Tuning Transition Dipole Moment Alignment via Bifunctional Ligands in Perovskite Nanocrystal Light‐Emitting Diodes

Perovskite quantum dot light-emitting diodes (PeQLEDs) are promising for display and lighting applications. As the internal quantum efficiency of the state-of-the-art PeQLEDs approaches unity, enhancing photon extraction becomes critical due to severe optical losses. The orientation of transition dipole moments (TDMs) plays a key role in determining the light outcoupling efficiency (η_out). Herein, the influence of nanocrystal (NC) shape and stacking behavior on the alignment of TDMs in the emissive layer is investigated. To modulate the TDMs, a facile ligand exchange strategy with bifunctional 1,5-naphthalenedisulfonic acid (NDSA) is introduced, which suppresses surface defects and improves carrier transport efficiency. More importantly, it enhances long-range NC stacking and modifies the dielectric environment of the emissive layer (EML), which increases the fraction of in-plane TDMs from 60% to 70%. This improved orientation is instrumental in η_out from 16.15% to 20.09%. As a result, devices with NDSA modified exhibit a peak external quantum efficiency (EQE) of 22.63% at 5127 cd m^- ² and a maximum luminance of 13 950 cd m^- ², significantly outperforming the pristine device. EQE remains above 20% across 400-8000 cd m^- ², and device lifetime improves by 400% under ambient conditions with encapsulation.