Enhanced Air Stability in 2D Tin Halide Perovskites Enabled by Rational Spacer Cation Design

Two-dimensional (2D) tin halide perovskites are lead-free semiconductors promising for photovoltaic, optoelectronic, and spintronic devices. However, their inherent instability to air and humidity relative to their toxic lead counterparts makes them more difficult to study and employ in applications. In this work, we report two new 2D tin perovskite crystal structures and systematically compare them among a series of Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) tin iodide perovskites to elucidate the spacer cation structural motifs that enable better air stability, as well as the underlying mechanism of how they protect the inorganic sublattice from degradation. Using solid-state NMR, we find 2D tin perovskites decompose into ligand salts and SnO₂ upon exposure to ambient air, which are localized at the crystal surfaces and produced at varying rates depending on the spacer cation. Direct comparisons between the studied phases demonstrate substantially enhanced air stability compared to the benchmark (PEA)₂SnI₄ (PEA = phenethylammonium) phase by use of halogenated aromatic spacer cations in RP perovskites or bulky ringed spacer cations in DJ perovskites. Interestingly, X-ray photoelectron spectroscopy depth profiling, complemented by synchrotron X-ray absorption spectroscopy, revealed that the long term bulk stability of 2D tin perovskites is not solely due to surface-level protection by the hydrophobic spacer cations as generally believed, but also due to effective subsurface protection from strong interlayer interactions that prevent penetration of air and water underneath the surface of the crystal. This study informs how we can rationally design 2D tin perovskites for enhanced air stability to enable further property studies and eventual applications in stable, high performance devices.