Among the emerging energy production technologies, metal halide perovskite solar cells (PSCs) have attracted extensive attention from both academia and industry as their power conversion efficiency (PCE) has increased from 3.8% to a certified >25% in a decade. This is even more remarkable considering that low cost solution processing technology is used. The main remaining challenge for the commercialization of PSCs lies in their limited lifetime due to a low stability of the material and devices. This is challenging because the understanding of the fundamental chemical reactions and electrical processes at the origin of the device degradation is complex and ageing tests are time consuming.

Beyond irreversible degradation due to moisture, oxygen, and thermal stress, there are partially reversible processes, which increase the complexity of stability tests and pose challenges for characterization and simulation of the operational principle of devices in general and during aging. Despite remarkable progress in improving the stability, reported lifetimes are still far from what is expected from a PV technology. Therefore, a lot of research is necessary on the sources of degradation within solar cell devices under operation. To speed up this research, it is important to get a better understanding of the degradation pathways, from a fundamental point of view, and to find ways to accelerate ageing tests and obtain rapid feedback, which is the goal of this project.In more detail, we intend to address the following questions:

• Which are the best stress factors for accelerated aging?
• How can degradation be monitored by non-destructive in-situ measurements, also considering the peculiarities of perovskite solar cells such as slow reversible effects?
• How can the observed changes be attributed to distinct physical parameters?
• How can acceleration factors be identified from the degradation patterns and be used to predict lifetime under operation?

Associated with these questions are the following research goals:

• Design of tailored experiments for accelerated aging and identification of the most suitable stress factors and their parameter range
• Development of suitable in-situ characterization based on opto-electronic measurements that allow distinguishing changes in charge transport and recombination
• Application of numerical device simulation based on drift-diffusion models to identify the most likely origin of degradation during aging by quantifying changes of material parameters
• Development of predictive models that based on the acceleration factors allow lifetime estimations for given operating scenarios and ambient conditions

To answer these questions and achieve our goal we are going to combine degradation studies on a statistically relevant sample size with in-situ characterization and paired with device modeling to identify degradation patterns and their underlying physical cause. To fabricate the solar-cell samples, a combination of laser etching, inkjet-printing, screen printing, lamination and thermal evaporation will be used. Concerning the ageing study samples will be stress in environmental chambers that allow a fine control of parameters such as light intensity, humidity and temperature. To characterize the samples, a combination of structural (SEM, XRD) and optoelectronic measurements (impedance spectroscopy, electroluminescence, steady-state and transient photocurrent and photovoltage as a function of illumination intensity, etc.) will be used.  

The results will be analyzed using available device simulators. These tools allow to solve numerically the semiconductor equations (continuity, drift, diffusion, Poisson) for the whole solar-cell stack.We anticipate that the results of this project will allow us to improve the knowledge regarding degradation mechanisms of perovskite photovoltaic devices and lead to an establishment of fast, reliable and cost-effective stability measurement standards, which will be applicable by the whole research community. On the long run, these outcomes should facilitate a faster development of stable perovskite-based PV helping this technology to penetrate more easily the energy market. This would help to accelerate the decarbonization of the economy, thus reducing the disastrous impact of global warming on our society.