Ligand Engineering for Precise Control of Strongly-Confined CsPbI3 Nanoplatelet Superlattices for Efficient Light-Emitting Diodes

Strongly-confined perovskite nanoplatelets (PeNPLs) offer promising opportunities for photonics and optoelectronics due to their narrowband emission, control over the transition dipole moment, and potential for polarized light emission. However, achieving uniform PeNPLs and effective surface passivation major challenges, especially for red-emitting iodide-based compounds. Here, we address these challenges with an ancillary ligand engineering strategy. Combining experimental and ab-initio modelling techniques, we demonstrate that ligands possessing an optimal backbone with strong surface binding and Pb-coordination not only passivate defects, but also improve thickness uniformity. The PeNPLs self-assemble into superlattices a single emission peak, indicative of uniform monolayer formation and orientation-dependent optical properties. Uniform-monolayer PeNPLs obtained with benzylphosphonic acid (BPAc) self-assemble into well-ordered superlattices with orientation-dependent optical properties. Face-down PeNPL thin films fabricated with solvent engineering exhibit Lambertian-like emission suitable for light-emitting devices, while edge-up oriented films exhibit linearly polarized emission. When integrated into light-emitting diodes, our PeNPL devices achieve a external quantum efficiency of 13.1%, the highest reported for strongly-confined PeNPL-based devices. Taken together, these results establish ancillary ligand-induced synthesis of uniform-monolayer PeNPLs as a decisive route to robust orientation control, positioning it as the central principle for advancing next-generation photonic and display technologies.