High-Irradiance Degradation Studies of Metamorphic 1eV GaInAs Solar Cells
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High-Irradiance Degradation Studies of Metamorphic 1eV GaInAs Solar Cells Ryan M. France and Myles A. Steiner National Renewable Energy Laboratory, Golden, Colorado 80401 ABSTRACT Initial tests are performed regarding the degradation of lattice-mismatched GaInAs solar cells. 1eV metamorphic GaInAs solar cells with 1-2×106 cm-2 threading dislocation density in the active region are irradiated with an 808 nm laser for 2 weeks time under a variety of temperature and illumination conditions. All devices show a small degradation in Voc that is logarithmic with time. The absolute loss in performance after 2 weeks illuminated at 1300 suns equivalent and 125°C is 7 mV Voc and 0.2% efficiency, showing these devices to be relatively stable. The dark current increases with time and is analyzed with a two-diode model. A GaAs control cell degrades at the same rate, suggesting that the observed degradation mechanism is not related to the additional dislocations in the GaInAs devices. INTRODUCTION The bandgap combination of multijunction solar cells determines the theoretical efficiency at a given operating condition. The ideal bandgap combination can be achieved by utilizing lattice-mismatched subcells, such as in the inverted metamorphic solar cell (IMM). IMM solar cells contain one1 or two2 lattice-mismatched junctions and have proven to be capable of very high efficiencies.3 The lattice constant is altered by intentionally introducing dislocations into an inactive region of the device. Dislocation glide is encouraged, which maximizes the amount of lattice constant change per dislocation and confines the majority of dislocations to the inactive buffer region, termed the compositionally graded buffer (CGB). However, some residual threading dislocation density in the active region is unavoidable. These dislocations could potentially lower the performance and reliability of the multijunction device, so specific studies on the performance and reliability of these cells is necessary. The ideal bandgap of the bottom cell in a triple junction IMM solar cell is approximately 1 eV. For a strain-free lattice-mismatched 1eV GaInAs subcell on a GaAs substrate, the metamorphic buffer must contain a misfit dislocation density (dislocations parallel to the growth surface) of 1 x 106 cm-1, distributed over the thickness of the buffer. High quality mismatched growth has been shown to limit the associated threading dislocation density in the active region of these cells to 1-2 x 106 cm-2. At this density, the subcell performs extremely well, achieving an open-circuit voltage (Voc) over 0.62 V with near-unity quantum efficiency.4 Therefore, the performance of lattice-mismatched solar cells is not significantly lower than the performance of lattice-matched solar cells. In this paper, we perform initial experiments regarding the reliability of high performance lattice-mismatched 1eV GaInAs solar cells under operating conditions of high illumination and elevated temperature. The experiment is designed to specifically focus on possible degradation me
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