Fabrication of a GaInN/GaInP/GaInAs/Ge four-junction solar cell using the wafer bonding technology

Abstract

We fabricated a GaInN/GaInP/GaInAs/Ge four-junction solar cell by wafer bonding a GaInN solar cell and a GaInP/GaInAs/Ge three-junction solar cell. We performed our wafer bonding at high pressure (500 N) and temperature (450 °C) by using p-type GaN and n-type GaAs. The open-circuit voltage (VOC), JSC, and fill factor of our four-junction solar cell under the condition of AM 1.5 G, i.e., 1 sun are 2.85 V, 0.219 mA cm−2, and 0.74, respectively. The improved VOC of our four-junction solar cells was confirmed by a series connection. The JSC is almost comparable to the theoretical value (itself based on the assumption of an ideal series junction), but the VOC is approximately 1 V less than that predicted by theory. Our research will improve the solar cell efficiency and help meet future energy needs.