Luminescent Intermediates and Humidity-Dependent Room-Temperature Conversion of the MAPbI₃ Perovskite Precursor

Preparation of metal-halide perovskites under room temperature attracts attention because of energy saving by removing thermal annealing. Room-temperature transformation of spin-cast wet films consisting of methylammonium (MA) iodide, PbI₂, and dimethylformamide toward solid MAPbI₃ perovskite proceeds via several intermediate crystalline states and is strongly dependent on ambient humidity. Light transmission and photoluminescence (PL) microscopy and spectroscopy were used to monitor the growth of crystals and transformation of their properties in time under nitrogen atmosphere at room temperature. Under low humidity, a highly luminescent intermediate phase with low absorption in the visible range appears, with the PL spectra composed of several bands in the range from 600 to 760 nm. We assign these bands to low-dimensional (nanocrystals and two-dimensional inclusions) MAPbI₃ intermediates, where the exciton confinement shifts the spectrum to higher energies in comparison with the bulk MAPbI₃. The intermediate levels of ambient humidity (10-50%) appear to catalyze the conversion of the intermediate phase to MAPbI₃. At a high ambient humidity (>80%), the initially formed MAPbI₃ is quickly transformed to the transparent hydrate phase of MAPbI₃. The role of ambient water catalyzing the material transformation by competing for Pb coordination with the solvent molecules is discussed.