In this study we propose an equivalent circuit model to describe S-shaped current–voltage (I–V) characteristics in inverted solar cells with a TiOx interlayer between the cathode and the poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester active layer. Initially the solar cells demonstrate S-shaped I–V characteristics resulting in a low fill factor (FF). Upon light soaking with UV radiation, the resistance of the TiOx interlayer decreases, the S-shape disappears, and the FF increases. Impedance spectroscopy was used to investigate the influence of the resistance of the TiOx layer on the shape of the I–V characteristics. We show that the equivalent circuit model can describe the voltage dependence of the data before and after light soaking in a range from −1 to +1.5 V well, demonstrating the robustness of the model. The equivalent circuit elements can be attributed to the distinct layers in the solar cell, therefore giving insight into the origin of the S-shape behavior in this solar cell architecture.