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

초록

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 degrees C) coupled with multimodal characterization to investigate A-site cation-dependent degradation pathways in mixed-halide perovskites, specifically FA0.85Cs0.15PbI3 (FACs) and FA0.85MA0.15PbI3 (FAMA). We identify three distinct stages of thermal decomposition dynamics with molecular-level precision: (1) grain boundary pre-collapse (25-175 degrees C), (2) halide segregation and Pb-I framework dynamics (200-325 degrees C), including PbI2-defective domains, PbI2 formation, and PbI2 degradation, and (3) terminal lattice disintegration (350-375 degrees C). FACs preferentially stabilizes CsPbI3 nanorods alongside amorphous beta-PbO, whereas FAMA exclusively generates crystalline beta-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 degrees C. In contrast, MA+ induces abrupt structural collapse via methylammonium volatilization, creating vacancy propagated disintegration channels. By correlating Raman fingerprint (e.g., beta-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.

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