Multilayer-Grown Ultrathin Nanostructured GaAs Solar Cells as a Cost-Competitive Materials Platform for III−V Photovoltaics Boju Gai,† Yukun Sun,§,∥ Haneol Lim,† Huandong Chen,† Joseph Faucher,§ Minjoo L. Lee,*,§,∥ and Jongseung Yoon*,†,‡ †
Department of Chemical Engineering and Materials Science and ‡Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089, United States § Department of Electrical Engineering, Yale University, New Haven, Connecticut 06520, United States ∥ Department of Electrical and Computer Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States S Supporting Information *
ABSTRACT: Large-scale deployment of GaAs solar cells in terrestrial photovoltaics demands significant cost reduction for preparing device-quality epitaxial materials. Although multilayer epitaxial growth in conjunction with printingbased materials assemblies has been proposed as a promising route to achieve this goal, their practical implementation remains challenging owing to the degradation of materials properties and resulting nonuniform device performance between solar cells grown in different sequences. Here we report an alternative approach to circumvent these limitations and enable multilayer-grown GaAs solar cells with uniform photovoltaic performance. Ultrathin single-junction GaAs solar cells having a 300-nm-thick absorber (i.e., emitter and base) are epitaxially grown in triple-stack releasable multilayer assemblies by molecular beam epitaxy using beryllium as a p-type impurity. Microscale (∼500 × 500 μm2) GaAs solar cells fabricated from respective device layers exhibit excellent uniformity (100) of substrate reclaims may not be strictly necessary for driving the cost reduction.9,10 However, the practical implementation of this idea in GaAs photovoltaics has been hampered by difficulties in preserving
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espite well-suited materials properties and the recordhigh efficiency approaching their theoretical limit, practical application of single-junction gallium arsenide (GaAs) solar cells in terrestrial photovoltaics has been largely precluded due to excessively high materials costs for growing device-quality epitaxial materials on an expensive native substrate.1−3 While epitaxial liftoff (ELO) has been proposed as a promising concept to potentially realize significant reduction of the cell cost by reusing the growth substrate,4−7 restoring an epi-ready condition of the used wafer and repeatedly performing epitaxial growth naturally involve many processing steps and thermal cycles, leading to the accumulation of crystalline defects and deterioration of © 2017 American Chemical Society
Received: November 10, 2016 Accepted: January 3, 2017 Published: January 11, 2017 992
DOI: 10.1021/acsnano.6b07605 ACS Nano 2017, 11, 992−999
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Figure 1. (a) Schematic illustration of the fabrication procedures for triple-stack ultrathin GaAs solar cells configured with a vertical-contact configuration. (b) Optical micrographs of arrays of GaAs microcells at various processing steps; after the n-type metal deposition (upper left), isolation of microcells (upper right), p-type metal deposition after the pick-up (lower left), and printing (lower right). (c) Photographic image of 10 × 10 arrays of vertical contact GaAs solar microcells printed on a glass substrate after the top-contact removal and bottom-contact exposure. Inset shows a magnified view.
performance of such ultrathin GaAs solar cells, when combined with optimized schemes of photon management, can be significantly enhanced to a level close to optically thick absorbers, thereby further improving the cost-effectiveness obtained from the reduced materials utilization.18−20 Motivated by these advances, herein we present a cost-competitive materials platform for multilayer-grown GaAs solar cells that can circumvent prior challenges and achieve uniform (
