Deciphering thermal degradation pathways of metal halide perovskite thin films under ambient conditions

Understanding the dynamic lattice collapse pathways under thermal stress is pivotal for designing stable perovskite and improving the power conversion efficiency (PCE) of perovskite solar cells (PSCs). However, the lack of dynamic molecular-level characterization techniques has limited a comprehensive understanding of the degradation pathways involved. In this study, we use temperature-dependent Raman spectroscopy (25-375 °C) coupled with multimodal characterization to investigate A-site cation-dependent degradation pathways in mixed-halide perovskites, specifically FA_0.85Cs_0.15PbI₃ (FACs) and FA_0.85MA_0.15PbI₃ (FAMA). We identify three distinct stages of thermal decomposition dynamics with molecular-level precision: (1) grain boundary pre-collapse (25-175 °C), (2) halide segregation and PbI framework dynamics (200-325 °C), including PbI₂-defective domains, PbI₂ formation, and PbI₂ degradation, and (3) terminal lattice disintegration (350-375 °C). FACs preferentially stabilizes CsPbI₃ nanorods alongside amorphous β-PbO, whereas FAMA exclusively generates crystalline β-PbO without intermediate phases. Mechanistically, Cs⁺ acts as a lattice "scaffold", facilitating dynamic Pb-I-Cs coordination reconfiguration and delaying the degradation of the perovskite by more than 25 °C. In contrast, MA⁺ induces abrupt structural collapse via methylammonium volatilization, creating vacancy-propagated disintegration channels. By correlating Raman fingerprint (e.g., β-PbO at 139 cm^-1) with XRD/PL decay kinetics, we establish a predictive framework for A-site cation selection. This study not only reveals the mechanism of lattice anchoring and dynamic coordination reconstruction regulated by A-site cations at the molecular level but also clarifies the previously ambiguous Raman band assignments. These findings highlight the potential of Raman spectroscopy as a dynamic roadmap for elucidating degradation mechanisms and guiding the design of thermally resilient perovskites.