Understanding S-Shaped Current-Voltage Characteristics in Organic Solar Cells Containing a TiOx Inter layer with Impedance Spectroscopy and Equivalent Circuit Analysis

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.