Analysis of the GaInP/GaAs/1-eV/Ge cell and related structures for terrestrial concentrator application

We analyze the potential of the GaInP/GaAs/1-eV/Ge four-junction solar cell to improve on the efficiency of the state-of-the-art GaInP/GaAs/Ge benchmark. We emphasize the following factors: (1) The newly proposed terrestrial concentrator spectrum has a lower ratio of red to blue light than does the AM1.5 direct spectrum. (2) Standard two-layer antireflection coatings do not provide near-zero reflectance over the full spectral range of interest for these devices. (3) Increasing temperature lowers the junction bandgaps, redistributing light to the top junctions. (4) GaInNAs junctions used to date for the 1-eV junction have quantum efficiencies less than ∼75%. These factors all limit the device current, adversely affecting the four-junction efficiency. We discuss strategies for ameliorating this problem, including going to alternate structures such as a GaInP/GaAs/0.9-eV three-junction device.