Ordering and Disordering Dynamics in Fluorinated Two Dimensional Perovskites under High Pressure

The incorporation of fluorinated 2D perovskites has contributed to some of the most efficient and stable perovskite solar cells; yet, the fundamental understanding of their structural behavior and optoelectronic properties remains incomplete. In this study, we explore the optical properties, disorder, and tunability of two fluorinated 2D perovskites, of monofluorinated (4-FPEA)2PbI4 and perfluorinated (F5PEA)2PbI4. Through a combination of ambient X-ray diffraction (XRD), in situ photoluminescence (PL), Raman spectroscopy, and density functional theory (DFT) calculations under high pressure, we emphasize key differences between these materials. Our high-pressure findings reveal significant PL tunability, with (F5PEA)2PbI4 showing a 420 meV shift up to 7 GPa, while (4-FPEA)2PbI4 demonstrates a larger shift of 440 meV within 5 GPa. Notably, (F5PEA)2PbI4 transitions from an initially disordered state to increased ordering under pressure, suggesting that high-pressure conditions better reveal structural features akin to parallel alignment of the rings, providing additional insight into the more complex ambient structure. These results underscore the role of cation alignment and disorder in achieving pressure-responsive PL behavior. By linking experimental data with theoretical models, this work contributes to a deeper understanding of fluorinated 2D perovskites, paving the way for the design of adaptable optoelectronic materials.