Stacking III-V p-n junctions on top of wafer based silicon solar cells is a promising way to go beyond the silicon single-junction efficiency limit. Indeed, realistic conversion efficiencies beyond 35% under 1 sun AM1.5G conditions can be expected for a triple-junction made of GaInP/AlGaAs//Si with ~ 4µm III-V materials on top of Si wafer. In our approach, triple-junctions GaInP/AlGaAs//Si solar cells were fabricated using surface activated direct wafer bonding. MOVPE grown GaInP/AlGaAs top cells were bonded at low temperature to independently prepared wafer based silicon cells. n-Si/n-GaAs bond interfaces with bulk-like bond strength, high transparency and conductivity were achieved. Moreover, by optimizing the process flow, it was possible to reach fully bonded and defect-free 4-inch Si/GaAs wafer pairs. We have used transfer matrix optical modeling to identify best design options to reach current-matched with different mid-cell bandgaps (1.47 and 1.52eV). Solar cells were characterized under calibrated AM1.5G 1-sun conditions. The best 2-terminal 4cm2 cell reaches efficiency beyond 30%.
Ref. Cariou, R. et al., Monolithic Two-Terminal III-V//Si Triple-Junction Solar Cells with 30.2% Efficiency Under 1-Sun AM1.5G. IEEE J. Photovolt. 7, 367-373 (2017).