Unravelling the environmental degradation mechanism of perovskite thin films

Abstract

Despite having remarkable efficiency, solar cells made of metal halide perovskites – such as methylammonium lead iodide (MAPbI₃) – are detrimental because of their innate instability. Since the precursor materials may have an impact on the degradation of MAPbI₃, the degradation needs to be studied at the precursor level. If they are not chemically stable or contain components that are prone to degradation, these effects can be felt on the resulting perovskite. For comprehending the degradation cycle, aging studies were conducted on precursors – methylammonium iodide (MAI) and lead iodide (PbI₂) powders, as well as on MAPbI₃ thin films. We perform aging studies starting from freshly synthesized samples over a span of 60 days on the precursors and over a span of 21 days on MAPbI₃ thin films. The degradation pathways of the precursors and MAPbI₃ are characterized by utilizing X-ray diffraction (XRD), FTIR spectroscopy, UV-Vis-NIR spectroscopy and field emission scanning electron microscopy (FESEM). The aging studies reveal that the synthesized organic precursors are unstable under ambient conditions and that the hybridized valence states of MAPbI₃ exhibit higher sensitivity under ambient conditions compared to that of their precursors. The material is observed to decompose into PbI₂ over the span of 21 days. We find that the structure of the MAI precursor degrades over time due to photooxidation-induced disintegration of the organic compound, while the PbI₂ precursor undergoes agglomeration while the structure is preserved. We explore the origin of the observed degradation and identify that the hybridized orbitals between MAI and PbI₂ initiate a sequence of chemical reactions responsible for this instability. Through ab initio simulations, we identify H₂O as the atmospheric molecule most readily incorporated in MAPbI₃, and this effect is corroborated by experimental observation of water-related degradation in the samples. On the other hand, incorporation of O₂ is shown to cause the most significant change to the electronic structure.