Interfacial Dipole Engineering for Energy Level Alignment in NiOx‐Based Quantum Dot Light‐Emitting Diodes

The solution‐derived non‐stoichiometric nickel oxide (NiOx) is a promising hole‐injecting material for stable quantum dot light‐emitting diodes (QLEDs). However, the carrier imbalance due to the misalignment of energy levels between the NiOx and polymeric hole‐transporting layers (HTLs) curtails the device efficiency. In this study, the modification of the NiOx surface is investigated using either 3‐cyanobenzoic acid (3‐CN‐BA) or 4‐cyanobenzoic acid (4‐CN‐BA) in the QLED fabrication. Morphological and electrical analyses revealed that both 4‐CN‐BA and 3‐CN‐BA can enhance the work function of NiOx, reduce the oxygen vacancies on the NiOx surface, and facilitate a uniform morphology for subsequent HTL layers. Moreover, it is found that the binding configurations of dipole molecules as a function of the substitution position of the tail group significantly impact the work function of underlying layers. When integrated in QLEDs, the modification layers resulted in a significant improvement in the electroluminescent efficiency due to the enhancement of energy level alignment and charge balance within the devices. Specifically, QLEDs incorporating 4‐CN‐BA achieved a champion external quantum efficiency (EQE) of 20.34%, which is a 1.8X improvement in comparison with that of the devices utilizing unmodified NiOx (7.28%). Moreover, QLEDs with 4‐CN‐BA and 3‐CN‐BA modifications exhibited prolonged operational lifetimes, indicating potential for practical applications. The effects of molecular configuration and orientation on tuning the energy levels of NiOx are analyzed. The 4‐cyanobenzoic acid‐treated NiOx exhibited deeper energy levels than 3‐cyanobenzoic acid‐treated NiOx, which can be attributed to the more perpendicular orientation of 4‐cyanobenzoic acid compared to 3‐cyanobenzoic acid on the surface. The optimal quantum‐dot light‐emitting diode achieved a maximum external quantum efficiency of 20.34%.