Hybrid organic-inorganic perovskites are highly promising absorber materials for third generation photovoltaic technologies. These materials have low production costs and enable solar cell efficiencies comparable to the best crystalline silicon PV cells [1]. The main obstacle for their widespread practical implementation is the low operational stability defined by both external and internal factors. External factors such as the action of oxygen and moisture from the atmosphere can be suppressed by encapsulating the photoactive layer. Intrinsic factors causing degradation of the materials include thermal and photochemical aging effects. In particular, the light-induced decomposition of MAPbI₃ leads to the formation of metallic lead and molecular iodine due to photolysis of PbI₂ [2]. 
Although the degradation mechanisms are usually considered as a bulk property, they are more likely to occur at the scale of individual grains and between the boundaries. Along with this, infrared scattering scanning near-field microscopy (IR s-SNOM) techniques are being actively developed to visualize the cation dynamics in lead halide perovskites [3]. Since changes in the concentration and localization of organic cations have a strong influence on the material optoelectronic characteristics and the solar cells performance, the IR s-SNOM provides great opportunities for experimental investigation of the photochemical degradation processes in these materials with nanoscale spatial resolution. Herein, we applied IR s-SNOM technique for the first time to follow the nanoscale dynamics of the light-induced decomposition of MAPbI₃ films under well-controlled anoxic conditions inside the glove box.
The obtained results revealed that the light-induced aging of the MAPbI₃ films has a spatially heterogeneous character. The perovskite decomposition is started at the grain boundaries and is accompanied by the formation of core-shell structures, where the perovskite grains are covered with a “skin” of PbI₂, which is the MAPbI₃ main aging product. It is shown that lead iodide does not decompose further into metallic lead and molecular iodine under the used experimental conditions (low temperature, LED light). On the contrary, PbI₂ recrystallization with the formation of big flat crystallites is observed. 
To summarize, we demonstrated a high potential of using IR s-SNOM as a method to study the photodegradation of perovskite films with nanoscale local spatial resolution. The application of this technique in combination with other complementary methods represents a promising approach for identifying the most stable absorber material compositions.