Halide-Site-Substituting Spacer Creates Quasi-2D Perovskites for Vapor-Deposited LEDs

Vapor deposition offers a scalable and industry-compatible route for perovskite light-emitting diodes (PeLEDs), yet the process remains challenging due to kinetically driven crystallization that produces mixed-dimensional phases and nanoscale heterogeneity. In particular, the lack of thermodynamic control leads to phase-disordered nanostructures, broadened energy landscapes, and limited device efficiency. Here, we report a thermodynamically guided vapor-phase synthesis of X-type quasi-two-dimensional (quasi-2D) perovskites, (CsPbBr3)n-1Cs2PbBr2X2, with controlled nanoscale phase distribution and interfacial coherence. By introducing halide-site-substituting organic spacer molecule that covalently bind to Pb2+ during in-situ deposition, we achieve selective crystallization of quasi-2D phases with high phase purity. A self-assembled hetero-scaffold of LiF and spacer molecule acts as a nanoscale growth template, promoting spatially uniform nucleation and minimizing phase segregation. Multimodal structural and spectroscopic analyses reveal dimensionally and spatially homogeneous films with high photoluminescence quantum yield (>85%) and reduced trap densities, enabling efficient exciton confinement and narrow emission. The resulting PeLEDs achieve an external quantum efficiency of 21.9%, an electroluminescence linewidth of 78.5 meV, and operational stability exceeding 1,500 minutes, with scalable pixel arrays demonstrated. These results provide a scalable route to high-efficiency vapor-deposited perovskite optoelectronics.