Monolithic Two-Terminal III-V//Si Triple-Junction Solar Cells With 30.2% Efficiency Under 1-Sun AM1.5g

Stacking III-V p-n junctions on top of waferbased silicon solar cells is a promising way to go beyond the silicon single-junction efficiency limit. In this study, triplejunction GaInP/AlxGa1-x As//Si solar cells were fabricated using surface-activated direct wafer bonding. Metal-organic-vaporphase- epitaxy-grown GaInP/AlxGa1-xAs top cells are bonded at low temperature to independently prepared wafer-based silicon cells. n-Si//n-GaAs interfaces were investigated and achieved bulk-like bond strength, high transparency, and conductivity homogeneously over 4-inch wafer area. We used transfer-matrix optical modeling to identify the best design options to reach current-matched two-terminal devices with different mid-cell bandgaps (1.42, 1.47, and 1.52 eV). Solar cells were fabricated accordingly and calibrated under AM1.5g 1-sun conditions. An improved Si back-side passivation process is presented, leading to a current density of 12.4 mA/cm(2) (AM1.5g), measured for a flat Si cell below GaAs. The best 4 cm(2) GaInP/GaAs//Si triple-junction cell reaches 30.2% 1-sun efficiency.