Assessing the origin of the S-shaped I–V curve in organic solar cells: An improved equivalent circuit model

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. In this work, we study the origin of S shape curve through an improved equivalent circuit model (ECM). The improved ECM involves a D:A junction as well as a rectifying junction to interpret the bias-dependent-recombination. [Display omitted] •We propose an improved equivalent circuit model to interpret the origin of S shape curve.•The improved equivalent circuit model involves a rectifying junction connected with the D:A junction in series.•The validity of this model is confirmed by good reproduction of the experimental results.•Effect of the formation of S shape curve on device parameters is analyzed through the equivalent model.•Detailed effect of the rectifying junction on the I–V curve of organic solar cells is simulated.