Firefly, in collaboration with Rochester Institute of Technology, proposes developing a space solar cell having record efficiency exceeding 40% (AM0) by the introduction of nanowires within the active region of the current limiting sub-cell. The introduction of these nanoscale features will enable realization of an intermediate band solar cell (IBSC), while simultaneously increasing the effective absorption volume that can otherwise limit short-circuit current generated by thin quantized layers. The triple junction cell follows conventional designs comprised of bottom Ge cell (0.67eV), a current-limiting middle GaAs (1.43eV) cell, and a top InGaP (1.90eV) cell. The GaAs cell will be modified to contain InAs nanowires to enable an IBSC, which is predicted to demonstrate ~45% efficiency under 1-sun AM0 conditions. The InAs nanowires will be implemented in-situ within the epitaxy environment, which is a significant innovation relative to conventional semiconductor nanowire generation using ex-situ gold nanoparticles. Successful completion of the proposed work will result in ultra-high efficiency, radiation-tolerant space solar cells that are compatible with existing manufacturing processes. Significant cost savings are expected with higher efficiency cells, enabling increased payload capability and longer mission durations.