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. This technique is considerably attractive for interconnecting different kinds of solar cells. We have demonstrated an InGaP/GaAs//InGaAsP/InGaAs (// indicates a smart stack interface) 4-junction solar cell with a high efficiency of 33.1% under 1 sun, as shown in Fig. 1. We also demonstrated an InGaP/GaAs//CIGS and InGaP/GaAs//Si 3-junction solar cells with efficiencies of 24.2 (under 1 sun) and 28.7% (×10 concentration), respectively. Our stacking method is highly useful to realize the ultra-high efficiency multijunction solar cells combined with III-V top and low-cost Si or CIGS bottom cells. The most important issue for the cost reduction is to grow low-cost III-V materials. We have newly developed a low-cost H-VPE system and fabricated a GaAs solar cell with an efficiency of 20.8%, as shown in Fig. 2. The next generation multijunction solar cells with a high efficiency and a low cost can be realized by using smart stacking technologies and H-VPE systems.
Figure.1 Schematic drawing of smart stack technology and I-V characteristics of a 4-junction solar cell.
Figure.2 H-VPE system and I-V curves of GaAs solar cells.