The CuPc/buffer interlayer/C60 heterojunction organic photovoltaic devices are fabricated by inserting a MoO3 or RB interlayer and investigated the impact of buffer interlayer on the performance of heterojunction organic photovoltaic devices. The results suggest that OPV with the buffer interlayer has higher open-circuit voltage and power conversion efficiency, lower short-circuit current density and fill factor. The open-circuit voltage increases from 0.39 V to 0.58 V and 0.55 V, respectively, and the power conversion efficiency reaches 0.44%, in comparison with a power conversion efficiency of 0.36% for OPV without interlayer. The short-circuit current density decreases from 1.92 mA/cm2 to 1.77 mA/cm2 and 1.81 mA/cm2, and from 0.48 to 0.43 and 0.44 in fill factor, respectively. Further research shows that the short-circuit current density strongly depends on the thickness of buffer layer. The short-circuit current density increases when the interlayer thickness is small, while it gradually decreases as the buffer layer thickness increasing. When the interlayer thickness is 10 nm, the short-circuit current density decreases to 0.35 mA/cm2. The open-circuit voltage enhances as the buffer layer thickness increasing, from 0.43 V with 1 nm interlayer to 0.63 V 10 nm interlayer. Open-circuit voltage enhancement and the short-circuit current density decrease are studied according to the integer charge transfer model and interfacial energy level theory, which provide a research foundation for the performance improvement of organic solar cells.