Nanoconfined Crystallization of MAPbI₃ to Probe Crystal Evolution and Stability

The effect of nanoconfinement on the crystallization of methylammonium lead iodide (MAPbI₃) was systematically studied using two-dimensional X-ray diffraction (2D XRD). Nanoconfined crystals were prepared by spin coating a cosolution of lead iodide (PbI₂) and methylammonium iodide (MAI) dissolved in N,N-dimethylformamide or dimethyl sulfoxide onto anodized aluminum oxide (AAO) templates with uniaxially aligned pores ranging from 20-200 nm in diameter. Upon spin coating, a metastable crystalline phase incorporating solvent molecules was observed. Analysis of 2D XRD patterns using refined lattice parameters revealed that these crystals adopt a preferential orientation with alternating sheets of PbI₂ and solvent molecules lying parallel to the long axis of the pores. Upon thermal annealing at temperatures up to 130 °C, the oriented PbI₂/solvent crystals converted to randomly oriented MAPbI₃ crystals, with the extent of conversion dependent on the characteristic pore diameter of the AAO template. Nanoconfinement was further observed to affect the stability of MAPbI₃ crystals exposed to air. Unconfined MAPbI₃ crystals degraded to PbI₂ within a period of 2 weeks of air exposure, accompanied by a significant change in crystal morphology. In contrast, MAPbI₃ crystals confined in AAO templates with a characteristic pore size of 100 nm were stable over the same period.