On the modeling of dispersive transient photocurrent response of organic solar cells

The current methods used for estimating the electrical parameters of organic solar cells (OSC) from time-domain measurements are based on integer-order impedance models. Meanwhile, in the frequency-domain, the adopted circuit models usually contain a constant phase element which is known to capture effectively the fractional-order dispersive behavior of these devices. Therefore, inconsistency arises between the two analyses. In this work, we derive the time-domain relaxation response of an OSC, found to follow a Mittag-Leffler function, using the same fractional-order impedance model. The classical integer-order model that results into the exponential decay of the photocurrent can be easily recovered and should be used as a valid approximation only when the dispersion coefficient is close to unity.