Nanowires and nanotubes can serve as electrodes for polymer and electrochemical photovoltaic cells [1,2] and building blocks in other novel optical devices [3,4]. Recently photovoltaic cells based on silicon nanowire arrays have been suggested as a promising candidate for solar energy harvesting [5]. The advantage of the silicon-nanowire photovoltaic cell lies in its short collection length for excited carriers, resulting in significant improvement in carrier collection efficiency. The optical absorption characteristics of nanowire arrays in the solar spectrum, one of the key factors that determine the efficiency of solar cells, remain unclear. A detailed analysis of the optical absorption will help the design and optimization processes of the silicon nanowire solar cells. In this letter, we numerically analyze the effects of wire diameter, length and filling ratio on the optical absorption of periodic nanowire arrays, and compare the cases with the results of silicon thin films. We found that, in comparison to thin films, nanowire array based solar cells have an intrinsic antireflection effect that increases absorption in short wavelength range.

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