Smart Stacking Technology
We have proposed a novel semiconductor bonding technology for mechanically stacked multijunction solar cells by using conductive nanoparticle alignment with low bonding resistances and minimal optical absorption losses, as shown in Fig.1. This technique is very attractive for interconnecting different kinds of solar cells. We have demonstrated an InGaP(Eg-1.89 eV)/GaAs(Eg-1.42 eV)//InGaAsP(Eg-1.05 eV)/InGaAs(Eg-0.75 eV) 4-junction solar cell with a high efficiency of 32.0% under AM1.5, as shown in Fig.2. We also demonstrated an InGaP/GaAs//CIGS and InGaP/GaAs// Si 3-junction solar cells with an efficiency of 24.2 and 23.1%, respectively. These results suggest that our stacking method is highly useful to realize the ultra-high efficiency (over 40%) multi-junction solar cells without concentration and various heterogeneous cell combinations, such as III-V, Si, and chalcogenide semiconductors. Moreover, the cost reduction can be expected because we can use inexpensive substrates for high efficiency III-V solar cells.
Figure.1 Schematic drawing of a stacking structure and a photograph of the stacked cell.
Figure.2 Schematic layer structure of a 4-junction solar cell and its I-V characteristics.