Air-Stable Methylammonium Lead Iodide Perovskite Thin Films Fabricated via Aerosol-Assisted Chemical Vapor Deposition from a Pseudohalide Pb(SCN)₂ Precursor

Ambient-air-stable methylammonium lead iodide (MAPI) perovskite thin films have been fabricated via one-step aerosol-assisted chemical vapor deposition (AACVD) from a pseudohalide Pb(SCN)2 precursor. We compare both the bulk and surface properties of the perovskite films grown using AACVD with those made by the widely used spin-coating method. Films with larger grain sizes and much better stability in ambient air can be obtained by AACVD. By addition of excess MAI to the precursor solution, MAPI films with negligible PbI2 impurities, as determined by X-ray diffraction, are obtained. The AACVD-grown MAPI films retain high phase purity with limited PbI2 formation after aging in air for approximately one month. The films exhibit an optical bandgap energy of ca. 1.55 eV and the expected nominal bulk stoichiometry (within error). In addition to probing bulk properties, we utilize X-ray photoelectron spectroscopy (XPS) to scrutinize the surface characteristics in detail. We find that the use of excess MAI results in formation of neutral CH3NH2 molecules at the surface. With aging time in air, the concentrations of iodine and nitrogen drop with respect to that of lead, but these changes are less severe in the AACVD-grown films compared to the counterparts made by spin-coating. Near-ambient pressure XPS is utilized to examine the surface stability of AACVD-grown films on exposure to 9 mbar H2O vapor. The formation of CH3NH2 molecules at the surface is observed, and the MAPI phase remains largely intact. The CH3NH2 molecules may passivate the surfaces and protect MAPI from degradation, providing a rationale for the observed stability of MAPI films fabricated from Pb(SCN)2 with excess MAI.