Band-Gap-Engineered Architectures for High-Efficiency Multijunction Concentrator Solar Cells

R. R. King, A. Boca, W. Hong, X.-Q. Liu, D. Bhusari, D. Larrabee, K. M. Edmondson, D. C. Law, C. M. Fetzer, S. Mesropian, and N. H. Karam Spectrolab, Inc., 12500 Gladstone Ave., Sylmar, CA 91342 USA ABSTRACT: Beginning with maximum theoretical efficiencies from detailed balance calculations, we evaluate the real-world energy loss mechanisms in a variety of high-efficiency multijunction cell architectures such as inverted metamorphic 3- and 4-junction cells, as a step toward closing the gap between theory and experiment. Experimental results are given on band-gap-engineered lattice-matched and metamorphic 3-junction cells, and on 4-junction terrestrial concentrator cells. A new world record 41.6%-efficient solar cell is presented, the highest efficiency yet demonstrated for any type of solar cell. Keywords: III-V Semiconductors, Concentrator Cells, High-Efficiency, Multijunction Solar Cell, Gallium Arsenide Based Cells, Lattice-Mismatched, Metamorphic 1 INTRODUCTION AR Multijunction solar cells for concentrator photovoltaic (CPV) systems can have over 70% theoretical efficiency. The key question for terrestrial electricity generation, however, is "how much of this efficiency potential can be realized in practical solar cells and converted to kilowatt-hours at the electric meter?" Experimental III-V multijunction solar cells have demonstrated efficiencies over 40% since 2006 [1], and are the highest efficiency PV technology known presently. But various mechanisms analyzed below still result in a chasm between the efficiencies promised from theoretical calculations, and those reached in practice. Beginning with maximum theoretical efficiencies from detailed balance calculations, we evaluate the real-world energy loss mechanisms in a variety of high-efficiency multijunction cell architectures such as inverted metamorphic 3- and 4-junction cells, as a step toward closing the gap between theory and experiment. Experimental results are given on band-gap-engineered lattice-matched and metamorphic 3-junction cells, and on 4-junction terrestrial concentrator cells. A new world record 41.6%-efficient solar cell is presented, the highest efficiency yet demonstrated for any type of solar cell. This experimental Spectrolab concentrator cell, with a III-V lattice-matched 3-junction design and reduced metal coverage fraction, has been independently verified at NREL under standard test conditions (25°C, AM1.5D, ASTM G173-03 spectrum), at 364 suns (36.4 W/cm