Adaptive Molecular Passivation of CsSnBr₃ Surfaces Enables Efficient and Stable LEDs

Tin halide perovskites are promising lead-free emitters, but their soft and chemically labile lattices make interfacial stabilization inherently difficult. In CsSnBr₃, fluctuating surface coordination and structural instability drive defect formation, non-radiative recombination and rapid device degradation. Current molecular passivation strategies rely largely on static binding motifs, including single-site coordination and rigid multidentate anchoring, which are poorly matched to a dynamically evolving perovskite surface. Here, we show that 2-furanethanaminium thiocyanate (FEASCN), an ionically linked multidentate passivator that combines strong intra-ion-pair correlation with bounded configurational compliance, enables adaptive stabilization of CsSnBr₃ emitters. FEASCN undergoes limited configurational relaxation in response to lattice vibrations and fluctuating surface coordination, maintaining persistent surface matching rather than detaching from the interface. This adaptive coordination suppresses defects, stabilizes the lattice, improves film quality and promotes more balanced carrier injection. CsSnBr₃ light-emitting diodes (LEDs) based on this strategy achieve a peak luminance of 1344 cd m^-2, a record external quantum efficiency of 2.64% and a sixfold improvement in operational stability. These findings establish dynamic surface matching as a route to efficient and stable lead-free perovskite electroluminescence.