Design principle for mitigating moisture-induced degradation in 2D halide perovskites

We establish a conceptual framework which explains and mitigates moisture-induced degradation in 2D perovskites. Degradation proceeds along defined pathways across the spacer cation iodide (A'I or A'I₂)-PbI₂-H₂O ternary phase diagram as moisture intercalates into the perovskite. Phases evolve as the composition of the system travels towards the H₂O and PbI₂ sides of the phase diagram. The structure and chemistry of the spacer cations control the availability and formation energies of non-perovskite phases, which in turn determines the degradation mechanism and rate in humid air. In particular, stable 2D perovskites typically can coexist in thermodynamic equilibrium with water, whereas unstable 2D perovskites typically cannot because they are impeded by hydrate phases. This framework establishes a design principle focused on tuning the spacer cation to not accomodate low-energy hydrate phases and other lead-rich phases, thereby suppressing moisture driven degradation and enhancing 2D perovskite stability. Leveraging this mechanism, we show that replacing linear butylammonium with branched isobutylammonium suppresses moisture-induced degradation of the associated 2D powders by a factor of six. The existence of mixed-organic cation non-perovskite phases can similarly destabilize mixed 3D-2D perovskite films, and suppressing such mixed phases improves stability.