The characteristics of optical absorption and photovoltaic properties of silicon(Si) wire solar cells with different aspect ratio have been studied. Vertically aligned Si wire arrays were fabricated by electrochemical etching and pen junction was prepared by spin-on-dopant (SOD) diffusion method. To demonstrate the effect of wire length, it was controlled with varying electrochemical etching time. We also investigated the optical properties of wire arrays with increasing wire length. 50-nm-thick Al-doped zinc oxide (AZO) film was used as a surface passivation and electrical contact layer that conformally deposited on the radial pen junction Si solar cell by atomic layer deposition (ALD) technique. We have observed efficiency enhancement with increasing wire length that increases the light trapping within the Si wire arrays, while the large surface area results in increased surface recombination. From the cell efficiency data, we identify that the surface recombination effect becomes more significant in the large surface area solar cells. To overcome this issue, we demonstrate the effect of a highly conformal AZO film grown by ALD to serve as surface passivation layer and top contact electrode. The conversion efficiency of the best sample is 7.1% and short circuit current (J(sc)) of 25.3 mA/cm(2) under 100 mW/cm(2) of air mass (AM) 1.5G illumination, when the Si wire length is 8 mu m with AZO coating. These results present that optimization of the wire length and surface passivation is necessary for silicon based solar cells prepared by electrochemical etching to enhance the solar cell energy conversion efficiency. Crown Copyright (C) 2010 Published by Elsevier B. V. All rights reserved.