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

Abstract

The solution-processed QLED (quantum dot lightemitting diode) has a great potential for low-cost and largescale 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 (BF 4 ) 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 À 1 and 12.9 lm W À 1 ) enhanced significantly compared to the structure of QLEDs ITO/PEDOT : PSS/PVK/CdSe@ZnS QDs/ZnO NPs/PEI/Al (9.7 cd A À 1 and 5.8 lm W À 1 ) and the structure of QLEDs ITO/ PEDOT : PSS/PVK/CdSe@ZnS QDs/ZnO NPs/Al (6.8 cd A À 1 and 4.1 lm W À 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 highperformance quantum dot light-emitting diodes have a high potential and impact on lighting and display technologies.

Improving the Device Stability by Controlling the Morphology of Quantum Dot Emissive Layer Via a Coating Process in Blue QLEDs

“…[8][9][10] It has been well known that unbalanced charge injection is the critical aspect of QD charging, and thus the non-radiative recombination that causes the Joule Heating and Auger recombination phenomena in QLED devices. [10][11][12][13][14] A lot of efforts have been presented to enhance the QLED device performance in terms of their EQEs and stability by providing the charge injection balance via controlling the charge transport ability of HTL and ETL, QD stability, and also device structure. [8,10,12,[15][16][17][18][19][20][21] Furthermore, the effects of the QD properties, such as surface ligands, core/shell structure, shell thickness, etc., on the QLED device performance have been investigated in many publications.…”

“…The unbalanced charge injection to the QD emissive layer (EML) from the charge transport layers, which is motivated due to the mismatch energy level of the hole transport layer (HTL) and the high electron mobility of the electron transport layer (ETL), have been addressed as the primary reasons for this poor device performances [8–10] . It has been well known that unbalanced charge injection is the critical aspect of QD charging, and thus the non‐radiative recombination that causes the Joule Heating and Auger recombination phenomena in QLED devices [10–14] . A lot of efforts have been presented to enhance the QLED device performance in terms of their EQEs and stability by providing the charge injection balance via controlling the charge transport ability of HTL and ETL, QD stability, and also device structure [8,10,12,15–21] .…”