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

We establish a novel conceptual framework which explains and mitigates moisture-induced degradation in 2D perovskites. Degradation proceeds along defined pathways across the spacer cation iodide (A′I)-PbI2-H2O ternary phase diagram as moisture intercalates into the perovskite. 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. Guided by this framework, we establish a spacer cation design principle focused on eliminating low-energy hydrate phases to suppress moisture driven degradation, thereby 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.