Japan develops multi-junction solar cell technology with a conversion efficiency of 24.2%

The Japan Industrial Technology Research Institute (AIST) on June 24, 2014 announced at the "AIST Photovoltaic Power Generation Research Report 2014" that it has developed the "Smart Stack" technology that can bond semiconductors of different types. The pn layer of solar cells. With this technology, III-V pn junctions can be stacked on Si-type and CIGS-type solar cells, so that high-efficiency solar cells can be manufactured at low cost.

The development of this technology is the advanced multi-junction device group of the AIST Photovoltaic Power Engineering Research Center. The team produced a power generation element in which GaAs and GaInP double junction solar cells were stacked on a CIGS solar cell, and confirmed that a conversion efficiency of 24.2% was obtained.

Multi-junction solar cells with more than four junctions are difficult to produce using conventional crystal growth techniques because of different lattice constants. Therefore, the research team began to develop the "Mechanical Stack" technology that does not use crystal growth, but rather physically bonds the separately fabricated battery cells (see this site's report).

The research institute's Smart Stack technology also belongs to this kind of technology. The biggest difference between the Smart Stack and the mechanical stack is that palladium particles with a diameter of 50 nm are arranged at a density of 1×10 10 /cm 2 on the adhesive surface. This eliminates the need for surface treatment of the bonding surface of the electron beam and the plasma as with the mechanical stack, and the required surface flatness is also greatly relaxed from 1 nm or less to about 10 nm.

Specifically, by using a self-organizing phenomenon of a polymer material such as polystyrene, palladium nanoparticles are arranged on the bottom cell at a pitch of 100 nm at substantially equal intervals, and then the polymer material is removed by plasma treatment.

Next, the top substrate of the top battery cell pasted thereon is peeled off, and bonded to the bottom cell by a weight bonding method, ie, a pressure bonding method.

The actual trial production of two kinds of solar cells. One is GaInP, GaAs, InGaAsP, InGaAs four-junction solar cells, and the other is GaInP, GaAs, CIGS triple-junction solar cells.

The bottom cell of a four-junction solar cell is an InGaAs and InGaAsP double junction solar cell fabricated on an InP substrate, and the top cell is a GaAs or GaInP double junction solar cell fabricated on a GaAs substrate. After the GaAs substrate was peeled off, two batteries were attached to each other. The solar cell conversion efficiency is 30.4% when the light is not concentrated, and the size of the battery cell is about 5 mm square.

The bottom cell of a three-junction solar cell is a CIGS solar cell fabricated on a glass substrate, and the top cell is a GaAs or GaInP double junction solar cell fabricated on a GaAs substrate. After peeling off the GaAs substrate of the top battery cell, it was fabricated by laminating two batteries. The conversion efficiency is 24.2%. According to the introduction of ZJM, "(conversion efficiency) is the world's highest value in this combination of solar cells."

Among these techniques, the GaAs substrate can be reused after being peeled off. Therefore, especially the latter three-junction solar cell can achieve high conversion efficiency while reducing the price to the same level as low-cost CIGS solar cells.

Production Research Institute said that the future task is to "optimize the substrate stripping technology and the enhanced adhesion method when the cell size is large." (Reporter: Nozawa Tetsuru, Nikkei Electronics)

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