The power conversion efficiencies of organic solar cells fabricated with Ag and Ti nanoparticle arrays using nanosphere lithography are studied. The Ag nanoparticle array exhibits a broad absorption spectrum peaked at 420 nm, which spectrally overlaps with the absorption band of the organic absorbing layer centered at 520 nm, while no peak presented for the Ti nanoparticle array. Power conversion efficiencies by the solar cells with Ag and Ti nanoparticle arrays are 2.42% and 1.68%, respectively. This efficiency improvement is proposed to originate from the strong surface plasmon resonant scattering of visible light by Ag nanoparticle arrays. (c) 2007 American Vacuum Society.