30% Efficient Three Junction Solar Cells for Space Concentrator Modules

While 30% AM0 efficient GaAs/GaSb mechanically stacked concentrator solar cells have been previously fabricated, these cells were designed for 100 suns concentration using point focus optics. These cells also used prismatic cover slides to reduce grid shading losses, a feature which is not practical when thick cover slides are used for radiation protection. When operated at 25 suns with line focus lenses and thick cover slides, the efficiency of these cells drops to 25%. Of this demonstrated 25%, 20% is associated with the GaAs top cell and 5% is attributable to the GaSb booster cell. Meanwhile, the InGaP/GaAs dual junction monolithic cell is expected to reach 25% at 25 suns AM0. In Phase I, InGaP/GaAs/GaSb three junction stacked cells will be fabricated and their performance will be measured over a concentration range of 10 to 30 suns. It is believed that a practical AM0 efficiency of 30% at 25 suns can be demonstrated for line focus concentrator applications. With very high efficiency and excellent radiation resistance, such a cell would be of significant value for low earth orbit applications. Targeted array performance of 300 Watts per square meter is achievable, with cover slides for high natural radiation resistance. The physical size of a satellite's solar array has a large and direct impact on luanch cost, and the 30% efficiency demonstrated here translates into smaller solar arrays for given power requirements. Launch cost savings and radiation resistance should induce both telecommunications and militarv customers.