NiO _<i>x</i> /SAM-Mediated Interface Engineering for High-Performance PEA ₂ SnI ₄ Pure-Red Perovskite Light-Emitting Diodes

Tin-based perovskite light-emitting diodes (PeLEDs) have emerged as promising candidates for environmentally benign optoelectronic applications. However, their practical performance remains limited by severe interfacial defect states, particularly at the buried interface. In this work, we introduce a self-assembled monolayer (SAM) molecule [4-(3,6-dibromo-9 H -carbazol-9-yl)butyl]phosphonic acid (Br-4P), combining NiO x to effectively mitigate interfacial defects for PEA 2 SnI 4 -based (PEA = phenethylammonium) pure-red PeLEDs. This SAM is meticulously designed with tailored dimensions to achieve interfacial structural synergy with PEA 2 SnI 4, serving as a bridging component between NiO x and the perovskite. The insertion of Br-4P promotes superior crystallinity and more uniform crystal orientation, enables the passivation of undercoordinated Sn 2+ species, and the phosphate group of Br-4P functions to anchor onto NiO x . As a result, the optimized PeLEDs achieved a peak external quantum efficiency of 6.19% at 623 nm with a narrow full width at half-maximum of 27 nm and a luminance of 226 cd·m –2 . This work highlights the critical role of interface engineering in unlocking the potential of Sn-based perovskite emitters for high-efficiency, nontoxic light-emitting applications.