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Research Center for Photovoltaics

 

Smart Stack Device Team ( SSDT )

 

Objective

Cost reduction and improved conversion efficiency are the most important issues facing the wide-scale deployment of solar photovoltaic systems. Although multijunction solar cells have the highest reported efficiencies and have been commercialized for space and concentrator applications, their use has not become widespread due to their high cost. Our objective is to develop advanced multijunction technologies with extremely low cost and high conversion efficiency.

 

Strategy

Multijunction solar cells enable ultra high efficiencies due to the effective utilization of a solar spectrum by interconnecting different kinds of solar cells. A traditional fabrication method is a monolithic epitaxial growth technique, which is very difficult and limits the choice of materials and cell combinations. We have developed a novel semiconductor bonding technology (smart stacking technology) to stack different kinds of solar cells with high flexibility.

 

Research Activities

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
  Figure.1 Schematic drawing of a stacking structure and a photograph of the stacked cell.          


figure 1

  Figure.2 Schematic layer structure of a 4-junction solar cell and its I-V characteristics.




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