Monolithic and Mechanical Multijunction Space Solar Cells

High-efficiency, light-weight, and radiation-resistant solar cells are essential to meet the large power requirements of the future space missions. Single junction cells are limited in efficiency. Higher cell efficiencies could be achieved by utilizing more of the sunlight energy spectrum. This could be realized by developing multijunction, multibandgap solar cells. Monolithic and mechanically stacked tandem solar cells surpassing single junction cell efficiencies have been fabricated. Two-junction, two-terminal GaAs/Ge; two-junction, three-terminal InP/Ga0.47In0.53As and three-junction, two-terminal AlGaAs/GaAs/InGaAs monolithic tandem cells have been developed. Four-terminal GaAs/GaSb and GaAs CuInSe2 (thin-film) mechanically stacked tandem cells have shown good promise towards high efficiency and specific power respectively. The results obtained are very encouraging and promise a great potential for use in future space power systems. With further improvements in cell material and processing it will be possible to achieve efficiencies near the theoretically predicted values (30 to 40% AM0) for two and three junction tandem cells. This paper surveys the current status of monolithic and mechanically stacked multibandgap space solar cells, and outline problems yet to be resolved. Both the monolithic and mechanically stacked cells have their own problems as to size, processing, current-voltage matching, weight, etc. More information is needed on the effect of temperature and radiation on the cell performance. Proper reference cells and full spectrum range simulators are also required to measure efficiencies correctly. Cost issues are not addressed, since the two approaches are still in the developmental stage.