How Charge Carrier Exchange between Absorber and Contact Influences Time Constants in the Frequency Domain Response of Perovskite Solar Cells

A model is derived for the frequency and time domain optoelectronic response of perovskite solar cells (PSCs) that emphasizes the role of charge carrier exchange, i.e., extraction and injection, from (to) the perovskite through the transport layer to (from) the collecting electrode. This process is described by a charge carrier exchange velocity that depends on the mobility and electric field inside the transport layer. The losses implied by this process are modeled in an equivalent circuit model in the form of a voltage-dependent transport layer resistance. The analysis of the model predicts that the voltage dependence of the measured time constants allows discriminating situations where the transport layer properties dominate the experimental response. Application of this method to experimental impedance spectroscopy data identifies charge extraction velocities between 1 and 100 cm/s under 1-sun open-circuit conditions for p-i-n PSCs with poly(triaryl amine) as the hole transport layer; this corresponds to transport layer mobilities between 10−4 and 3 × 10−3 cm2 V−1 s−1. The model paves the way for an accurate estimation of the photocurrent and fill factor losses in PSCs caused by low mobilities in the transport layers, using small-perturbation measurements in the time and frequency domain