Optimization and Characterization of Low-Cost Substrates for III-V Photovoltaics

The highest achieved photovoltaic power conversion efficiency (approximately 47% under concentration) is available from III-V multijunction solar cells made from subcells of descending bandgap that optimize light collection from the solar spectrum. Unfortunately, both III-V multijunction and single-junction solar cells are expensive, limiting their use to niche concentration or space power applications and precluding their competitiveness in the terrestrial flat-plate market. The majority of the III-V solar cell cost is attributed to the thick, monocrystalline substrates that are used as a platform for epitaxial growth, and to the throughput, precursors, and utilization of those precursors associated with traditional growth reactors. Significant cost reduction to approach $1/W for total photovoltaic system cost is imperative to realize III-V solar cells that are cost-competitive with incumbent silicon solar cells, and can include techniques to develop inexpensive substrates directly; enable multiple reuses of a pristine, expensive substrate without the need for polishing; and enhance the throughput by increasing the semiconductor growth rate during epitaxy. This dissertation explores two main techniques to achieve low-cost substrates for III-V photovoltaics: aluminum-induced crystallization to create polycrystalline germanium thin films, and remote epitaxy through graphene to enable monocrystalline substrate reuse without polishing. This dissertation also demonstrates record III-V growth rates exceeding 0.5 mm/h using a potentially lower-cost III-V growth technique, which would increase throughput in production reactors. The ability to reduce the costs associated with both substrates and epitaxy will be imperative to decreasing the total system cost of III-V PV