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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 in the wide-scale deployment of solar photovoltaic systems. Even though multijunction solar cells have the highest reported efficiencies and have been commercialized for space and concentrator applications, they are not used widely because of their high cost. Our objective is to develop advanced multijunction technologies with extremely low cost and high conversion efficiency.

 

Strategy

Multijunction solar cells provide ultra-high efficiencies by the effectively using the solar spectrum through the interconnection of different kinds of solar cells. A conventional fabrication method is a monolithic epitaxial growth technique, which is considerably difficult and limits the selection of materials and cell combinations. We proposed a novel semiconductor bonding technology (smart stacking technology) to stack different kinds of solar cells with high flexibility. We have also developed hydride vapor phase epitaxy (H-VPE) system to grow extremely low-cost III-V compound semiconductor materials.

 

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. 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
  Figure.1 Schematic drawing of smart stack technology and I-V characteristics of a 4-junction solar cell.           

figure 1

  Figure.2 H-VPE  system and I-V curves of GaAs solar cells.




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