Yttrium doped zinc oxide (ZnO) nanowires prepared on seedless indium tin oxide (ITO) substrates were used for nanostructured bulk heterojunction organic solar cells (OSCs). Without patterning by a mask, the optimized areal density of ZnO nanowires decreased the reflection of incident light impinging upon the substrate by more than 50%, resulting in better incident-photon to current efficiencies (IPCEs) than for solar cells without ZnO nanostructures. OSC decorated with yttrium doped ZnO nanostructures showed an increase in current density from 8.9 mA/cm(2) to 9.9 mA/cm(2) resulting in 15% better device performance than a planar bulk heterojunction OSC. It was found that the antireflective effect due to ZnO nanowires was maximized with an areal density of 50 nanowires per 100 mu m(2). The areal density was optimized by incorporation of 0.02 mM yttrium chloride into the growth solution during nanostructure formation. In addition, OSC that incorporated the optimized areal density and length of ZnO nanostructures showed a 25% improved power conversion efficiency of 2.5% compared to OSC prepared on an ITO substrate without the ZnO nanostructures. Our results indicate that growth of densitycontrolled ZnO nanowire arrays on seedless ITO substrates using yttrium dopant is simple, low cost, and easily controllable method that enables low temperature fabrication, without patterning, of antireflective structures for significantly enhanced efficiency bulk heterojunction OSCs. Published by Elsevier B.V.