Efficient polymer solar cells were fabricated with regioregular poly 3-hexylthiophene (P3HT):(6,6)-phenyl C61 butyric acid methyl ester (PCBM)) as active layer and molybdenum trioxide (MoO3) and (N,N'-diphenyl)-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (NPB) as buffer layers. The results of transient photocurrent and electrochemical impedance spectrometry of device indicate that the insertion of NPB layer between the active layer and MoO3 layer is critical to the enhanced performance. It can effectively prevent charge recombination at the interface of MoO3 hole extraction layer, reduce interfacial resistance due to the formation of Ohmic contact and enhance the exciton dissociation because of the newly formed NPB/PCBM dissociation interface. The optimized thickness of NPB layer is 5 nm, resulting in maximized power conversion efficiency (PCE) of 3.94% under AM1.5G 100 mW cm(-2) illumination.