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

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.