NiO_x/SAM-Mediated Interface Engineering for High-Performance PEA₂SnI₄ 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-9H-carbazol-9-yl)butyl]phosphonic acid (Br-4P), combining NiO_x to effectively mitigate interfacial defects for PEA₂SnI₄-based (PEA = phenethylammonium) pure-red PeLEDs. This SAM is meticulously designed with tailored dimensions to achieve interfacial structural synergy with PEA₂SnI₄, 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 Sn²⁺ 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.