Ultra-High-Efficiency, Multi-Junction Solar Cells for Space Applications

ABSTRACT: There is a continuing demand for higher efficiency space-qualified solar cells capable of powering spacecraft with growing power demand. For over a decade this demand has been met with lattice-matched GaAs based solar cells evolving from single junction GaAs/Ge with a BOL conversion efficiency of roughly 18% under 1 sun AM0 conditions to triple junction InGaP/GaAs/Ge cells with conversion efficiencies close to 30%. The objective of this investigation is to build upon the success of this technology by developing a multi-junction space solar cell platform incorporating a GaInNAsSb dilute nitride solar cell. This material system provides a straightforward approach to achieving a>40% 1 sun AM0 solar cell. Early measurements of triple-junction solar cells GaInP/GaAs/GaInNAsSb grown on GaAs substrates have demonstrated AM0 efficiencies in excess of 30% AM0 and preliminary results from proton radiation testing indicate the structure is sufficiently radiation hard for use in space applications. BENEFIT: The ever growing demand for increased platform power combined with the requirement for high-reliability form the need for a>40% 1 sun AM0 solar cell in a lattice-matched (LM) configuration, the configuration of all previous multi-junction solar cells. Due to fundamental technical extendibility issues facing existing manufacturers, cell suppliers are departing lattice-matching in favor of an inverted metamorphic (IMM) design. IMM introduces new potential risks and technical challenges: mitigating defects found within the graded buffer layer moving to the active region, a complex manufacturing process, potential cost increases as compared to an LM cell, new panelization methods and possible radiation softness. In contrast, Solar Junction"s GaInNAsSb cell maintains lattice-matching throughout the roadmap and is a drop-in replacement since the cell is grown monolithically on a rigid substrate. In addition, our GaInNAsSb material has a tunable bandgap, from 0.8 to 1.42 eV, thereby allowing multiple versions of the material with appropriate bandgaps for a five and six-junction cell solution. The extendibility of the material provides an efficiency roadmap exceeding 40% 1 sun AM0 while remaining lattice-matched. The manufacturing process consists of standard III-V methods utilized to produce semiconductors found in cellular telephones, LEDs or multi-junction solar cells. In the future, the GaInNAsSb cell architecture could also be adapted for substrate reuse for customers driven principally by Watts / kilogram. Potential commercial applications of the Research and Development, once manufacturing scale up has occurred, include all existing satellite manufacturers both commercial and military - utilizing multi-junction solar cells to power the payload, particularly those who are most risk averse due to reliability concerns.