Abstract
Hybrid perovskite materials have attracted much attention recently in the solar cell community as promising materials capable of developing into high performance photovoltaic devices at a low cost. While these materials have been routinely characterized with techniques like X-ray diffraction (XRD), scanning electron microscope (SEM), etc., not much attention has been paid to the issues related to solid state reaction in areas like the compositional evolution and reaction mechanism. Herein, we would like to report on an investigation of solid state reaction between lead halide and methylammonium iodide (MAI) to form perovskites by means of synchrotron-based high resolution X-ray photoemission spectroscopy that yields the elemental makeup information of the materials (HR-XPS) as well as mass spectrometry that analyzes the content of evolved gaseous species. The first set of measurements focus on the investigation of the initial stage of MAPbClxI3-x perovskite formation with either reactant film of PbCl2 (or MAI) prepared ex-situ and the other reactant MAI (or PbCl2) vacuum-deposited incrementally on the first reactant in UHV. For MAI deposition on PbCl2 film, the conversion from PbCl2 to MAPbClxI3-x perovskite is facile and at a sufficiently large dosage of MAI, a complete transformation to perovskite at 25 °C can be observed. Interestingly, over the course of annealing to 100 °C, XPS Pb 4f and I 4d spectra exhibit no change in spectral feature. For the perovskite formation from reverse deposition sequence, namely, a small amount of PbCl2 deposited on top of MAI film, the perovskite formation is incomplete at 25 °C and a thermal annealing at 60 °C is needed for a perovskite transformation. It seems that the deposition sequence can influence the perovskite formation reaction. The second set of measurements deal with the perovskite formation from a stoichiometrically correct reactant mixture prepared from the so-called one-step solution preparation procedure. The lead halides of PbCl2, PbBr2, PbI2 were mixed with MAI at a molar ratio of 1:3, 1:3, and 1:1, respectively. The solution preparation, as opposed to the vacuum deposition, is believed to result in a completed perovskite reaction owing to a much better control of the amount of MAI. Both XPS and mass spectrometry measurements could be simultaneously performed as the reactant mixtures were gradually heated to 250 °C during a course of about 8 h. The observation of gaseous species including H2, HX, CH3NH2, etc. signals the stage of thermal decomposition of reactants and intermediate products. Careful curve fitting of XPS Pb 4f and I 4d spectra provides clues to the progress of perovskite formation and the chemical composition profile of the perovskites. Taken together, plausible reaction mechanism for various characteristic reaction temperatures are proposed.