High efficiency three terminal GaAsP/SiGe dual junction solar Cell on silicon substrate

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Embargoed until 2018-02-28
Copyright: Wang, Li
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Abstract
III-V solar cells are well known for the world record efficiencies in photovoltaic field. Integrating III-V solar cells on silicon (Si) substrates can lead to a significant increase in efficiency over the present best monocrystalline Si laboratory device. The challenge is the lattice-mismatch between III-V material and Si. A leveraging technology of metamorphic silicon: germanium (SiGe) buffer between the Si and the III-V material provides a low-dislocation interface for III-V nucleation. The SiGe layer may then serve as a high-quality current-matched bottom cell to form a tandem dual-junction structure. A GaAsP cell with optimum material composition was selected as the top solar cell, due to its contribution to a lattice-matched GaAsP/SiGe tandem device on Si with 40% theoretical efficiency. This research first describes the design, fabrication, analysis and improvement of the GaAsP top solar cell of various generations. GaAsP top cell efficiencies have been improved from 8.4% to 18.4%, with bandgap voltage offsets (Woc) of 0.48 V and 0.31 V under concentration factors of 1X and 20X, respectively. This progress is made by improved III-V material quality, reduced series resistance and addition of an anti-reflection coating. Next, the progress of a three-terminal (3T) GaAsP/SiGe tandem solar cell on Si is demonstrated. A corrected three-terminal tandem efficiency of 20.6% under 1X illumination is achieved, a relative improvement of approximately 9% compared to the 18.9% efficiency reported previously. The difference between short circuit current measured outdoor and indoor leads to an approximate 1.5% relative difference in 3T tandem efficiencies. Current matching is realized under 2.1X illuminations. These are achieved by the improved tandem structure, optimized fabrication process, adjustable light source, and effect from the bottom cell area. Pathways to achieve the realistic efficiency of over 29% under 1X illumination and over 32% under 20X illuminations are also provided. Finally, the GaAsP solar cell has been characterized by photoluminescence (PL) and electroluminescence (EL) techniques. The band-to-band absorption coefficient and the external quantum efficiency (EQE) near the band edge have been extracted. The wavelength dependent PL spectra emitted from various III-V film stacks is also analyzed and discussed.
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Author(s)
Wang, Li
Supervisor(s)
Barnett, Allen
Perez-Wurfl, Ivan
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Publication Year
2017
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Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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