The effect of copper phthalocyanine (CuPc) buffer layer on open circuit voltage in indium tin oxide/zinc phthalocyanine (ZnPc)/C60/CuPc/Al devices is investigated. A dependence of the open circuit voltage on the thickness of CuPc layer is observed. We analyze the current-voltage curves under illumination based on semiconductor device theory. A high ideality factor is found as the reason for the high open circuit voltage. We propose that both ZnPc/C60 junction and CuPc/Al contact contribute the high ideality factor. Our result shows that the open circuit voltage may be improved by introducing an extra rectifying buffer/cathode junction.