Researchers led by Japan's National Institute of Advanced Industrial Science and Technology (AIST) have fabricated a three-junction solar cell based on indium gallium phosphide (InGaP), gallium arsenide (GaAS) and copper, indium, gallium and selenium (CIGS) with a mechanical stacked design.
“We are currently increasing our efforts to improve the cell efficiency and the development of the mass production technology,” researcher Kikuo Makita told pv magazine, noting that this kind of cell has the potential to achieve efficiencies close to 35%.
The scientists built the cell with a two-junction InGap-GaAs upper cell with a bandgap of 1.49 eV, based on a rear-emitter heterojunction structure developed by Japanese manufacturer Sharp, and a CIGS bottom device with a bandgap of 1.01 eV, with improved surface roughness. They connected the cells through a modified smart stack with palladium (Pd) nanoparticles and adhesive.
The research group improved the bottom cell's surface via wet etching. They used a bromine-based solution and modified its thin transparent conducting oxide (TCO) layer.
“Surface roughness leads to an increase in the gap width at the bonding interface,” they explained, noting that this roughness, combined with the TCO thickness may lead to reflection loss. “Therefore, in this study, we focused on minimizing surface roughness and TCO thickness.”
The academics tested the performance of the cell under standard illumination conditions. They found it achieved a power conversion efficiency of 29.3% for the aperture area (31.0% for the active area), an open-circuit voltage of 2.97 V, a short-circuit current density of 12.41 mA/cm2, and a fill factor of 0.80.
They said the obtained efficiency of 29.3% is superior to that of the group's previous results. They claimed it was the highest value ever reported for any two-terminal GaAs.CIGSe-based multijunction solar cell.
“We examined the costs of the cells using Smart stack technology and, according to our simulation, they may result in a final module cost of US$ 2/W,” Makita said. “GaAs cell cost, CIGSe cell cost, bonding cost, and modulization cost are 86%, 7%, 3%, and 4%, respectively.
The GaAs cell, especially the GaAs substrate and GaAs epi-growth, is the main factor affecting device-fabrication costs.
“In our project, device epitaxial lift-off (ELO) and substrate reuse techniques are studied to reduce the GaAs substrates costs,” Makite said. “In addition, the AIST has developed a hydride vapor phase epitaxy (H-VPE), which is a new growth method for GaAs cells. H-VPE is a low-cost technique compared to the general metal-organic chemical vapor deposition (MOCVD) technique. We think that the development of these fabrication technologies contributes to the cost reduction of expensive GaAs cells.”
The researchers presented the cell design in “Mechanical stacked GaAs//CuIn1−yGaySe2 three-junction solar cells with 30% efficiency via an improved bonding interface and area current-matching technique,” which was recently published in Progress in Photovoltaics. The cost of producing solar cells based on compounds of III-V element materials – named according to the groups of the periodic table that they belong to – has confined such devices to niche applications, including drones and satellites, where low weight and high efficiency are more pressing concerns than costs in relation to the energy produced.
Fraunhofer ISE researchers recently achieved a 35.9% conversion efficiency for a III-V monolithic triple-junction solar cell based on silicon. In August 2020, the research institute announced a 25.9% conversion efficiency rate for a III-V tandem solar cell grown directly on silicon. This cell is a modified version of a 34.5%-efficient III-V solar cell that is manufactured through a process known as direct wafer bonding, where the III-V layers are first deposited on an aluminum gallium arsenide (GaAs) substrate and then pressed together.
Researchers at Tampere University in Finland recently developed a III-V multi-junction solar cell that purportedly has the potential to reach a power conversion efficiency of close to 50%. The National Renewable Energy Laboratory (NREL) in the United States announced a 32.9% efficiency for a tandem cell device utilizing III-V materials last year.
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