Quantum-dot light-emitting diodes with Fermi-level pinning at the hole-injection/hole-transporting interfaces

Quantum-dot light-emitting diodes (QLEDs) are multilayer electroluminescent devices promising for next-generation display and solid-state-lighting technologies. In the state-of-the-art QLEDs, hole-injection layers (HILs) with high work functions are generally used to achieve efficient hole injection. In these devices, Fermi-level pinning, a phenomenon often observed in heterojunctions involving organic semiconductors, can take place in the hole-injection/hole-transporting interfaces. However, an in-depth understanding of the impacts of Fermi-level pinning at the hole-injection/hole-transporting interfaces on the operation and performance of QLEDs is still lacking. Here, we develop a set of NiO_x HILs with controlled work functions of 5.2–5.9 eV to investigate QLEDs with Fermi-level pinning at the hole-injection/hole-transporting interfaces. The results show that despite that Fermi-level pinning induces identical apparent hole-injection barriers, the red QLEDs using HILs with higher work functions show improved efficiency roll-off and better operational stability. Remarkably, the devices using the NiO_x HILs with a work function of 5.9 eV demonstrate a peak external quantum efficiency of ~ 18.0% and a long T₉₅ operational lifetime of 8,800 h at 1,000 cd·m⁻², representing the best-performing QLEDs with inorganic HILs. Our work provides a key design principle for future developments of the hole-injection/hole-transporting interfaces of QLEDs.