Au-Cu alloy nanoparticles (NPs) are synthesized for triggering localized surface plasmon resonance (LSPR) in organic photovoltaic devices (OPVDs). Because Cu is readily oxidized, alloying with Au enhances the chemical stability of the NPs, thereby simplifying the fabrication processes. The electrical characterizations indicate that the alloy NPs improve the device performance under both one-sun illumination and indoor lighting conditions due to the effects of LSPR. Finally, the result of the stability test reveals that the use of the Au-Cu NPs would not affect the device stability. We anticipate that the results in this work open up a new avenue for plasmonic-enhanced OPVDs featuring low cost, stable nanostructures.