The Japan Industrial Technology Research Institute (AIST) announced on June 24, 2014, during its "AIST Photovoltaic Power Generation Research Report 2014," the development of a groundbreaking technology called "Smart Stack." This innovation allows for the bonding of different types of semiconductor materials, specifically the pn junctions used in solar cells. By leveraging this technology, AIST has made it possible to stack III-V type pn junctions onto both silicon and CIGS-based solar cells, enabling the production of high-efficiency photovoltaic devices at a significantly lower cost.
This advancement is part of the AIST Photovoltaic Power Engineering Research Center's efforts to push the boundaries of multi-junction solar cell design. The team successfully created a power generation element by stacking GaAs and GaInP double-junction solar cells on a CIGS cell, achieving a remarkable conversion efficiency of 24.2%.
Traditionally, creating multi-junction solar cells with more than four layers has been challenging due to differences in lattice constants between materials. Conventional crystal growth methods often fail to accommodate these mismatches. To address this, the research team developed the "Mechanical Stack" technique, which avoids crystal growth by physically bonding pre-fabricated cells. AIST’s Smart Stack technology is an evolution of this approach.
What sets Smart Stack apart is the use of palladium nanoparticles—50 nm in diameter—arranged at a density of 1×10¹Ⱐper cm² on the bonding surface. Unlike mechanical stacking, which requires precise surface treatments such as electron beam or plasma processing, Smart Stack reduces the need for such intensive preparation. It also relaxes the requirement for surface flatness from less than 1 nm to approximately 10 nm, making the process more efficient and scalable.
The process involves using a self-organizing polymer like polystyrene to arrange the palladium nanoparticles at a 100 nm pitch. Afterward, the polymer is removed through plasma treatment. The top cell, which has been fabricated separately, is then bonded to the bottom cell using a weight or pressure bonding method.
In practical trials, two types of solar cells were produced: one was a four-junction cell consisting of GaInP, GaAs, InGaAsP, and InGaAs, while the other was a three-junction cell combining GaInP, GaAs, and CIGS. The four-junction cell achieved a conversion efficiency of 30.4% without light concentration, with each cell measuring about 5 mm in size. The three-junction cell reached 24.2%, which, according to reports, is the highest efficiency recorded for that specific combination.
Importantly, the GaAs substrate used in the top cell can be recycled after peeling, making the three-junction design particularly cost-effective. This allows for high efficiency while maintaining the low-cost advantages of CIGS technology.
Looking ahead, AIST aims to refine the substrate removal process and improve adhesion techniques for larger cell sizes. These improvements will be crucial for scaling up the technology for commercial applications. (Reported by Nozawa Tetsuru, Nikkei Electronics)
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