Department of Chemical and Biological Engineering, Gachon University, Seongnam, Gyeonggi 461-701, Korea Among metallic nanomaterials used as plasmonic structures in organic photovoltaics (OPVs), gold (Au) and silver (Ag) nanoparticles (NPs) have attracted significant attention owing to their remarkable localized surface plasmon resonance optical properties and relatively high scattering efficiency. In this study, we propose rough-surface Au@Ag core-shell NPs (RSAu@Ag NPs) as an advanced plasmonic material to combine the high scattering power of Ag NPs and broad spectral response of Au NPs in the long-wavelength range. We coat an Au nanoflower core with a thin Ag shell to fabricate the RSAu@Ag NPs. Highly efficient plasmonic organic solar cells were fabricated by embedding RSAu@Ag NPs at the bottom of the organic active layer consisting of a polymer blend of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]. The size, controlled morphology, and distribution of the metallic NPs are considered to determine the plasmonic effect, as these are the main factors contributing to the light absorption enhancement and power conversion efficiency of an OPV device.