Tunnel-junction-free GaAs/Si tandem solar cells by self-aligned wafer bonding technology

GaAs/Si double junction solar cells are demonstrated by novel self-aligned wafer bonding techniques. The GaAs top junction is grown by metalorganic chemical vapor deposition (MOCVD) and the Si bottom junction is achieved by ion implantation. The two-junction solar cells are connected in series by Au/Si eutectic bonding, where the top and bottom cells were self-aligned by utilizing the Au/Si eutectic alloy as the interconnection metals and GaAs top cells as the etching mask. The Au/Si eutectic bonding was characterized by transmission electron microscope (TEM), scanning electron microscope (SEM) and current-voltage (I-V) measurements. A moderate conversion efficiency improvement of the GaAs/Si tandem cells compared to GaAs single junction cells is measured, while tandem cells with different top and bottom cell areal ratios are also investigated to achieve better current matching.