Organic solar cells (OSCs) with the structure of ITO/MoO3(6 nm)/Rubrene(30 nm)/C70(30 nm)/PTCBI(x nm)/Al(150 nm) are fabricated. Role of perylenebisimide with extended pi system (PTCBI) modified cathode layer in Rubrene/C70 based organic solar cells is investigated. Experimental results show that the insertion of PTCBI between C70 and Al electrode can significantly improve the performance of the devices. PTCBI contributes to an Ohmic contact between the C70 layer and Al cathode, which enhances the built-in potential in OSCs. Furthermore, PTCBI avoids the contact between the excitons and the Al electrode, and reduces the damage of high energy Al ions to C70 in the cathode preparation process. The effect of PTCBI thickness on the performances of OSC is also studied. The results indicate that the optimized PTCBI thickness is 6 nm. Compared with the performances of OSC without PTCBI, the open circuit voltage (VOC), fill factor (FF), short current density (JSC), and power conservation efficiency (P) of the optimum device are ameliorated by 70.4%, 55.5%, 125.1%, 292.2%, respectively. The cause of S-shape J-V curve in organic solar cells with thick modified cathode layer is analyzed. The modified cathode layer can be divided into two regions: the PTCBI layer and the Al permeated PTCBI layer. The electron mobility of PTCBI layer is lower than the hole mobility of Rubrene layer, which results in the charge accumulation on the unaffected PTCBI layer. When the thickness value of PTCBI layer is small, the whole modified cathode layer is permeated by Al ions, and this layer has better electron mobility than the unaffected one. When the thickness of PTCBI layer is 6 nm or more, the series resistance of OSC will increase and the S-shape J-V curve appears.