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