Metamorphic epitaxy for multijunction solar cells
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Introduction Multijunction solar cells combine multiple semiconducting materials into a single device to achieve efficiencies well beyond what is possible with traditional single-junction devices. While single-junction GaAs solar cells have achieved solarto-electric conversion efficiencies of 29%, efficiencies up to 46% have already been demonstrated using four-junction devices under concentrated sunlight illumination.1,2 Currently, multijunction cells are primarily used by the space industry for power generation on satellites or spacecraft, where the high efficiency lowers the required weight of the solar array for a given power. These cells are also used for terrestrial power generation in concentrator photovoltaic systems, where the amount of solar cell material is reduced by the concentration factor (the ratio of illuminated area to cell area), and the concentration improves device efficiency due to a logarithmic increase in the voltage with light intensity. The performance increase under concentrated light, combined with the natural high performance of multijunction solar cells, has led to the highest reported photovoltaic efficiencies of any technology.3 While many multijunction solar cell designs exist, such as wafer-bonded2 or printed approaches,4 this article focuses on metamorphic multijunction solar cells. Metamorphic epitaxy provides access to a wide selection of III–V compounds as potential absorber materials, enabling an optimal bandgap
combination within a multijunction solar cell. However, metamorphic solar cells contain dislocations that can significantly reduce the solar cell performance if their density is not carefully controlled. We discuss the benefits and drawbacks of metamorphic materials for photovoltaic applications, describe the materials science of metamorphic buffers for multijunction solar cells, and demonstrate the designs and performance of different metamorphic multijunction approaches.
Background Motivation for metamorphic multijunction solar cells The efficiency advantage of multijunction solar cells over singlejunction solar cells is the reduction of losses due to carrier thermalization and photon transmission.5–8 An incoming photon with energy above the semiconductor bandgap (i.e., hν ≥ Eg, where h is Planck’s constant, ν is the photon frequency, and Eg is the bandgap energy) is absorbed and generates an electron– hole pair that may contribute to the device current. The excess energy of the photon above the bandgap is lost as heat as the photogenerated electron–hole pair quickly thermalizes to the semiconductor band edge, and photons with energy less than the bandgap are not absorbed. In a properly designed multijunction device, each subcell receives a portion of the solar spectrum with energy only slightly greater than its bandgap,
Ryan M. France, National Renewable Energy Laboratory, USA; [email protected] Frank Dimroth, Department III–V Epitaxy and Solar Cells, Fraunhofer Institute for Solar Energy Systems ISE, Germany; [email protected] Tyler J. Grassman, Departme
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