The influence of the cathode electrode on the characteristics of pentacene/perylene derivatives based organic solar cells was analysed by means of absorption, photoluminescence, and X-ray spectroscopies. We report the characteristics of a series of organic solar cells fabricated with Al, Ag, and Au electrodes for the interface between metals and organic semiconductors, which play a central role in the physics of organic solar cells. Donor and acceptor layers of a solar cell were pentacene and N,N′-dioctyl-3,4,9,10- perylenetetracarboxylic diimide (PTCDI-8C) and N,N′-ditridecyl-3,4,9,10- perylene-tetracarboxylic diimide (PTCDI-13C) respectively. Two organic solar cells with pentacene/PTCDI-8C and pentacene/PTCDI-13C heterojunctions as active layers were fabricated to compare the influence of power conversion efficiency among perylene derivatives with various numbers of carbon molecules by means of J-V measurements. Under the sunlight simulator with an AM1.5G filter and power of 100 mW/cm2, the solar cells of the pentacene/PTCDI-13C heterojunction with the Ag cathode had J-V characteristics of short-circuit current density of 0.415 mA/cm2, open-circuit voltage of 0.413 V, fill factor of 0.55, and power conversion efficiency of 0.1%, which were better than those of the pentacene/PTCDI-8C heterojunction. Moreover, according to the thin film analysis, the PTCDI-13C thin film's excitons at the interface of the heterojunction for dissociation were more, and the probability of radiative recombination of the electron-hole pair was less than for the PTCDI-8C. The PTCDI-13C thin-film possessed better carrier mobility than PTCDI-8C. Therefore, we could conclude that the factors mentioned above are keys to the pentacene/PTCDI-13C-based solar cells' better power conversion efficiency. The carrier transportation mechanism of these solar cells is discussed clearly.