We investigated the optical characteristics and expected photocurrent of 10-mu m-thick Si solar cells with surface nanowire arrays. The diameter and filling ratio of the array influenced not only the optical absorption, but also the spatial distribution of the optical generation rate. Proper design of the nanowire array could locate the high optical field region far from the surface, avoiding serious carrier collection loss and raising the energy conversion efficiency. (C) 2014 Elsevier B.V. All rights reserved.