Effect of Light and Voltage on Electrochemical Impedance Spectroscopy of Perovskite Solar Cells: An Empirical Approach Based on Modified Randles Circuit

The voltage (V) and light-power (P) dependences of electrochemical impedance spectroscopy (EIS) curves of perovskite solar cells (PSCs) are investigated over the entire (V, P) space rather than limited to short-circuit or open-circuit conditions. For the first time, we report that under fixed V and increasing P, the EIS curves show complicated behaviors. Employing a modified Randles circuit, we successfully fitted the data using three newly proposed empirical equations. From the behaviors of the fitting parameters in the (V, P) space, we conclude that the dynamics inside PSCs consists of two parts: (1) the migration of mobile ions/vacancies and their capturing of the free carriers at the perovskite layer boundary and (2) a classical diode involving the migration and recombination of free carriers. Our calculated diffusivity and mobility agree with those of previous reports. We further found that the mobile ion density inside the perovskite layer is proportional to light power, which could explain the inverse linear power dependence of the amplitude of the slower EIS feature and the resonance time and amplitude of the faster one.