Formation of S-shaped I-V curve or the so-called kink has been shown detrimental to organic solar cells (OSC) performance. Previous researches have indicated that a variety of reasons could count for the origin of the S-shaped I-V curve. However, its origin is still not clear. In this contribution, we investigated the origin of S-shaped I-V curve from the view of an equivalent circuit model (ECM) in OSCs. The proposed ECM involves a rectifying junction connected with a donor/accepter (D/A) junction in series. OSCs with and without a Schottky barrier that was a rectifying junction were fabricated to verify the modeled results. And the good reproduction of experimental results confirmed the validity of our model. The results indicate that the origin of S-shaped I-V curve in OSCs is associated with the rectifying junction. With this model, the effects of the rectifying junction on the shape of I-V characteristic and its effect on device parameters are analyzed: fill factor (FF) dropped, short circuit current density decreased, open circuit voltage however, remained. Also, from simulation, we varied the parameters of the rectifying junction to study their influence on the device performance. (C) 2013 Elsevier B.V. All rights reserved.