Monolithic crystalline multijunction solar cell development and analysis at the US Air Force research laboratory

As satellite payload electrical power system requirements continue to grow, satellite systems employing flat panel arrays have reached limits set by either on-orbit dynamics that limit the size and shape of the deployed array, mass constraints set by the launch vehicle, or by the limits set by the volume constraints of the launch shroud. This has caused several satellite programs to approach power margin limits early in the design cycle, and to either compromise on satellite capabilities or perform costly redesigns. A very leveraging parameter for raising satellite power levels and reducing costs is the efficiency of the solar cells employed by satellite systems. State of the art efficiencies have reached 26.5% efficiency at load, and 30.1% for prototype cells, and solar arrays using GaAs based multijunction solar cells have achieved deployed solar array power densities of 70 W/kg and stowed volume power densities of 8 kW/m3. A simplified approach to the unwieldy dark current electrical analysis of multijunction solar cells has been developed, correlated with the performance of dual and triple junction solar cells, and explains ideality factors and reverse saturation currents that appear large. It was found that introducing a fourth junction with modest performance could raise the efficiency of multijunction solar cells to 31.5% efficiency at load, raise total power levels to 22 kW, raise the power densities to 100 W/kg and 9 kW/m3 with no impact to the configuration or operation of satellite solar arrays.