Browsing by Author "Metcalf, Isaac W"

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    Understanding and Suppressing Degradation in Mixed-Dimensional Halide Perovskites
    (2025-04-25) Metcalf, Isaac W; Mohite, Aditya; Ajayan, Pulickel
    A new theory of 2D and 3D-2D perovskite degradation in humid air is demonstrated, introducing the novel concept of an A-site cation – spacer cation – PbI2 – H2O quaternary phase diagram. The degradation pathway of a given perovskite is determined by the path charted across the phase diagram as water is introduced to the three-component (n=1) or four-component (n>1) system, often moving through several multi-phase regions that include hydrate phases as nodes. Because of the tendency of organic cations to escape the sample through solvation with the ambient humid air, the effective composition of the system tends towards the PbI2 side of the phase diagram and away from the organic side over time, while also increasing in H2O content. With this degradation picture in mind, a new set of design principles can be conceptualized focusing on suppressing the formation of hydrates and other “avoidable” degradation phases besides PbI2. This new conceptual approach has been explored using the series of linear alkylammonium 2D perovskite spacer cations from methylammonium to octylammonium, and the linear alkyldiammonium spacer cations from butyldiammonium to decyldiammonium, both of which form series of quasi-1D ribbon-like hydrate phases. The new conceptual framework can explain the lower moisture stability of Dion-Jacobson phase 2D perovskites, and also explains the various pathways seen for degradation of higher-n-value 2D and mixed 3D-2D perovskites in humid air. I exploit the quantized ribbon length of the hydrate structure to engineer spacer cations so as not to fit within the organic site of any ribbon-like hydrate, reducing the number of “avoidable” phases in the phase diagram and as a result improving the moisture stability.