AnnaMaria T. Pal

Rare Earth Doped High Temperature Ceramic Selective Emitters

As a result of their electron structure, rare earth ions in crystals at high temperature emit radiation in several narrow bands rather than in a continuous blackbody manner. This study develops a spectral emittance model for films of rare earth containing materials. Although there are several possible rare earth doped high temperature materials, this study was confined to rare earth aluminum garnets. Good agreement between experimental and theoretical spectral emittances was found for erbium, thulium and erbium-holmium aluminum garnets. Spectral emittances of these films are sensitive to temperature differences across the film. Emitter efficiency is also a sensitive function of temperature. For thulium aluminum garnet the efficiency is 0.38 at 1700 K but only 0.19 at 1262 K.

Rare earth doped yttrium aluminum garnet (YAG) selective emitters

Abstract As a result of their electron structure, rare earth ions in crystals at high temperature emit radiation in several narrow bands rather than in a continuous blackbody manner. This study presents a spectral emittance model for films and cylinders of rare earth doped yttrium aluminum garnets. Good agreement between experimental and theoretical film spectral emittances was found for erbium and holmium aluminum garnets. Spectral emittances of films are sensitive to temperature differences across the film. For operating conditions of interest, the film emitter experiences a linear temperature variation whereas the cylinder emitter has a more advantageous uniform temperature. Emitter efficiency is also a sensitive function of temperature. For the holminum aluminum garnet film the efficiency is 0.35 at 1446 K, but only 0.27 at 1270 K.

Final results from the MISSE5 GaAs on Si solar cell experiment

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 1cm2 and 4 cm2 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.

Thermal cycle testing of GaAs on Si and metamorphic tandem on Si solar cells

GaAs on Si (GaAs/Si) solar cells grown on Si substrates coated with a step graded buffer of Si/sub x/Ge/sub 1-x/ alloys graded to 100% Ge have demonstrated AM0 efficiencies in excess of 17%. Recently, 4 cm/sup 2/ devices were developed in preparation for on-orbit testing aboard Materials International Space Station Experiment number 5 (MISSES). In preparation for flight, thermal cycling life testing and thermal shock testing have been conducted to examine the stability of these thermal coefficient of expansion mismatched structures. Six thousand (6000) thermal cycles, equivalent to one year in LEO, from -80/spl deg/C to +80/spl deg/C have been completed with no discernable degradation in the electrical performance. The use of metamorphic III-V materials, of larger lattice parameter, has demonstrated the ability to dramatically reduce the micro crack density, presumably through strain balancing. Recently, the first demonstration of metamorphic tandem devices (1.6 eV InGaP / 1.1 eV InGaAs) on Si substrates was accomplished. The devices appeared micro crack free as determined by Nomarski optical microscopic examination and electroluminescence image analysis. Thermal shock testing of GaAs/Si devices showed micro crack formation and device electrical degradation. Thermal cycle testing of metamorphic structures (InGaP/InGaAs/Si) demonstrated a general lack of micro crack formation and improved thermal stability compared to GaAs/Si control devices.

Recrystallization of Ge for III-V photovoltaic substrates

Amorphous germanium (Ge) is RF sputtered onto 25 µm thick molybdenum (Mo) foils and recrystallized at 675 °C under an AsH3 environment. After annealing, the Ge is polycrystalline with grain boundary regions that range from 1 um2 to 0.5 cm2. The polycrystalline surface behaves electrically much like that of commercial single crystal Ge. This polycrystalline Ge on thin Mo foil will serve as a substrate for a high efficient, high mass-specific-power, photovoltaic device.

Lattice mismatched dual junction tandem cells

High performance solar cells with capabilities covering a broad range of mission parameters are of great interest to the space photovoltaic community. Current areas of interest include improving efficiency of multi-junction cells by adjusting bandgaps to more optimum values, adding junctions to existing structures and investigating the effects of various substrate materials. The goal is to merge the highest efficiency multijunction solar cell with a low cost, lightweight substrate. This paper focuses on developing a multijunction solar cell with optimum bandgaps by relaxing the constraint for lattice matching between the substrate and the epitaxial cell structure. A III-V lattice mismatched dual junction solar cell composed of a 1.6 eV InGaP top cell and a 1.1 eV InGaAs bottom cell has been grown with an Air Mass Zero (AM0) efficiency of 16.4% without an antireflective coating (ARC). An AM0 efficiency of 23% is anticipated when a dual layer antireflective coating is applied. Both sub-cells are lattice matched to each other but mismatched to the GaAs substrate. Accommodation of the lattice strain was accomplished via an InGaAs buffer structure. Extension of the lattice mismatched approach to three junction devices holds the promise to demonstrate AM0 efficiencies in excess of 30%.

Progress Towards III-V Photovoltaics on Flexible Substrates

Presented here is the recent progress of the NASA Glenn Research Center OMVPE group’s efforts in the development of high efficiency thin-film polycrystalline III-V photovoltaics on optimum substrates. By using bulk polycrystalline germanium (Ge) films, devices of high efficiency and low mass will be developed and incorporated onto low-cost flexible substrates. Our progress towards the integration of high efficiency polycrystalline III-V devices and recrystallized Ge films on thin metal foils is discussed.