, and encapsulating a space solar panel module is described that uses large-area multi-cell transparent covers. Coverglass replacement materials were evaluated and tested in a variety of environments, including a flight test on MISSE-7, and found to be durable in the LEO environment, with approaches to extending durability to GEO and other high radiation environments. The multi-cell cover approach enables assembling advanced cells, such as IMM, into a thin array using a laminating approach that implements co-planar front contact cells with thermosonic wire bonding interconnection. Coupons of the encapsulated panels were fabricated and found to have good durability in thermal cycling.
    2013 IEEE 39th Photovoltaic Specialists Conference (PVSC); 06/2013
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    ABSTRACT: A flexible space solar cell coverglass replacement called Pseudomorphic Glass (PMG) has been under investigation in hopes of providing a robust, high transmissivity replacement for conventional coverglass. PMG is composed of ceria doped borosilicate or fused silica beads incorporated in a variety of polymer matrices. The glass beads provide the primary radiation protection and the polymer matrix provides the mechanical integrity. PMG development has recently focused on optimization of optical performance (transmissivity and scattering), bead material and development of optical coatings.
    2013 IEEE 39th Photovoltaic Specialists Conference (PVSC); 06/2013
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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.
    38th IEEE Photovoltaic Specialists Conference (PVSC), Austin, TX; 06/2012
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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: The insertion of nanostructured materials (such as quantum wells, wires, and dots) into the intrinsic region of p-i-n solar cells introduces an intermediate band within the bandgap of the host material. It has been shown that the sub-bandgap conversion provided by the nanostructured materials, enhances the short circuit current as well as the overall efficiency of InAs quantum dots (QD) imbedded in GaAs superlattice (SL) solar cells [1]. As a contender for space applications, it is necessary to subject these solar cell structures to temperatures encountered in the Low Earth Orbit (LEO), probing for any material degradation. Herein, we focus on temperature dependent characterization using high resolution X-ray diffraction (HRXRD) of InAs QD enhanced GaAs solar cell structures with varying growth parameters. The structures characterized can be classified into three groups: (1) GaP strain compensation coverage, (2) GaAs barrier coverage, and (3) InAs coverage for QD formation. HRXRD rocking curves of each structure focusing around the GaAs peak are analyzed at a range of temperatures up to 200˚C. Although no noticeable shifts in the SL peaks are detected, interfacial diffusion decreased the resolution of fringes produced by reflections at the SL interfaces in test structures with varying InAs QD coverage. Unbalanced strain in the same structures shows a distortion in the GaAs peaks.
    MRS Online Proceeding Library 01/2012; 1432. DOI:10.1557/opl.2012.1139
  • [Show abstract] [Hide abstract]
    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
  • N. Walmsley, T. Stern, D. Wilt
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    ABSTRACT: Implementing new solar panel technology is often hampered by the difficulty in obtaining flight heritage and integrating experimental technology into conventional panels. Modular solar panel technology overcomes this limitations by allowing substitution of an experimental module into a modular flight arrays using mostly conventional modules. The approach also allows the benefits of full-scale testing when test article sizes are limited, and easier repairability by allowing damaged modules to be unplugged and repaired off-line. The Modular Solar Array with Integrated Construction (MOSAIC) technology described in this paper is being flight tested on an experiment called MATRS (Modular Array Technology for Reconfigurable Spacecraft). The flight test unit includes modules with conventional space cells, as well as advanced modules incorporating 4-junction IMM cells to demonstrate the ease of ground integration and replacement of the modular approach, incorporation of experimental modules in flight, and life cycle performance.
    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; DOI:10.1109/PVSC.2011.6186335
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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: Inverted Metamorphic Multijunction (IMM) technology has demonstrated excellent energy conversion efficiency, 32% AM0. In addition to high conversion efficiency, this technology also offers the potential for ultra-high mass specific power at the blanket level. Because the substrate is removed, the thin, flexible epitaxial cell can be incorporated in a variety of novel blanket structures. Several novel array technologies have been proposed which would take advantage of the flexible nature of the IMM by incorporating rolled stowage for launch. The flexibility of the IMM may lead one to assume that the IMM is a much higher efficiency drop-in replacement for conventional thin-film photovoltaics (ex. amorphous silicon, copper indium gallium diselenide). An important differentiation between these technologies is the radiation hardness of the different technologies to the space environment. This paper presents a study to examine the photovoltaic blanket specific mass achievable with IMM technology depending upon the orbit of interest and the end-of-life performance requirement. The impact of radiation shielding, both front and back, is assessed.
    Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
  • 6th International Energy Conversion Engineering Conference (IECEC); 07/2008
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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: 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: 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: 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: 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; 92(12-92):123512 - 123512-3. DOI:10.1063/1.2903699 · 3.52 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. (C) 2007 American Institute of Physics.
    Applied Physics Letters 10/2007; 91(18):183108. DOI:10.1063/1.2803854 · 3.52 Impact Factor
  • 5th International Energy Conversion Engineering Conference and Exhibit (IECEC); 06/2007
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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
    02/2007; 890(1):335-345. DOI:10.1063/1.2711751

Publication Stats

757 Citations
74.15 Total Impact Points

Institutions

  • 2000–2008
    • Rochester Institute of Technology
      • Department of Biomedical Engineering
      Rochester, New York, United States
  • 1997–2008
    • NASA
      • Glenn Research Center
      Вашингтон, West Virginia, United States
  • 2002–2005
    • The Ohio State University
      • Department of Electrical and Computer Engineering
      Columbus, OH, United States
  • 1994
    • Cleveland State University
      • Department of Electrical and Computer Engineering
      Cleveland, Ohio, United States