Advanced-Architecture High-Efficiency Solar Cells for Low Irradiance Low Temperature (LILT) Applications

The scientific community and NASA view the outer solar system as a high-priority destination for space exploration and research. The cost and feasibility of future missions to 5-10 AU and beyond would greatly benefit from the availability of high-efficiency solar arrays optimized for low-irradiance low-temperature (LILT) environments. We have therefore set out to develop an advanced-architecture solar cell technology that will produce over 20% more power per unit area at LILT than the current state of practice. This technology will leverage recent advances in bandgap engineering originally developed for 1 AU, such as inverted or upright metamorphic multijunction device architectures. In addition, the new technology will feature a LILT-optimized, radiation-hard design. This paper reports on initial results from JPL's advanced-architecture LILT cell development project, specifically ground-based laboratory measurements of temperature-dependent illuminated current-voltage (LIV) and external quantum efficiency (EQE). Test conditions included irradiances corresponding to Sun distances from 1 AU to 9.5 AU, and junction temperatures between +28 °C and -165 °C. Through such measurements, multiple advanced architectures have been identified which demonstrate superior performance even prior to completing the design optimization for LILT, at both beginning of life (BOL, pre-radiation) and at end of life (EOL, post 1e15 1 MeV e-/cm²). For example, we measured 5.5 AU -140 °C average efficiencies of ~35% at BOL and ~31-33% at EOL for inverted metamorphic four-junction cells from SolAero and upright metamorphic triple-junction cells from Spectrolab. Further efficiency improvements are to be expected in the near future as we investigate, identify and eliminate from the device design those mechanisms that limit the cell performance at LILT.