The recent developments in the metal halide perovskite solar cells were able to achieve power conversion efficiencies comparable to silicon solar cells. Perovskite materials have emerged as promising contenders in the field of photovoltaic technology, offering a cost-effective and energy-efficient alternative to traditional silicon-based solar cells. Their low-temperature fabrication processes and earth-abundant precursor materials position them as highly attractive for scalable solar energy solutions [1]. Despite achieving an impressive performance, perovskite solar cells still face significant challenges for outdoor implementation due to limited reliability. Although there are many external factors at play such as humidity, temperature and even the prolonged sun exposure, the interfaces between the halide perovskite absorber layer and adjacent charge transport films play a big part in the inherent stability of the cell component [2].

Here, we studied the effect of external stressors such as the exposure to air and humidity on double cation, Cs_0.3FA_0.7Pb(Br_0.2I_0.8)₃  and  triple cation, Cs_0.05(MA_0.17FA_0.83)_0.95Pb(Br_0.2I_0.8)₃  perovskites, where MA and FA stand for methylammonium (CH3NH3) and formamidinium (CH₂(NH₂))₂ as well as perovskite crystallisation by in-situ Grazing Incidence Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) and photoluminescence (PL) spectroscopy measurements [3]. We carried out GIWAXS measurements whilst depositing perovskite on different substrates (e.g. SnO₂ and NiO) to study perovskite crystallisation, where we were able to detect differences in perovskite crystallisation. Furthermore, to advance our understanding of degradation, we carried out real-time GIWAXS and concomitant PL measurements as the perovskite degrades under different conditions where it was possible to detect different perovskite crystal structures, present heterogeneously, diminish with time as the PbI2 concentration increased at the interface. We observed that different conditions triggered unique defect routes with different reaction kinetics. In air, we observed phase instabilities such as the breakdown of both double and triple cation perovskites into non-perovskite organic iodide species (CH₃NH₂I) as well as cesium iodide, CsI. These phase instabilities were not present under 100% humidity without oxygen. Whilst the phase instabilities were present in air for both double cation and triple cation, we observed no intrinsic phase instabilities when 2D interlayer (4-FPEAI) was deposited on top of the perovskite. For 100% humidity, we observed  partially reversible electron transfer from I^-  to Pb²⁺ leading to irreversible  Pb⁰ formation  for double and triple cation perovskite cells.