Triple-junction solar cells from III-V compound semiconductors have thus far delivered the highest solar-electric conversion efficiencies. Increasing the number of junctions generally offers the potential to reach even higher efficiencies, but material quality and the choice of bandgap energies turn out to be even more importance than the number of junctions. Several four-junction solar cell architectures with optimum bandgap combination are found for lattice-mismatched III-V semiconductors as high bandgap materials predominantly possess smaller lattice constant than low bandgap materials. Direct wafer bonding offers a new opportunity to combine such mismatched materials through a permanent, electrically conductive and optically transparent interface. In this work, a GaAs-based top tandem solar cell structure was bonded to an InP-based bottom tandem cell with a difference in lattice constant of 3.7%. The result is a GaInP/GaAs//GaInAsP/GaInAs four-junction solar cell with a new record efficiency of 44.7% at 297-times concentration of the AM1.5d (ASTM G173-03) spectrum. This work demonstrates a successful pathway for reaching highest conversion efficiencies with III-V multi-junction solar cells having four and in the future even more junctions.

GaInP/GaAs//Si solar cells with three active p-n junctions were fabricated by surface activated direct wafer bonding between GaAs and Si. The direct wafer bond is performed at room temperature and leads to a conductive and transparent interface. This allows the fabrication of high-efficiency monolithic tandem solar cells with active junctions in both Si and the III-V materials. This technology overcomes earlier challenges of III-V and Si integration caused by the large difference in lattice constant and thermal expansion. Transmission electron microscopy revealed a 5-nm thin amorphous interface layer formed by the argon fast atom beam treatment before bonding. No further defects or voids are detected in the photoactive layers. First triple-junction solar cell devices on Si reached an efficiency of 23.6% under concentrated illumination.

Highly efficient III-V/Si triple-junction solar cells were realized by a fabrication process based on direct wafer bonding: Ga 0 .51 In 0 .49 P/GaAs dual-junction solar cells were grown inverted by metal organic vapor phase epitaxy on GaAs substrates and bonded to separately fabricated Si solar cells. The fast atom beam activated direct wafer bond between highly doped n-Si and n-GaAs enabled a transparent and electrically conductive interface. Challenges arising from the different thermal expansion coefficients of Si and the III-V semiconductors were circumvented, as the bonding was performed at moderate temperatures of 120 °C. The external quantum efficiency and current-voltage characteristics of the wafer-bonded triple-junction solar cells were thoroughly investigated, and a maximum efficiency of 30.0% was found for a concentration factor of 112.

The terrestrial photovoltaic market is dominated by single‐junction silicon solar cell technology. However, there is a fundamental efficiency limit at 29.4%. This is overcome by multijunction devices. Recently, a GaInP/GaAs//Si wafer‐bonded triple‐junction two‐terminal device is presented with a 33.3% (AM1.5g) efficiency. Herein, it is analyzed how this device is improved to reach a conversion efficiency of 34.1%. By improving the current matching, an efficiency of 35% (two terminals, AM1.5g) is expected.

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