Nanoconfined Crystallization of MAPbI3 to Probe Crystal Evolution and Stability

The effect of nanoconfinement on the crystallization of methylammonium lead iodide (MAPbI(3)) 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 degrees C, the oriented PbI2/solvent crystals converted to randomly oriented MAPbI(3) 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 MAPbI(3) crystals exposed to air. Unconfined MAPbI(3) crystals degraded to PbI2 within a period of 2 weeks of air exposure, accompanied by a significant change in crystal morphology. In contrast, MAPbI(3) crystals confined in AAO templates with a characteristic pore size of 100 nm were stable over the same period.