The progress on the performance of perovskite solar cells performance has been spectacular since today. In parallel our understanding of the special properties of perovskite has been improving but lot of debate and some controversies are still ongoing due to the complexity of the behaviour of this semiconductor. Perovskite presents electron, hole and ionic transport at the same time and furthermore, when measuring its electrical properties, some special effects such as an extremely large capacitance or, its opposite, a low frequency inductance, also called negative capacitance, appear. To make it worse, illumination or application of a bias potential produce radical changes in these properties. In this talk we will explore, with the help of impedance spectroscopy, how some of the parameters that control the behaviour of perovskite solar cells change due to effects such as humidity or light and how they affect performance parameters such FF or Voc. ^{1- 4}

The current status of electrochemical impedance spectroscopy (EIS) and related analysis on perovskite solar cells (PSC) is still unsatisfactory. The provided models are still vague and not really helpful for guiding the efforts to develop more efficient and stable devices. This study provides fully validated impedance spectra and presents reproducible EIS timeseries at open circuit voltage (VOC) for more than 20 hours, with a total of 150 analysed spectra. We conclude that the observed changes stem from a temporary reduction of the electronically active area of the devices, as can be deduced from the inverse behaviour of resistance and capacitance. The changes in these values is almost 100% reversible if the devices are kept in the dark for only one day, while the time constant of the high-frequency process remains unchanged throughout the whole characterization procedure. The tested devices are full PSC devices that have proven to be stable over more than 500 hours. We compare those results to our latest generation of PSC that feature a self-assembled monolayer (SAM) of a fullerene derivative that have shown less than 1% of degradation over 1000 hours.

The results presented here are helpful for the diagnosis of PSC degradation and to derive new stability criteria and countermeasures against degradation. We also point out the importance of a stable operating point during measurements for a reliable impedance analysis.

Fundamental working mechanisms of perovskite solar cells (PSCs) remain an elusive topic of research. Impedance spectroscopy (IS) has been key to characterize the main physical processes governing the behavior of dye and quantum dot sensitized solar cells, which are considered, in many aspects, predecessor technologies. IS is a non-destructive characterization technique that can help in the understanding of these devices. Impedance spectroscopy is a characterization method in the frequency domain that allows to decouple physical processes with different characteristic times at the working conditions i.e. under illumination and applied bias. However, IS application to perovskite-based devices generates uncommon features and misleading outputs, mainly due to the lack of a stablished model for the results interpretation. We have systematically studied IS by the analysis of a series of perovskite-based devices ranging from sensitized to thin-film perovskite solar cells by the control of the perovskite precursors concentration. This transition is characterized by a change in the working principles, determined by an evolution of the dominant capacitance: from the intermediate frequency chemical capacitance of TiO₂ in devices with isolated perovskite domains, to a large low-frequency capacitance which divides the spectra in two sections. This study gives access to the characteristic features of the system, which we leverage to provide a rationalized IS model focusing especially on transport and recombination processes. The model, by means of an AC branch, paves the way to study the material and interfacial properties distinctive of perovskite solar cells. Concurrently, it provides for the first time the distribution of transport-related and recombination-related losses, a crucial tool for further development of perovskite photovoltaics. We analyze the implications in the characterization of PSCs. We show different examples in which impedance spectroscopy provide an excellent guide to determine the limiting processes in different kind of PSCs, providing consequently important clues to experimentalist to optimize its performance.

Mixed-halide perovskites like MAPb(Br_xI_1-x)₃ are the material of choice for perovskite/silicon tandem solar cells because of the easy bandgap tunability. However, under illuimination these materials separate into iodide-rich and bromide-rich domains. This segregation is goverend by migrtion of halide ions. Here I show that under pressure the segregation of the halide ions is slower, because the activation energy for ion migration is increased. I will then discuss that this kinetic stabilization helps to make the material more stable, but is not sufficient for long-term stability. Instead, thermodynamic stabilization is desired. I show that the unit cell size is a key factor for the thermodynamic stability of the mixed halide perovskites. By tuning the unit cell size, either by external pressure or by mixing the A-site cations, any halide ratio can be stabilized thermodynamically.

Metal halide perovskites are mixed electronic-ionic semiconductors with an extraordinary rich optoelectronic behavior and the capability to function very efficiently as active layers in solar cells, with a record efficiency surpassing 24% nowadays. In this talk, we use as case study an impedance spectroscopy and small-perturbation analysis of two perovskite solar cells with quite distinct optical and electrical characteristics, i.e. MAPbI₃ and CsPbBr₃-based devices. The main aim of the talk is to establish how, regardless the inherent complexity of the spectrum due to ionic effects, information like ideality factors, recombination losses and the collection efficiency can be qualitative and quantitatively assessed from impedance experiments at operating conditions. Open questions and remaining issues will also be discussed.