Photovoltaic device performance depends on efficient conversion of absorbed photons to electronic charge. Any mismatch of refractive indices between air and the solar cell front surface results in reflection of incident sunlight and reduced device performance. We detail a new approach for texturing silicon surfaces over arbitrarily large areas, combining selfassembly of block copolymer thin films and plasma-based etching. This process creates densely packed arrays of sub-wavelength size cones, whose tapered profile grades the refractive index transition between air and bulk silicon. The gradual change in refractive index greatly reduces reflection at the air/silicon interface from more than 40 percent in a flat film to less than 1 percent over a broad wavelength range from 350 nm to 1000 nm. Reducing the nanostructure separation distance to 42 nm allows us to achieve less than1 percent broadband reflectance using nanocones as short as 155 nm, with the height necessary for 1 percent reflectance increasing linearly with pitch. These antireflective structures maintain less than 5 percent broadband reflectance for incident light angles as high as 60 degrees. Optical simulations of the nanostructured surfaces using the Transfer Matrix Method qualitatively reproduce the observed antireflection behavior. However, the experimentally measured reflectance values are smaller than the simulation. Local measurements of electron energy loss spectra are consistent with a change in dielectric constant along the nanocone axial direction. Using these values improves the match between optical reflectance simulations and the experimental measurements.