Planarizing Conjugated Ligands with Delocalized π Electrons for All-Bromine Blue Perovskite Light-Emitting Diodes

While extensive studies have focused on the electronic or coordination effects of organic ligands on perovskites, their spatial effects remain systematically underexplored. Herein, we designed and synthesized two conjugated ligands with nearly identical chemical compositions but distinct spatial configurations: benzophenone oxime (BPO) and 9-fluorenone oxime (FRO). In BPO, the two phenyl rings adopt a dihedral angle of 70.16° due to steric repulsion. In contrast, FRO features a planar structure, enhancing π-electron delocalization, which effectively passivated perovskite defects, optimized phase distribution, and promoted exciton radiative recombination. The FRO-doped all-bromine quasi-two-dimensional blue perovskite device achieved a peak external quantum efficiency of 15.25%, alongside improved spectral stability and operational lifetime. Notably, the dihedral angle of BPO decreased by 11.35° upon adsorption, providing direct and quantitative evidence for the existence and non-negligible role of π-bond interactions. This work highlights the critical importance of ligand spatial design for high-performance perovskite optoelectronic devices.