Complex phenomena are prevalent during the formation of materials, which affect their processing-structure-function relationships. Thin films of methylammonium lead iodide (CH3NH3PbI3, MAPI) are processed by spin coating, antisolvent drop, and annealing of colloidal precursors. The structure and properties of transient and stable phases formed during the process are reported, and the mechanistic insights of the underlying transitions are revealed by combining in situ data from grazing-incidence wide-angle X-ray scattering and photoluminescence spectroscopy. Here, we report the detailed insights on the embryonic stages of organic-inorganic perovskite formation. The physicochemical evolution during the conversion proceeds in four steps: i) An instant nucleation of polydisperse MAPI nanocrystals on antisolvent drop, ii) the instantaneous partial conversion of metastable nanocrystals into orthorhombic solvent-complex by cluster coalescence, iii) the thermal decomposition (dissolution) of the stable solvent-complex into plumboiodide fragments upon evaporation of solvent from the complex and iv) the formation (recrystallization) of cubic MAPI crystals in thin film.

Extensive studies have focused on improving the operational stability of perovskite solar cells but few surveyed the fundamental degradation mechanisms. One aspect overlooked in earlier works is the effect of the atmosphere on the device performance during operation.Here, we investigate the degradation mechanisms of perovskite solar cells operated under vacuum and a nitrogen atmosphere using synchrotron radiation-based operando grazingincidence X-ray scattering methods. Unlike what was seen in previous reports, we find that light-induced phase segregation, lattice shrinkage, and morphology deformation occur under vacuum. Under nitrogen, only lattice shrinkage appears during the operation of solar cells resulting in a better device stability. The different behavior in nitrogen is attributed a larger energy barrier for lattice distortion and phase segregation. Finally, we find that the migration of excessive PbI2 to the interface between the perovskite and the hole transport layer degrade the performance of devices either under vacuum or nitrogen.3 Solution-processed hybrid halide perovskite materials have attracted strong interest for next-generation thin-film photovoltaic applications due to their high power conversion efficiency (PCE) and low fabrication costs compared to silicon photovoltaics 1 . With solvent engineering, compositional tuning, and surface passivation 2-4 , the highest PCE of perovskite solar cells (PSCs) has reached 25.5 % 5 . Moreover, possibility of fabricating PSCs on flexible substrates opens up promising manufacturing routes, and novel application fields are explored, such as lightweight photovoltaic devices for space applications. Previous studies showed that PSCs were successfully operated in space with low vacuum conditions such as on a highaltitude balloon and a suborbital rocket [6][7][8] . Although these pioneers confirmed the possibility of operating PSCs in space, the operational stability of PSCs is unknown under such conditions.In terrestrial studies, vacuum conditions play a major role in the performance loss of PSCs during operation. Thus, although there is such a rapid increase in the PCE, very significant challenges remain. More research is required to increase the stability of the materials and the longevity of the devices, as long-term operational stability remains the main challenge for realworld applications of hybrid halide perovskite materials. Therefore, investigating the performance degradation mechanism of PSCs under different atmospheric conditions is one key approach to further improving the long-term operational stability of PSCs 9 .Exposure to above-bandgap illumination can cause a loss of phase and structure stability for perovskite materials. For instance, phase segregation introduced by lattice distortion, halide migration, and crystalline reorganization can cause an open-circuit voltage penalty arising from halide segregation 10 . In addition, several studies have indicated that a lattice distortion under illumination originates from light excitation or therm...

To increase the moisture stability of hybrid perovskite photovoltaics, a combination of three-dimensional (3D) and a thin layer of two-dimensional (2D) perovskite incorporating long-chained organic cations is often employed as photoabsorber. However, the detailed interaction between water and 3D/2D perovskite heterojunctions has not been elucidated yet. Using in situ neutron and X-ray scattering techniques, we reveal surprisingly strong water uptake into 3D/2D perovskite films despite the presence of hydrophobic bulky cations. Our results show rapid disproportionation of the initial 2D phase (number of layers, m = 5) in methylammonium lead iodide (MAPI)/2D films into lower m phases under humidity. Nevertheless, the 2D perovskite inhibits the irreversible PbI 2 formation, which suggests that the suppression of I − and MA + ion migration and consequently of MAI escape is related to the improved moisture stability of MAPI/2D perovskite films. In comparison, quadruple-cation perovskites including Rb + exhibit poor stability toward phase segregation upon exposure to moisture regardless of the 2D perovskite layer.