Understanding the Mechanisms of Methylammonium-Induced Thermal Instability in Mixed-FAMA Perovskites

Despite a recent shift toward methylammonium (MA)-free lead-halide perovskites for perovskite solar cells, high-efficiency formamidinium lead iodide (FAPbI₃) devices still often require methylammonium chloride (MACl) as an additive, which evaporates away during the annealing process. In this article, it is shown that the residual MA⁺, however, triggers thermal instability. To investigate the possibility of an optimal concentration of MA⁺ that may improve thermal stability, the intrinsic thermal stability of pure FA, FA-rich, MA-rich, and pure MA perovskite films (FA_1-xMA_xPbI₃, FAMA) is studied. The results show that the thermal stability of FAMA perovskites decreases with more MA⁺, under degradation conditions that isolate the intrinsic thermal stability of the material (i.e., without moisture and oxygen effects). X-ray diffraction (XRD), proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS), photoluminescence (PL) and UV-visible spectroscopy, and depth-profiling X-ray Photoelectron Spectroscopy (XPS) are employed to show that the observed trend is mainly due to the decomposition of the MA⁺ cation, as opposed to other effects such as the precursor solvent and film morphologies. It is also found that the surfaces of these FAMA films are MA⁺ rich, although this phenomenon does not appear to affect thermal stability. Finally, it is demonstrated that this trend is unaffected by the presence of Spiro-OMeTAD atop the film, and thus solar cell devices should preserve this trend.