Design, fabrication, and characterization of solar cells for high temperature and high radiation space applications

In this work, novel III-V photovoltaic (PV) materials and device structures are investi- gated for space applications, specifically for tolerance to thermal effects and ionizing radia- tion effects. The first focus is on high temperature performance of GaP solar cells and on performance enhancement through the incorporation of InGaP/GaP quantum well structures. Temperature dependent performance of GaP solar cells is modeled and compared to a modeled temperature dependence of GaAs. The temperature model showed that a GaP cell should have a normalized efficiency temperature coefficient of -1.31 *10⁻³°C⁻¹, while a standard GaAs cell should have a normalized temperature coefficient of -2.23*10⁻³°C⁻¹, representing a 42% improvement in the temperature stability of efficiency. Both GaP and GaAs solar cells were grown using metal organic vapor phase epitaxy and fabricated into solar cell devices. An assortment of optical and electrical characterization was performed on the solar cells. Finally, GaP solar cell performance was measured in an environment simulating the temperatures and light concentrations seen in sub 1 AU solar orbits, simulating the effects on a solar cell as it approaches the sun. A positive normalized temperature coefficient of 2.78*10⁻³°C⁻¹ was measured for a GaP solar cell, indicating an increase in performance with increasing temper- ature. In addition, comparing results of GaP solar cells with and without quantum wells, the device without MQWs had an integrated short circuit current density of 1.85 mA/cm² while the device containing quantum wells has a short circuit current density of 2.07 mA/cm² or a 12.4% short circuit current increase over that of the device without quantum wells, showing that quantum wells can be used effectively in increasing the current generation in GaP solar cells. The second focus of this thesis is on the ionizing radiation tolerance of epitaxially lifted off (ELO) InP and InGaAs (lattice-matched to InP) for the purpose of assessing device lifetime in high-radiation Earth orbits. Solar cells are characterized through spectral responsivity as well as illuminated and dark current-voltage (I-V ) measurements before being subjected to exposure to a 5 mCi ²¹⁰Po alpha source and a 100 mCi ⁹⁰Sr beta source. Device performance is measured with increasing particle fluences. Previously reported results showed epitaxially grown InP solar cells to generate 76.5% of the beginning-of-life (BOL) maximum power under AM0 at a 1MeV beta fluence of 6*1015 e/cm2. In this study, a degradation to 71.1% unirradiated maximum power was seen at a 1MeV beta fluence of 3.19*10¹⁵ e/cm². This demonstrates that ELO InP cells degrade comparably to bulk InP cells under ionizing radiation. An InGaAs cell was measured under 5.4 M eV alpha radiation and had a 50% BOL performance point at 4.7*10⁹ 5.4MeV alpha/cm². The 50% BOL performance point for an InP cell in the same conditions was 1.9*10¹⁰ alpha/cm², showing similar degradation at 4x the alpha fluence

Development and Characterization of Novel III-V Materials for High Efficiency Photovoltaics

Photovoltaics (PV) are an enabling technology in the field of aerospace, allowing satellites to operate far beyond the technological limitations of chemical batteries by providing a constant power source. However, launch costs and payload volume constraints result in a demand for the highest possible mass and volume specific power generation capability, a proxy for which is device power conversion efficiency. Enhancing the efficiency of III-V PV devices beyond the single-junction Shockley Queisser (SQ) limit has been a driving goal in PV development. Two competing loss mechanisms are thermalization, where photon energy in excess of the absorbing material’s bandgap is lost to heat, and transmission or non-absorption, where a photon has too little energy to generate an electron-hole pair in the semiconductor. A further complication regarding the longevity of PV on satellites is damage due to exposure of high energy particle radiation limiting the operational life of the satellite via gradual degradation in efficiency. In this work, two approaches to achieving higher power conversion efficiency are explored. The first, for devices at beginning of life, is towards the development of a prototype intermediate band solar cell (IBSC) where the spectrum is split into three optical transitions via the formation of an intermediate band between the conduction and valence bands of a wide bandgap host material. Towards this goal, an InAs/AlAsSb quantum dot solar cell (QDSC) capable of enabling sequential absorption is demonstrated via a two-step photon absorption measurement and photoreflectance is used to demonstrate the presence of intraband optical transitions. The second approach, focusing on power generation at end of life, utilizes multijunction photovoltaics where successively higher bandgap materials are stacked in series to optically split the solar spectrum to reduce both thermalization and transmission loss. The addition of InAs/GaAs QDs to a GaAs subcell and InGaAs strain balanced quantum well superlattices to inverted metamorphic multijunction (IMM) devices are explored in order to improve device current retention as material is damaged due to knock-on events displacing atoms from the crystalline lattice. A third section of this work focuses on reducing costs by demonstrating a model for performance of III-V devices grown on polycrystalline virtual substrates considering two primary extended defects: the effects of crystal grain boundaries and the effects of antiphase boundaries induced by growing polar III-V materials on nonpolar Ge substrates

GaSb on GaAs interfacial misfit solar cells

The GaAs/GaSb interface misfit design can achieve comparable efficiency to conventional inverted metamorphic multijunction cells at up to 30% cost reduction. In this pre-liminary work, GaSb single junctions were grown via molecular beam epitaxy on both GaSb and GaAs substrates to compare and fine tune the interfacial misfit growth process. Current vs voltage results show that the best homo-epitaxial cell achieved 5.2% under 35-sun concentration. TEM did not reveal any threading dislocations in the hetero-epitaxial cells, however, device results indicated higher non-radiative recombination than expected, likely due to unpassivated surface states. Improvements to cell processing will be explored and more characterization is planned to determine the cause of degraded hetero-epitaxial cell performance

GaSb solar cells grown on GaAs via interfacial misfit arrays for use in the III-Sb multi-junction cell

Growth of GaSb with low threading dislocation density directly on GaAs may be possible with the strategic strain relaxation of interfacial misfit arrays. This creates an opportunity for a multi-junction solar cell with access to a wide range of well-developed direct bandgap materials. Multi-junction cells with a single layer of GaSb/GaAs interfacial misfit arrays could achieve higher efficiency than state-of-the-art inverted metamorphic multi-junction cells while forgoing the need for costly compositionally graded buffer layers. To develop this technology, GaSb single junction cells were grown via molecular beam epitaxy on both GaSb and GaAs substrates to compare homoepitaxial and heteroepitaxial GaSb device results. The GaSb-on-GaSb cell had an AM1.5g efficiency of 5.5% and a 44-sun AM1.5d efficiency of 8.9%. The GaSb-on-GaAs cell was 1.0% efficient under AM1.5g and 4.5% at 44 suns. The lower performance of the heteroepitaxial cell was due to low minority carrier Shockley-Read-Hall lifetimes and bulk shunting caused by defects related to the mismatched growth. A physics-based device simulator was used to create an inverted triple-junction GaInP/GaAs/GaSb model. The model predicted that, with current GaSb-on-GaAs material quality, the not-current-matched, proof-of-concept cell would provide 0.5% absolute efficiency gain over a tandem GaInP/GaAs cell at 1 sun and 2.5% gain at 44 suns, indicating that the effectiveness of the GaSb junction was a function of concentration

Modifying Different Components of Distillers Grains and the Impact on Feedlot Performance

A finishing study evaluated the effects of altering distillers grains composition on feedlot performance and carcass characteristics. Replacing dried distillers grains with isolated bran, solubles, and protein did not result in performance similar to commodity dried distillers grains. Exchanging bran for non- pelleted treated corn stover increased intake, reduced efficiency, and decreased 12th rib fat. Cattle fed pelleted treated corn stover had decreased intakes, but similar efficiency and gain as non- pelleted treated corn stover. As solubles increased and replaced protein, intakes increased quadratically and 12th rib fat linearly decreased however, all other performance and carcass characteristics were not different