Homogeneous self‐Assembled Monolayer for Efficient Deep Blue Mixed‐Halide Perovskite Light‐Emitting Diodes

Self‐assembled monolayer (SAM) has been regarded as an effective interfacial layer to improve the performance of perovskite light‐emitting diodes (PerLEDs). However, the self‐aggregation clusters and low surface coverage of SAM still need to be solved. Herein, a novel SAM (3‐(3,6‐di(naphthalen‐2‐yl)‐9H‐carbazol‐9‐yl)propyl)phosphonic acid (Nap‐3PACz) is designed and synthesized by expanding the carbazole core unit with double naphthalene groups. The Nap‐3PACz can alleviate self‐aggregation clusters due to suitable steric hindrance, which can also modulate work function, promote hole transport, and improve device performance. Furthermore, a similar‐molecule‐induced co‐assembly strategy (SMIA) is developed to modify NiO x (named NiO x /SMIA) to achieve higher surface coverage. The results indicate that the similar molecule can modulate the arrangement behavior of Nap‐3PACz and enhance the adsorption energy, which can promote the formation of a uniform and full molecular layer to passivate surface defects and improve interfacial contact. Meanwhile, NiO x /SMIA can simultaneously enhance the hole conductivity and reduce the energy barrier, thereby balancing carrier injection. As a consequence, the champion deep‐blue PerLEDs with NiO x /SMIA modification achieve a maximum external quantum efficiency (EQE max ) of 6.72% at 469 nm, representing one of the state‐of‐the‐art deep blue PerLEDs. These results demonstrate the effectiveness of the molecular design strategy and the great potential of PerLEDs for further application.