Layered hybrid halide perovskites (LHHPs) are an emerging type of semiconductors considered to be a more stable and versatile materials for the application in LEDs, solar cells, and other optoelectronic devices. However, the solution processing of high-quality LHHPs films with desired optical properties and phase composition is a challenging task. In our recent work we demonstrate that deviations of optical properties and X-ray diffraction in (C4H9NH3)2(CH3NH3)n−1PbnI3n+1 (further BA2MAn−1PbnI3n+1) LHHPs films from the ideal picture occur due to the presence of continuous defects – Stacking Faults (SFs). Upon analyzing the experimental data and modeled XRD patterns of a possible set of stacking faults (SFs) in the BA2MAPb2I7 phase, we uncovered the most plausible type of SFs, consisting of the thickness variation within single perovskite slabs. We also demonstrate that formation of SFs could be completely or partly suppressed via admixing of excess BAI into the perovskite solution [1].

            Systematic stability study of BA2MAn−1PbnI3n+1 homologous LHHPs row along with 3D MAPbI3 perovskite revealed one more curiosity of this class of materials – nonmonotonic photostability dependence on n number with “stability island” around n ≥ 3. Such nonlinear behavior is observed only under visible light irradiation in completely inert atmosphere (<10ppm O2) and could be explained by two counter-directional factors: (1) the intensification of internal I−/I0 and Pb2+/Pb0 redox processes along with perovskite band gap value which decreases with n and (2) the higher volatility of MA+ in MA-abundant compositions which increases with n (e.g., MAPbI3). But such a response of layered perovskites to long-term light soaking completely changes in the presence of > 10 ppm O2 in the surrounding atmosphere, affecting both the photodegradation mechanism and its rate for perovskite thin films and solar cells. The presence of >10 ppm O2 during LHHPs light soaking tests causes a dramatic decrease of materials stability due to the massive deprotonation and loss of alkylammonium cations by reactive oxygen species. This degradation mechanism is highly pronounced for all layered perovskite members in contrast to 3D APbI3 perovskites [2].

            Summing up, LHHPs based on monoalkylammonium cations demonstrate several intrinsic limitations such as tendency to crystallize with stacking faults and poor exploitation stability under visible light, especially in oxidizing atmosphere which should hinder using these materials in optoelectronic devices.