Assigning ionic properties in perovskite solar cells; a unifying transient simulation/experimental study

Abstract

Kinetic modelling has proven to be essential to understand the time and spatial dependence of charge carriers in solar cells. Traditional drift–diffusion simulations have generally been employed to describe static steady-state conditions, whereas recently the transient counterpart has been able to reveal more detailed information regarding carrier kinetics. In addition to customary electron and hole dynamics, perovskite materials are known to also be strongly affected by the displacement of lattice vacancies, charged atoms or even entire molecules. Such ionic motion transpires on vastly different time scales compared to free charges and are generally not straightforward to simultaneously account for in one transient simulation tool. Here, based on coding in Julia, we develop and use such a tool accounting for both the fast dynamics of free charges subjected to radiative, Shockley–Read–Hall and surface recombination, simultaneously as the very slow displacement of ions is properly accounted for. We show that interconnected steady-state parameters such as V_OC and j_SC as well as transient experimental voltage decay measurements can be accurately reproduced by our simulations. The vast time window, ranging from sub-microseconds to hours, provided by high-resolution open circuit voltage decay (OCVD) measurements combined with the steady-state parameters, is identified as a reliable gauge of artifact-free ionic dynamics in perovskite solar cells. With the knowledge gained from our simulations we can thus provide a straightforward experimental method providing direct access to values of ion concentration, and diffusivity as well as its thermal activation energy, parameters that are crucial for the future development and commercialization of perovskite-based photovoltaics.