Discerning recombination mechanisms and ideality factors through impedance analysis of high-efficiency perovskite solar cells

The ubiquitous hysteresis in the current-voltage characteristic of perovskite solar cells (PSCs) interferes in a proper determination of the diode ideality factor (n), a key parameter commonly adopted to analyze recombination mechanisms. An alternative way of determining n is by measuring the voltage variation of the ac resistances in conditions of negligible steady-state dc currents. A reliable analysis of n based on the determination of the resistive response, free of hysteretic influences, reveals two separated voltage exponential dependences using different perovskite absorbers (3D perovskites layer based on CH3NH3PbI3 or mixed Cs(0.1)FA(0.74)MA(0.13)PbI(2.48)Br(0.39)) and a variety of interlayers (2D perovskite thin capping). The dominant resistive element always exhibits an exponential dependence with factor n approximate to 2, irrespective of the type of perovskite and capping layers. In addition, a non-negligible resistive mechanism occurs at low-frequencies (with voltage-independent time constant similar to 1 s) which is related to the kinetic properties of the outer interfaces, with varying ideality factor (n = 2 for CH3NH3PbI3, and n = 1.5 for Cs(0.1)FA(0.74)MA(0.13)PbI(2.48)Br(0.39)). Our work identifies common features in the carrier recombination mechanisms among different types of high-efficiency PSCs, and simultaneously signals particularities on specific architectures, mostly in the carrier dynamics at outer interfaces.