Abstract
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.
Highlights
Complex phenomena are prevalent during the formation of materials, which affect their processing-structure-function relationships
We report on the characteristics of the metastable structure formed by the first-order phase transition occurring during antisolvent-induced nucleation from the colloidal precursor sol
By virtue of the high miscibility of chlorobenzene with DMSO and DMF9 and its poor solubility with the perovskite solid precursors, a phase separation process occurs as excess solvent molecules are displaced from the sample by the antisolvent stream, ensuing a marked increase in the concentration of the solute species within the system creating the conditions for a phase transition process to transpire
Summary
Beyond 120 s, there is a second, slow decrease in the diffraction intensities of the solvent-complex (Fig. 3a) up to 144 s, which is attributed to the removal of the solvent complex from the deeper parts of the thin film, which require longer annealing times for complete solvent removal Such an observation confirms insights on the structural gradients of the structure within thin films[56], where crystallization occurs at different rates within varying thicknesses of the film by solvent evaporation and interdiffusion[57] and resultant colloidal assembly[58]. The final phase IV (t >144 s) represents exclusive diffraction signals from cubic MAPI at q = 1.00, 1.42, 1.74, 2.00, 2.24, 2.46, 2.84, 3.01, and 3.48 Å−1 In this phase, the intensity of the perovskite peak increases (Fig. 3a) up to 260 s, while the peak width narrows (Fig. 3b), suggesting enhanced crystallinity and reduced lattice strain on longer annealing. Long-term annealing and cooling of a sample reflect the presence of a PbI2 phase (q~0.9 Å−1) as well as of a tetragonal MAPI phase (q~1.4 Å−1) (Fig. S3)
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