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, we propose the use of a short-chain fluoroaromatic ligand, 4-fluoro-phenethylammonium bromide (FPEABr) via a facile spin-casting method to fine-tune the refractive index (n) and horizontal dipole ratio values of the perovskite emitter layer and simultaneously suppress the defects formed during film deposition. After FPEABr ligand exchange, the FAPbBr3 NCs films exhibite a refractive index n significantly lower (by about 0.4) than bulk (2D/quasi 2D or 3D) perovskite films and show an enhanced  value of 77%. Therefore, we successfully produce ultra-pure green PeLEDs with a maximum current efficiency of ~ 50 cd A-1, a maximum luminance of 21304 cd m-2, and a peak external quantum efficiency of 11.33% at a high luminance of 2804 cd m-2, approaching the theoretical value of 11.90% given the structure, photoluminance quantum yield, and horizontal dipole ratio. 

https://onlinelibrary.wiley.com/doi/10.1002/adom.202370066