Current stability tests in the lab mainly focus on maintaining the solar cells under constant ambient and operational conditions such as controlled temperature, one-sun equivalent illumination, and maximum power point [1]. However, solar cells under operation face volatility, in particular regarding illumination and temperature. Additionally, they might be exposed to open circuit or negative voltages in case of partial shading of a module with devices connected in series. Furthermore, slow reversible process triggered by light and accelerated by temperature, for instance due to ion migration, play an important role in the performance of perovskite solar cells.

In this talk I will give an overview on stability measurements under fluctuating ambient conditions. I will discuss the main factors influencing stability and the effect of reversible degradation. Furthermore, I will comment on the role of the voltage during degradation. I will highlight the advances that have been made since the initial studies [2,3] and address open challenges.

Perovskite solar cells (PSCs) are one of the most efficient and cost-effective PV technologies with efficiencies reaching the 26 % mark. They have attracted substantial interest due to their light-harvesting capacity combined with low-cost manufacturing. However, unsolved questions of perovskite stability are still a concern, challenging their potential of widespread commercialization. Factors able to induce degradation to metal halide perovskite (MOHP) materials and devices, such as humidity, atmosphere, bias voltage, temperature, or light exposure, have been the centre of multiple studies and debates for many years. The ISOS stability assessment protocols, elaborated after a consensus among leading international laboratories, have been recently upgraded to incorporate stressors involving the peculiarities of PSCs. However, the understanding of the different degradation mechanisms taking place in materials and solar cells will only be effective if the applied stressors are as closed as possible to an actual functioning solar cell under real (outdoor) conditions. The application of real outdoor conditions (ISOS-O protocol) implies that multiple stressors are imposed to the sample with large variability and with different dosages. In this talk, I will show our most recent publications related to the issues observed for the analysis of PSCs under outdoor stability analysis (e.g. encapsulation). I will also show our most recent results on the interface and bulk modification of PSCs with the aim of enhancing device stability (indoor and outdoor), in this case our work is focus on the use of organic additives and MXenes. Finally, I will briefly describe our current work on in-situ and operando characterization, under single or multiple stressors with time, on PSCs to elucidate degradation mechanisms in these devices.

Halide perovskite solar cells is a very promising technology as an alternative to low cost and high-performance photovoltaics. Some of the main challenges of bringing perovskite technologies to market are efficiency, low cost and technically viable manufacturing, and long-term stability. Each of the key aspects has its own development; for example, efficiency has already surpassed values of 25%, and there are various publications demonstrating scalability and solution processing with very promising results particularly using slot die techniques [1]. This presentation will focus on two aspects: first, the analysis of ideality factor (nID) [2] for perovskite mini modules measured under outdoor conditions as a tool to follow up the performance and to understand the device degradation. Second, to mitigate this degradation process, some strategies to encapsulate the mini modules have been successfully implemented and evaluated.

Encapsulation tests have been carried out on minimodules fabricated on rigid and flexible substrates. In both cases, the edge-sealing structure for encapsulation was polyisobutylene (PIB) sealant by Quanex (SolarGain®) [3], [4]. In addition, glass and polymers coextruded with EVO were used as barrier material for rigid and flexible encapsulation, respectively. The encapsulation methodology developed is compatible with flexible substrates and allows the process to be carried out at a temperature of 100°C without compromising the thermal stability of the mini-modules. Finally, devices were tested under ambient conditions with MAPI perovskite solar cells in PIN structure and T80 efficiency was achieved after 1000 h of exposure.

Halide perovskite solar cells is a very promising technology as an alternative to low cost and high-performance photovoltaics. Some of the main challenges of bringing perovskite technologies to market are efficiency, low cost and technically viable manufacturing, and long-term stability. Each of the key aspects has its own development; for example, efficiency has already surpassed values of 25%, and there are various publications demonstrating scalability and solution processing with very promising results particularly using slot die techniques [1]. This presentation will focus on two aspects: first, the analysis of ideality factor (nID) [2] for perovskite mini modules measured under outdoor conditions as a tool to follow up the performance and to understand the device degradation. Second, to mitigate this degradation process, some strategies to encapsulate the mini modules have been successfully implemented and evaluated.

Encapsulation tests have been carried out on minimodules fabricated on rigid and flexible substrates. In both cases, the edge-sealing structure for encapsulation was polyisobutylene (PIB) sealant by Quanex (SolarGain®). In addition, glass and polymers coextruded with EVO were used as barrier material for rigid and flexible encapsulation, respectively. The encapsulation methodology developed is compatible with flexible substrates and allows the process to be carried out at a temperature of 100°C without compromising the thermal stability of the mini-modules. Finally, devices were tested under ambient conditions with MAPI perovskite solar cells in PIN structure and T80 efficiency was achieved after 1000 h of exposure.

Halide perovskites quickly overrun research activities in new materials for cost-effective and high-efficiency photovoltaic technologies.  Since the first demonstration from Kojima and co-workers in 2007, several perovskite-based solar cells have been reported and certified with rapidly improving power conversion efficiency, now approaching the theoretical limit.  Recent reports demonstrated that perovskites outperform the most efficient photovoltaic materials to date. At the same time, they still allow solution processing as a potential advantage in delivering a cost-effective solar technology.

The most stable and efficient perovskites contain lead, among the most toxic elements on earth. Lead-free alternatives have been reported with impressive progress in power conversion efficiency for tin-based (lead-free) perovskites. However, the stability of tin-based perovskite solar cells is still unexplored. In the present talk, we will focus on the stability of tin-based (lead-free) perovskite solar cells. We will show how the use of tin can actually improve the stability of perovksite solar cells. 

The dynamic response of metal halide perovskite devices shows a variety of physical responses that need to be understood and classified for enhancing the performance and stability and for identifying physical behaviours that may lead to developing new applications. Beyond the well-established characteristics of regular impedance arcs, we address the appearance of inductor effect at high voltage in perovskite solar cell. We present a physical model in terms of delayed recombination current that explains the evolution of impedance spectra and the evolution of current-voltage curves. A multitude of chemical, biological, and material systems present an inductive behavior that is not electromagnetic in origin. Here, it is termed a chemical inductor. We show that the structure of the chemical inductor consists of a two-dimensional system that couples a fast conduction mode and a slowing down element. Therefore, it is generally defined in dynamical terms rather than by a specific physicochemical mechanism. The impedance spectra announce the type of hysteresis, either regular for capacitive response or inverted hysteresis for inductive response. We address the characterization of electron diffusion and radiative emission in halide perovskites using a range of light stimulated techniques as IMPS, IMVS, and voltage controlled light emission technique (LEVS).

Perovskite is well known for its composition tunability allowing bandgap optimization according to the application requirements. Composition and cations stoichiometries have an impact on the type and concentration of ionic charge carriers as well as on performance stability under continuous stressing conditions.

First, we will analyze the intrinsic instability of Methylammonium lead iodide (MAPbI3) based films under constant illumination. The analysis shows that the film transmittance increases (“bleaching”) within the first few hours but then saturates. The photoluminescence signal shows an initial increase – linked to the transmission – before it decreases. Interestingly, the speed and amount of the bleaching depend on the hole-transport layer and potential post-treatment steps.

We then proceed with investigations on the characteristics of FA-based perovskite solar cells with non-stochiometric compositions. A FA excess of 1-1.5% leads to improved stabilized device performance and higher ionic conductivity as measured with electrical impedance spectroscopy. According to simulations of IV and impedance data, carried out with the simulation software Setfos, it appears that increased conductivity correlates with an increase in the density of ions rather than mobility.[1]

Finally, we compare the performance under continuous stressing conditions of high bandgap MA- and FA-based PSCs, which are suitable candidates as sub-cells for tandem solar cell technologies. In this study, we tested encapsulated PSCs of various compositions with bandgaps higher than 1.57eV and an efficiency of up to 20%. The stressing experiments were carried out at different temperatures from 25°C up to 85°C, at MPP conditions with equivalent AM1.5G illumination (ISOS-L1 conditions) using the benchtop instrument Litos. Additionally, by performing in-situ JV scans every 60 minutes, we could follow the evolution of the PV parameters in parallel with stressing. The decay of the MPP correlates with the short-circuit current decay, thus indicating the development of charge generation and collection issues with aging. We performed additional characterization techniques before and after the stressing experiments using the Paios measurement platform to gain further insight into the degradation mechanisms.