A one-pot synthesis of large size and high quality AuAg alloy nanoparticles (NPs) with well controlled compositions via hot organic media is demonstrated. Amid the synthesis, complexation between trioctylphosphine (TOP) and metal precursors is found, which slows down the rate of nucleation and leads to the growth of large-size AuAg nanoalloys. The wavelength and relative intensities of the resulting plasmon bands are readily fine-tuned during the synthetic process using different Au/Ag precursors molar ratios. In the polymer solar cells, a key step in achieving high efficiency is the utilization of 1% Au11Ag89 alloy NPs embedded in the active layer to promote the power conversion efficiency (PCE) up to 4.73%, which outperforms the reference device based on the control standard device of poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PC61BM) under identical conditions. Corresponding increases in short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and incident photon-to-current efficiency (IPCE) enable 31% PCE improvement due to the enhancement of the light-trapping and the improvement of charge transport in the active layer. The findings advance the fundamental understanding and point to the superiority of Au11Ag89 nanoalloys as a promising metallic additive over Au, Ag, and Au28Ag72 alloy NPs to boost the solar cell performance.