Highly-Efficient III-V // Si Wafer-Bonded Triple-Junction Solar Cells

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In this paper triple junctions GaInP/AlGaAs//c-Si were fabricated usinf surface activated direct wafer bonding. The GaInP/AlGaAs top cells are grown by MOVPE and then bonded at low temperature (120 °C – exposition to an argon fast atom beam + surface deoxidization using Ar atoms – mechanical pressure with force of 10 kN) to independently prepared silicon cells using n-Si//n-GaAs bond. To ensure good bond mechanical quality roughness of the surface was reduced by mechanical polishing and special care was also given to avoid any particulate deposition on the surface.

The n-GaAs//n-Si interface is especially important. Low specific resistance and ohmic characteristic are achieved (< 3 mOhm.cm²) thanks to high doping (NSi=5x1019 cm-3 & NGaAs=1x1019 cm-3) even with the presence of a thin amorphous layer (2-3 nm) at the surface.

New world record of 26.2% efficiency on 4 cm² was obtained using Al0.035Ga0.965As middle cell for a monolithic tandem cell on c-Si bottom cell. However, the very harsh conditions needed to create the bonding for this cell make really difficult the implementation of such a device in high throughput mass production, even more on bottom wafers which are not mirror polished (not the standard in c-Si PV industry).

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Wafer-bonded III-V / Si multi-junction solar cell with 30.2 percent efficiency. ©Fraunhofer ISE/A. Wekkeli