Optical Engineering of FAPbBr₃ Nanocrystals via Conjugated Ligands for Light‐Outcoupling Enhancement in Perovskite Light‐Emitting Diodes

Abstract Formamidinium lead bromide (FAPbBr₃) nanocrystals (NCs) that exhibit ultra‐pure green emission are the most promising candidates for future displays. Despite the rapid development of light‐emitting diodes (LEDs) based on perovskite NCs (PeNCs), there is limited research detailing their intrinsic light outcoupling. Herein, the use of a short‐chain fluoroaromatic ligand, 4‐fluoro‐phenethylammonium bromide (FPEABr), via a facile spin‐casting method is proposed to fine‐tune the refractive index (n) and horizontal dipole ratio values (Θ_H) of the perovskite emitter layer and simultaneously suppress the defects formed during film deposition. After FPEABr ligand exchange, the FAPbBr₃ NC films exhibit a refractive index n that is significantly lower (by about 0.4) than bulk (2D/quasi‐2D or 3D) perovskite films and show an enhanced Θ_H value of 77%. Therefore, ultra‐pure green perovskite LEDs are successfully produced with a maximum current efficiency of ≈50 cd A⁻¹, a maximum luminance of 21 304 cd m⁻², and a peak external quantum efficiency of 11.34% at a high luminance of 2804 cd m⁻², approaching the theoretical value of 11.90% given the structure, photoluminescence quantum yield, and Θ_H.