III-V Multi-Junction Materials and Solar Cells on Engineered SiGe/Si Substrates
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III-V Multi-Junction Materials and Solar Cells on Engineered SiGe/Si Substrates Steven A. Ringel,1 Carrie L. Andre,1 Matthew Lueck,1 David Isaacson,2 Arthur J. Pitera,2 Eugene A. Fitzgerald2 and David M. Wilt3 1 Department of Electrical and Computer Engineering The Ohio State University 2015 Neil Avenue, Columbus, OH 43210 USA 2 Department of Materials Science and Engineering Massachusetts Institute of Technology 77 Massachusetts Avenue, Cambridge, MA 02139 USA 3 Photovoltaic and Space Environments Branch NASA Glenn Research Center 21000 Brook Park Road, Cleveland, Ohio 44135 USA ABSTRACT The monolithic integration of high efficiency III-V compound solar cell materials and devices with lower-cost, robust and scaleable Si substrates has been a driving force in photovoltaics (PV) basic research for decades. Recent advances in controlling mismatchinduced defects that result from structural and chemical differences between III-V solar cell materials and Si using a combination of SiGe interlayers and monolayer-scale control of III-V/IV interfaces, have led to a series of fundamental advances at the material and device levels, which establish that the great potential of III-V/Si PV is within reach. These include demonstrations of GaAs epitaxial layers on Si that are anti-phase domain-free with verified dislocation densities at or below 1x106 cm-2 and negligible interface diffusion, minority carrier lifetimes for GaAs on Si in excess of 10 ns, single junction GaAs-based solar cells on Si with open circuit voltages (Voc) in excess of 980 mV, efficiencies beyond 18%, and area-independent PV characteristics up to at least 4 cm2. These advances are attributed in large part to the use of a novel “engineered Si substrate” based on compositionally-graded SiGe buffers such that a high-quality, low defect density, relaxed, “virtual” Ge substrate could be developed that can support lattice-matched III-V epitaxy and thus merge III-V technology based on the GaAs (or Ge) lattice constant with Si wafers. This paper focuses on recent results that extend this work to the first demonstration of high performance III-V dual junction solar cells on SiGe/Si. Open circuit voltages in excess of 2 V at one-sun have been obtained for the conventionally “lattice-matched” In0.49Ga0.51P/GaAs dual junction cells on inactive, engineered SiGe/Si; to our knowledge is the first demonstration of > 2V solar power generation on a Si wafer. Comparisons with identical cells on GaAs substrates reveal that the Voc on engineered Si retains more than 94% of its homoepitaxial value, and that at present both DJ/GaAs and DJ/SiGe/Si cells are similarly limited by current mismatch in these early cells, and not fundamental defect factors associated with the engineered Si substrates. INTRODUCTION Recent work using relaxed buffer layers consisting of step-graded SiGe has shown enormous potential for achieving large area, low-cost direct integration of III-V compound
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semiconductor photovoltaics based on the GaAs lattice constant [1], and III-V optoelectro
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