Impedance Spectroscopy with Variable Voltages and Illuminations to Reveal Recombination Routes of Free Carriers in Perovskite Solar Cells

The voltage (V)- and light power (P)-dependencies 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. Employing a modified Randles circuit, we successfully fitted the EIS results. From the behaviors of the fitting parameters in the (V, P) space, we conclude that the dynamics inside PSCs consist of two parts, 1) the migration of mobile ions/vacancies and their capturing the free carriers at the boundary and 2) a classical diode involving the migration and recombination of free carriers. We further found that the mobile ion density inside the perovskite layer is proportional to light power, which could explain the reciprocal power-dependence of the amplitude of the slower EIS feature and the lifetime and amplitude of the faster one.