Spacer Cation Design Motifs for Enhanced Air Stability in Lead-Free 2D Tin Halide Perovskites

Two-dimensional (2D) tin halide perovskites are promising tunable lead-free semiconductors for photovoltaic and optoelectronic devices. However, their poorer air stability relative to that of their toxic lead counterparts limits their applications. Here, we systematically compare a series of four 2D Ruddlesden-Popper and Dion-Jacobson tin iodide perovskites with different organic spacer cations to elucidate the spacer cation structural motifs that enhance air stability. Using several complementary characterization techniques, we find that 2D tin perovskites that contain rigid spacer cations that are halogenated or divalent greatly improve air stability compared to the reference (PEA) 2 SnI 4 (PEA = phenethylammonium) and show minimal degradation even after several weeks of ambient air exposure. We find that there is a correlation between interlayer space filling and air stability, suggesting that tightly packed interlayers impede oxidation beyond the surface and enable high bulk stability. This study informs design principles for enhanced air stability in 2D tin halide perovskites for stable, high performance devices.