Physical understanding of defects in perovskites for a science-based approach to improve the stability of perovskite solar cells

Perovskite solar cells (PSCs) were introduced in 2009, only achieving 3.8% power conversion efficiency (PCE). A rapid increase in effiency has followed ever since, achieving already 23% PCE nowadays. In this perspective, it can already start to compete with crystalline silicon (c-Si) solar cells, which represent more than 90% of the market. Also, the abundancy of the materials and their low cost, combined with the low-temperature, simple processing make PSCs an interesting technology for low cost energy. Furthermore, they can provide applications which cannot be provided by c-Si solar cells, such as semi-transparent solar cells for building-integrated photovoltaics (BIPV), e.g. in the form of colored (orange) windows that can convert solar energy in usable energy. However, the stability of PSCs is significantly inferior to the stability of c-Si solar cells, which can maintain 80% of the initial PCE after 20 years. The PCE of PSCs rapidly decrease when exposed to oxygen, humid environments, elevated temperatures and continuous (UV) irradiation, which results in degradation mechanisms. Although that many improvements have been achieved, e.g. by using mixtures of cations and halides in the perovskite compound or by using different materials as selective contacts, additional improvements in the overall stability are still required for succesfull commercialization. Some believe that  the passivation of the perovskite layer is the key to improvement of the stability. Passivation means that defects present within this layer are reduced (or made chemically inactive). It has been shown already that the use of additives during the processing can improve the performance and stability of PSCs. However, the approaches applied are rather empirical. In this work, the nature of the defects will be investigated in order to provide a science-based approach to passivate the defects that are detrimental for the stability.