Metal nanoparticles (NPs) have attracted much attention owing to their particular characteristics such as localized surface plasmon resonance (LSPR) and scattering properties, which can improve the light-harvesting ability of photovoltaic cells. However, modification of the metal NP surface is needed to prevent particle aggregation and photoinduced charge trapping. Surface modification of silica-coated Ag NPs with oligothiophene (OT) provides silver/silica core–shell (Ag@SiO₂–OT) NPs, which are well dispersed in nonpolar organic solvents and miscible with the bulk-heterojunction (BHJ) layer of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) (P3HT:PCBM). Incorporation of the Ag@SiO₂–OT NPs into the P3HT:PCBM layers as the active layer of photovoltaic devices improves the light-harvesting ability and enhances the photo-conversion efficiency (PCE) by about 18%. There is no significant change in the open-circuit voltage (V_oc) value even when up to 30 wt% of Ag@SiO₂–OT NPs are incorporated, confirming that the OT-modified silica layer on silver NPs contributes to improving light absorption and photo-current without causing aggregation and photo-induced charge trapping. Moreover, the deliberately designed transmission electron microscopy (TEM) investigation of the composite films of P3HT:PCBM and Ag@SiO₂–OT NPs reveals that Ag@SiO₂–OT NPs are mainly located in the P3HT domain owing to the favorable interaction between the similar molecular structures of OT and P3HT.