The future of high efficiency, multi-junction space solar cells

Space power generation has involved the continual improvement of device designs to achieve higher efficiencies. The improvements have involved both evolutionary as well as revolutionary changes in device design. Currently, the lattice matched, GaInP2/GaAs/Ge multi-junction solar cell is the device of choice for space power generation. The current design of this architecture has reached the 30% efficiency level, the expected maximum performance level for this architecture in a manufacturing environment. However, the band gaps for the GaInP2/GaAs/Ge device are not the ideal set of three junctions for maximum power conversion. We present theoretical modeling for 3-, 4-, 5-, and 6-junction devices. Other criteria for space power generation, including reliability, weight, and cost are discussed. Different approaches to higher efficiencies are discussed and are evaluated not only in the light of power conversion efficiency, but also in light of the other criteria.