Wafer-bonded GaInP/GaAs/GaInAs//GaSb four-junction solar cells with 43.8% efficiency under concentration

III-V based multi-junction solar cells with 4 to 6 junctions reach the highest efficiencies for the conversion of solar energy today. We present the latest results on the development of a GaInP/GaAs/GaInAs//GaSb four-junction solar cell. The implemented devices show an efficiency of $43.8 \pm 2.6 \%$ at a concentration of 796 suns and the efficiency decreases only marginally for higher concentrations up to 1378 suns. The remaining loss mechanisms of the device are analyzed and quantified. The Sb-based four-junction cell has the potential of reaching 50% conversion efficiency in future.