Lumped Parameter Modeling of Organic Solar Cells’ S-Shaped I–V Characteristics

This paper addresses lumped parameter equivalent circuit modeling of the S-shape anomaly observed in many experimental organic solar cells´ I-V characteristics measured under illumination. It presents a convenient exact analytic expression for the cell´s terminal voltage as an explicit function of its terminal current according to a model previously proposed by other authors. A new model is also recommended here in order to improve the previous model so that it may also describe the exponential-like upward bend observed at large forward voltages in the first quadrant of the I-V characteristics beyond the S-shape kink. The explicit nature of the proposed equations avoids numerical iteration and eases fitting to experimental data when extracting the cell´s model parameters. It also facilitates the derivation of other analytic expressions, such as the temperature dependence of the open-circuit voltage.