In this work, we demonstrate a thorough device design, fabrication, characterization, and analysis of biomimetic antireflective structures implemented on a Ga
P/GaAs/Ge triple-junction solar cell. The sub-wavelength structures are fabricated on a silicon nitride passivation layer using polystyrene nanosphere lithography followed by anisotropic etching. Both the reflectance spectroscopy and external quantum efficiency measurements confirm the improved optical absorption in the ultraviolet/blue and near-infrared wavelengths. Hence, the conversion efficiency of cell with sub-wavelength structure (SWS) is enhanced by 46.1% and 3.4% due to much improved current-matching, compare to cells without an ARC and SL ARC, respectively. We further employ RCWA method to analyze the reflectance by tuning structural factors such as periodicity or height of the SWS.