The further evolution of PV power generation is an important issue of global energy policy. According to ‘NEDO PV Challenges’ , the target of PV electricity at 2030 is to achieve both efficiency of >25% and power generation cost of <0.1 $/kWh. Against this background, heterogeneous multi-junction (MJ) solar cell is one of the strategies to realize this target. The mechanical stacking, as fabrication method of MJ solar cells, enables high efficiency and low-cost because of flexible and most appropriate combination of different cells. In here, GaAs//Si and GaAs//CuInGaSe MJ solar cell are the most realistic combination. GaAs-based top cell essentially has high efficiency. Si and CuInGaSe bottom cells have high sensitivity for long wavelength region and hold the promise of low cost bottom cell. According to our theoretical prediction, these cells have a potential to reach more than 30% efficiency.
In this paper, we show the way to practical use of GaAs//Si and GaAs//CuInGaSe MJ solar cells with mechanical stacking method. Our key technology is a powerful bonding method using Pd nano-particle alignment, which is named “Smart Stack” technology -. Using this method, InGaP/GaAs//Si and InGaP/GaAs//CuInGaSe 3-junction solar cells were fabricated. It was revealed that the total efficiency at 1 sun (AM1.5g) was 25.1% and 24.2% for GaAs//Si and GaAs//CuInGaSe solar cell, respectively. And also, we examined the low concentration performance for each solar cell. Low concentration system contributes to the expansion of the theoretical limit of conversion efficiency and the cost reduction of high expensive GaAs cell. As a result, we obtained the maximum efficiency of 23.7% at 8.0 suns and 25.3% at 5.7 suns for GaAs//Si and GaAs//CuInGaSe solar cell, respectively. This performance is acceptable level for low concentration system below 10 suns. In addition, we have developed the technology for practical application. As the mass productive mounting method, we have developed “Individual Transport Technology”, namely Epitaxial Lift-off (ELO) GaAs top cells prepared using Supercritical Rinse & Dryer equipment are transported and bonded to Si bottom cell individually and freely. Using this method, the large area of solar cell enables becoming it. On the other hand, we attempted the initial reliability verification with accelerated aging test and thermal cycling test. For GaAs//Si MJ solar cells, long-term stability was confirmed. Appling the general lifetime estimation method with Arrhenius plot, the life time was estimated to be more than 1.5×105 hours at 60°C. These results strongly suggested the high potential of MJ solar cells with Smart Stack technology for practical use. Reference:  www.nedo.go.jp/news/press/AA5_100318.html.  H. Mizuno, et. al., Appl. Phys. Lett., 101, 191111 (2012). H. Mizuno, et. al., Appl. Phys. Express, 10, 072301(2017).  K. Makita, et. al., 33rd EU PVSEC, 1AO.3.1 (2017).