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.