Wafer bonding for use in mechanically stacked multi-bandgap cells

Two and three junction monolithic two-terminal solar cells have been developed that have 1-Sun, AM0 efficiencies of greater than 25%. In order to reach 1-Sun efficiencies of 30% and greater, solar cells with more junctions are required. Mechanically stacking junctions with different band gaps provides a means of developing such a cell. The authors propose a four-junction, mechanically stacked cascade solar cell structure that projects to a 34.8% AM0 efficiency. Wafer bonding provides a means of mechanically joining semiconductor materials with different lattice constants. They present optical, electrical and mechanical data on wafer bonding GaAs and InP substrates. The data indicate that wafer bonding can be used to develop a four-junction device.