The technology was developed by the Advanced Multijunction Devices Group at the AIST Photovoltaic Engineering Research Center. This group fabricated a power generation device that stacks GaAs and GaInP dual-junction solar cells on CIGS-type solar cells, confirming a conversion efficiency of 24.2%.
Multi-junction solar cells with four or more junctions are difficult to fabricate using conventional crystal growth techniques due to differences in lattice constants and other factors. Therefore, the research team began developing a "MechanicalStack" technique that does not utilize crystal growth but instead physically bonds separately fabricated cell units.
The SmartStack technology developed by the American Institute in Taiwan (AIST) also falls into this category. The biggest difference between SmartStack and MechanicalStack is that SmartStack uses palladium particles with a diameter of 50nm arranged at a density of 1×10¹⁰ particles/cm² on the bonding surface. As a result, there is no need for surface treatment of the bonding surface for electron beam and plasma bonding as required by previous MechanicalStack technologies, and the required surface flatness is significantly reduced from below 1nm to around 10nm.
Specifically, the method utilizes the self-organization phenomenon of polymer materials such as polystyrene to arrange palladium nanoparticles at approximately equal intervals of 100 nm on the bottom battery cell, and then removes the polymer materials through plasma treatment.
Next, the substrate of the top battery unit is peeled off and bonded to the bottom battery unit using a weighted bonding method, i.e., pressure bonding.
Two types of solar cells were successfully tested. One is a GaInP, GaAs, InGaAsP, InGaAs quad-junction solar cell, and the other is a GaInP, GaAs, CIGS triple-junction solar cell.
The bottom cell of the quad-junction solar cell is an InGaAs/InGaAsP double-junction solar cell fabricated on an InP substrate, while the top cell is a GaAs/GaInP double-junction solar cell fabricated on a GaAs substrate. The two cells are bonded together after the GaAs substrate is peeled off. The solar cell has a conversion efficiency of 30.4% without focusing, and the cell size is approximately 5mm square.
The bottom cell of the triple-junction solar cell is a CIGS-type solar cell fabricated on a glass substrate, while the top cell is a GaAs/GaInP double-junction solar cell fabricated on a GaAs substrate. The two cells are bonded together after the GaAs substrate of the top cell is peeled off. The conversion efficiency is 24.2%, which, according to the Advanced Industrial Science Research Institute (AIST), is "the highest in the world among solar cells using this combination."
In these technologies, the GaAs substrate can be reused after being peeled off. Therefore, the latter, especially the triple-junction solar cell, can achieve high conversion efficiency while reducing the price to the same level as low-cost CIGS solar cells.
The Institute for Advanced Industrial Science and Technology (AIST) stated that future research will focus on "optimizing substrate peeling technology and weighted bonding methods for larger battery cell sizes."
