The optics of core / shell nanowire solar cells was investigated. The optical wave propagation was studied by finite difference time domain simulations using realistic interface morphologies. The interface morphologies were determined by a 3D surface coverage algorithm, which provides a realistic film formation of amorphous silicon films on zinc oxide nanowire arrays. The influence of the nanowire dimensions on the interface morphology and light trapping was investigated and optimal dimensions of the zinc oxide nanowire were derived.