We fabricate semiconductor//metal grid/semiconductor junctions by using surface activated bonding (SAB) of heavily-doped Si and GaAs substrates to metal grids. The metal grids are self-aligned to SiO2 layers on other Si substrates. The current–voltage characteristics of these junctions show linear properties. A low junction resistance of 1.99 mΩ cm2 is achieved for an n+-GaAs//metal grid/n+-Si junction by successively annealing the junction at 300 °C for 1 h and 400 °C for 1 min in N2 gas ambient. This value is much smaller than an interface resistance of SAB-based GaAs//ITO/Si junctions. These results demonstrate that metal grids could be useful for bonding subcells with low parasitic resistances in fabricating hybrid multijunction solar cells by SAB.

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Using surface-activated bonding technologies, we bond InGaP/GaAs double-junction (2J) solar-cell structures invertedly-grown on GaAs substrates to metal grids formed on conductive Si substrates. The metal grids are made of 170-nm SiO2 layers and 310-nm Ti/Au metal layers. 2J cells are fabricated by eliminating the GaAs substrates and forming emitter and base contacts. The emitter contacts are aligned to the metal grids. The fabricated 2J cells reveal a lower series resistance in comparison with cells directly bonded to Si substrates, which is likely to be due to a lower resistance across GaAs//metal grid junctions than that across GaAs//Si junctions. A higher open-circuit voltage and a larger internal quantum efficiency are also observed for the 2J cells on metal grids. These features as well as the oscillation in reflectance spectrum suggest that the 2J cells work as optically-isolated thin-film cells.