Firefly, in collaboration with Rochester Institute of Technology, proposes an STTR program for the development of 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 which enables increased payload capability and longer mission durations.