The theoretical optical response of 2D surface silicon nanowire arrays was studied to create design guidelines for silicon photovoltaic devices with minimal reflection and maximal absorption by the nanowire bodies. The optical response depends on the shape of the tips and effective fill factor of the array in addition to the grating periodicity. The interplay between these factors is interpreted in terms of the graded index matching and the fill factor. Using nanowires with conic-shaped tips of the same height and base diameter, we found the maximal attainable conversion efficiency to be 29% (4% larger than without such head modifications) for 5-μm nanowires, which is better than a virtual-reflection-free silicon layer. © 2011 IEEE.