Mechanical stacked GaAs//CuIn1-yGaySe2 three-junction solar cells with 30% efficiency via an improved bonding interface and area current-matching technique

Multijunction (MJ) solar cells have attracted considerable attention as next-generation solar cells. III-V-based MJ solar cells connected to heterogeneous cells, such as GaAs//Si and GaAs//CIGSe (CuIn1-yGaySe2), are expected to be highly efficient and inexpensive. Consequently, we have developed a mechanical stacking method with Pd nanoparticles and silicone adhesives for the bonding interface, which was modified for our previous "smart stack" technology. Using this "modified smart stack," we previously demonstrated an InGaP/Al0.06Ga0.94As//CIGSe three-junction solar cell with an efficiency of 28.06%. In this study, we fabricated an InGaP/GaAs//CIGSe three-junction solar cell. The total efficiency is 29.3% for the aperture area (31.0% for the active area) under AM1.5G solar spectrum illumination, which is the optimal value reported for two-terminal GaAs//CIGSe-based tandem solar cells thus far. This superior performance was realized by using a specialized CIGSe cell with a flattened surface via a wet etching process and a thin In2O3;Ce,H transparent conducting oxide layer. Using an area current-matching technique, the efficiency increased to 30%. In addition, the bonding resistivity of the fabricated solar cell was estimated to be less than 1 omega cm(2) from solar-concentrated measurements. These results suggest the potential of III-V//CIGSe-based tandem solar cells with modified smart stack technology as next-generation photovoltaic cells for applications such as vehicle-integrated photovoltaics.