Instability analysis of perovskite solar cells via short-circuit impedance spectroscopy: A case study on NiOx passivation

Abstract

Perovskite solar cells (PSCs) continue to be the “front runner” technology among emerging photovoltaic devices in terms of power conversion efficiency and versatility of applications. However, improving stability and understanding their relationship with their ionic–electronic transport mechanisms continue to be challenging. In this work, a case study of NiOx-based inverted PSCs and the effect of different interface passivating treatments on device performance is presented. Impedance spectroscopy (IS) measurements in short-circuit conditions were performed under different illumination intensities, as well as bias-stress operational stability tests under constant illumination intensity. Surface treatments that involved bulky Lewis bases resulted in better and more stable performance. In contrast, acidic anion donors could induce both an initial performance decrease with a characteristic three-arcs impedance Nyquist plot and a subsequent instability during light exposure. Drift–diffusion simulations suggest strong modifications of surface recombination at the interface with the hole transport material, and for the ion concentration and mobilities in the perovskite. Importantly, capacitance and resistance are shown to peak maximum and minimum values, respectively, around mobile ion concentration (Nion) of 1016 and 1017 cm−3. These features relate to the transition from a drift-, for low Nion below a threshold value, to a diffusion-dominated transport in the bulk of the perovskite, for high Nion beyond the threshold value. Our results introduce a general route for characterization of instability paths in PSCs via IS performed under short-circuit conditions.