Abstract We report the enhanced performance and stability of polymer solar cells based on regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) blend using lithium benzoate (C 6 H 5 COOLi) as cathode buffer layer between the active layer and the Al cathode. The effects of the C 6 H 5 COOLi thickness on the performance of polymer solar cell are also investigated. Under 100 mW/cm 2 white light illumination, the device with 1 nm thick C 6 H 5 COOLi as cathode buffer layer exhibits power conversion efficiency (PCE) as high as 3.41±0.07% and the device stability is greatly extended. Compared to the solar cell with LiF/Al cathode, the PCE is increased ca. 9.4%. Introduction of C 6 H 5 COOLi buffer layer effectively increases the shunt resistance and improves the photo-generated charge collection. The improved performance may attribute to the dissociation of semi-conducting C 6 H 5 COOLi upon deposition to liberate Li with a low work function, which reduces the interface resistance of the active layer and the cathode and enhances the interior electric field that may result in efficient charge transportion. In addition, the C 6 H 5 COOLi layer may serve as an effective oxygen and moisture diffusion barrier for the organic solar cells. Therefore, C 6 H 5 COOLi is a promising candidate as an interlayer to improve the efficiency of electron collection and to reduce the ambience influence on the stability of polymer solar cells.