Improved Current Efficiency of Quantum Dot Light-Emitting Diodes by Utilizing ZnO Nanoparticles and an Organic Ionic Interlayer

The solution-processed QLED (quantum dot light-emitting diode) has a great potential for low-cost and large-scale displays. However, the solution-processed QLED has a limitation in the current efficiency due to the rough surface of the quantum dot layer causing shunt leakage. This paper reports solution-processed highly current-efficient QLED utilizing zinc oxide nanoparticles (ZnO NPs) and an organic ionic interlayer as an electron injection layer. The organic ionic materials create the permanent interface dipole by shifting the vacuum energy using tetrabutylammonium (TBA) cations and tetrafluoroborate (BF4) anions under the applied electric field. The dipole effectively reduces the electronic injection barrier to provide charge carrier balance of the holes and electrons in the emissive layer. As a result, the maximum current efficiency and power efficiency for the device structure of QLEDs ITO/PEDOT : PSS/PVK/CdSe@ZnS QDs/ZnO NPs/TBABF(4)+PEG/Al (23.9 cd A(-1) and 12.9 lm W-1) enhanced significantly compared to the structure of QLEDs ITO/PEDOT : PSS/PVK/CdSe@ZnS QDs/ZnO NPs/PEI/Al (9.7 cd A(-1) and 5.8 lm W-1) and the structure of QLEDs ITO/PEDOT : PSS/PVK/CdSe@ZnS QDs/ZnO NPs/Al (6.8 cd A(-1) and 4.1 lm W-1). The improvement of the current efficiency can be analyzed under the Space Charge-Limited Current regime. In addition, this paper examines the existence of parasitic resistances including series resistance and shunt resistance by modeling the QLED device as a single exponential diode model. Finally, we can evaluate the influence of the ionic interlayer on the entire QLED device in reducing the leakage current and the current-voltage (I-V) characteristic of the fabricated QLED devices. We believe the presented high-performance quantum dot light-emitting diodes have a high potential and impact on lighting and display technologies.