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

Abstract 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.