Wafer Bonding and Layer Transfer Processes for High Efficiency Solar Cells

A wafer-bonded four-junction cell design consisting of InGaAs, InGaAsP, GaAs, and Ga0.5In0.5P subcells that could reach one-sun AM0 efficiencies of 35.4% is described. The design relies on wafer-bonding and layer transfer for integration of non-lattice-matched subcells. Wafer bonding and layer transfer processes have shown promise in the fabrication of InP/Si epitaxial templates for growth of thebottom InGaAs and InGaAsP subcells on a Si support substrate. Subsequent wafer bonding and layer transfer of a thin Ge layer onto the lower subcell stack can serve as an epitaxial template for GaAs and Ga0.5In0.5P subcells. Additionally, wafer bonded Ge/Si substrates offer the possibility to improve the mechanical performance of existing triple-junction solar cell designs, while simultaneouslyreducing their cost. Present results indicate that optically active III/V compound semiconductors can be grown on both Ge/Si and InP/Si heterostructures. Current-voltage electrical characterization of the interfaces of these structures indicates that both InP/Si and Ge/Si interfaces have specific resistances lower than 0.1 ..omega..cm2 for heavily doped wafer bonded interfaces, enabling backsurface power extraction from the finished cell structure.