A chlorinated organic cation enables stable 2D/3D tin iodide perovskite photovoltaics.

Tin halide perovskites (THPs) offer narrower bandgaps and improved environmental safety compared with the widely studied lead-based perovskites, but their air sensitivity has hindered their progress and demands new material design to enable their practical applications. Here we report stable and efficient 2D/3D tin perovskite solar cells enabled by new ultrastable 2D and quasi-2D THPs based on the 4-chloro-phenethylammonium (4ClPEA) cation. The stronger π-stacking interactions and tighter interlayer packing in (4ClPEA)2SnI4 among (4XPEA)2SnI4 structures (X = H, F, Cl, Br) substantially impede oxygen and water diffusion, enabling superior air and moisture stability and bright photoluminescence lasting several months in ambient air. The addition of 4ClPEA markedly improves 2D/3D THP film crystallinity and orientation, leading to 16.2% efficient 2D/3D THP solar cells that show prolonged storage stability and operational stability at 55 °C surpassing 1,000 h. This study establishes a new strategy for designing stable and efficient THPs towards their practical applications.