Effect of Oxidizing Agent on the Synthesis of ZnO Nanoparticles for Inverted Phosphorescent Organic Light-Emitting Devices without Multiple Interlayers.

Inverted organic light-emitting devices (OLEDs) have been aggressively developed because of their superiorities such as their high stability, low driving voltage, and low drop of brightness in display applications. The injection of electrons is a critical issue in inverted OLEDs because the ITO cathode has an overly high work function in injecting electrons into the emission layer from the cathode. We synthesized hexagonal wurtzite ZnO nanoparticles using different oxidizing agents for an efficient injection of electrons in the inverted OLEDs. Potassium hydroxide (KOH) and tetramethylammonium hydroxide pentahydrate (TMAH) were used as oxidizing agents for synthesizing ZnO nanoparticles. The band gap, surface defects, surface morphology, surface roughness, and electrical resistivity of the nanoparticles were investigated. The inverted devices with phosphorescent molecules were prepared using the synthesized nanoparticles. The inverted devices with ZnO nanoparticles using TMAH exhibited a lower driving voltage, lower leakage current, and higher maximum external quantum efficiency. The devices with TMAH-based ZnO nanoparticles exhibited the maximum external quantum efficiency of 19.1%.

Enhancing Efficiency in Inverted Quantum Dot Light-Emitting Diodes through Arginine-Modified ZnO Nanoparticle Electron Injection Layer.

Many quantum dot light-emitting diodes (QLEDs) utilize ZnO nanoparticles (NPs) as an electron injection layer (EIL). However, the use of the ZnO NP EIL material often results in a charge imbalance within the quantum dot (QD) emitting layer (EML) and exciton quenching at the interface of the QD EML and ZnO NP EIL. To overcome these challenges, we introduced an arginine (Arg) interlayer (IL) onto the ZnO NP EIL. The Arg IL elevated the work function of ZnO NPs, thereby suppressing electron injection into the QD, leading to an improved charge balance within the QDs. Additionally, the inherent insulating nature of the Arg IL prevented direct contact between QDs and ZnO NPs, reducing exciton quenching and consequently improving device efficiency. An inverted QLED (IQLED) utilizing a 20 nm-thick Arg IL on the ZnO NP EIL exhibited a 2.22-fold increase in current efficiency and a 2.28-fold increase in external quantum efficiency (EQE) compared to an IQLED without an IL. Likewise, the IQLED with a 20 nm-thick Arg IL on the ZnO NP EIL demonstrated a 1.34-fold improvement in current efficiency and a 1.36-fold increase in EQE compared to the IQLED with a 5 nm-thick polyethylenimine IL on ZnO NPs.

Highly Efficient All-Solution-Processed Quantum Dot Light-Emitting Diodes Using MoOx Nanoparticle Hole Injection Layer.

This paper presents a study that aims to enhance the performance of quantum dot light-emitting didoes (QLEDs) by employing a solution-processed molybdenum oxide (MoOx) nanoparticle (NP) as a hole injection layer (HIL). The study investigates the impact of varying the concentrations of the MoOx NP layer on device characteristics and delves into the underlying mechanisms that contribute to the observed enhancements. Experimental techniques such as an X-ray diffraction and field-emission transmission electron microscopy were employed to confirm the formation of MoOx NPs during the synthesis process. Ultraviolet photoelectron spectroscopy was employed to analyze the electron structure of the QLEDs. Remarkable enhancements in device performance were achieved for the QLED by employing an 8 mg/mL concentration of MoOx nanoparticles. This configuration attains a maximum luminance of 69,240.7 cd/cm2, a maximum current efficiency of 56.0 cd/A, and a maximum external quantum efficiency (EQE) of 13.2%. The obtained results signify notable progress in comparison to those for QLED without HIL, and studies that utilize the widely used poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) HIL. They exhibit a remarkable enhancements of 59.5% and 26.4% in maximum current efficiency, respectively, as well as significant improvements of 42.7% and 20.0% in maximum EQE, respectively. This study opens up new possibilities for the selection of HIL and the fabrication of solution-processed QLEDs, contributing to the potential commercialization of these devices in the future.

Performance Improvement of Quantum Dot Light-Emitting Diodes Using a ZnMgO Electron Transport Layer with a Core/Shell Structure.

Highly efficient and all-solution processed quantum dot light-emitting diodes (QLEDs) with high performance are demonstrated by employing ZnMgO nanoparticles (NPs) with core/shell structure used as an electron transport layer (ETL). Mg-doping in ZnO NPs exhibits a different electronic structure and degree of electron mobility. A key processing step for synthesizing ZnMgO NPs with core/shell structure is adding Mg in the solution in addition to the remaining Mg and Zn ions after the core formation process. This enhanced Mg content in the shell layer compared with that of the core X-ray photoelectron spectroscopy showed a higher number of oxygen vacancies for the ZnMgO core/shell structure, thereby enhancing the charge balance in the emitting layer and improving device efficiency. The QLED incorporating the as synthesized ZnMgO NP core/shell A exhibited a maximum luminance of 55,137.3 cd/m2, maximum current efficiency of 58.0 cd/A and power efficiency of 23.3 lm/W. The maximum current efficiency and power efficiency of the QLED with ZnMgO NP core/shell A improved by as much as 156.3% and 113.8%, respectively, compared to the QLED with a Zn0.9Mg0.1O NP ETL, thus demonstrating the benefits of ZnMgO NPs with the specified core/shell structure.

The Effect of Particle Size on the Charge Balance Property of Quantum Dot Light-Emitting Devices Using Zinc Oxide Nanoparticles.

Zinc oxide nanoparticles (ZnO NPs) have been widely used as an inorganic electron transport layer (ETL) in quantum dot light-emitting devices (QLEDs) due to their excellent electrical properties. Here, we report the effect of ZnO NPs inorganic ETL of different particle sizes on the electrical and optical properties of QLEDs. We synthesized ZnO NPs into the size of 3 nm and 8 nm respectively and used them as an inorganic ETL of QLEDs. The particle size and crystal structure of the synthesized ZnO NPs were verified by Transmission electron microscopy (TEM) analysis and X-ray pattern analysis. The device with 8 nm ZnO NPs ETL exhibited higher efficiency than the 3 nm ZnO NPs ETL device in the single hole transport layer (HTL) QLEDs. The maximum current efficiency of 19.0 cd/A was achieved in the device with 8 nm ZnO NPs layer. We obtained the maximum current efficiency of 17.5 cd/A in 3 nm ZnO NPs device by optimizing bilayer HTL and ZnO NPs ETL.

Effect of Electron Injection Layer on the Parasitic Recombination at the Hole Transport Layer/Quantum Dot Interface in Quantum Dot Light-Emitting Diodes.

Zinc oxide (ZnO) nanoparticles layers are used as a substitute for organic electron transport layer due to high electron mobility, higher thermal stability and less sensitivity to the oxygen/moisture. In this study, we investigated the electron injection properties of ZnO nanoparticles in QLED compared with TPBi commonly used as injection layer in OLEDs. The expected electron injection barrier from energy diagram is similar in both devices, but the current density of the ZnO injection layer was slightly high compared with the TPBi injection layer. The current efficiency of ZnO and TPBi devices were 5.21 cd/A and 2.24 cd/A, respectively. The current efficiency of TPBi device is below half of ZnO device. We found that the electron-hole recombination occurs not only in the QD but also in the poly-TPD for TPBi device.

High Efficiency Quantum Dot Light-Emitting Diode by Solution Printing of Zinc Oxide Nanoparticles.

Quantum dot light-emitting diodes (QLEDs) have attracted considerable attention owing to the narrow emission spectra, wide color gamut, high quantum yield and size-controlled emission wavelength. Zinc oxide nanoparticles have been widely used as an electron transport layer (ETL) in QLEDs due to their excellent electrical properties. In this study, we compared the efficiency of QLEDs with organic and zinc oxide ETLs in viewpoint of the charge balance. The QLEDs were constructed using ZnO nanoparticles with an average particle size of 3 nm or 3TPYMB as the ETL materials. CdSe/ZnS quantum dots and poly-TPD were used as a light-emitting elements and hole transporting material, respectively. The QLED with 3TPYMB ETL exhibited current efficiency of 7.71 cd/A, while the efficiency of the QLED using ZnO nanoparticles was significantly improved to 38.76 cd/A. Such a substantial improvement of emission efficiency in the QLEDs with ZnO ETL was attributed to the much better charge balance by the ZnO.

Effect of Poly-TPD Molecular Weight on the Characteristics of Quantum-Dot Light-Emitting Devices.

We have investigated the effect of poly-TPD molecular weight (Mw) on the characteristics of CdSe/ZnS quantum-dot light-emitting devices (QD-LEDs). The poly-TPDs with Mw = 24000 and 84000 were used for hole-transporting layer. The Mw = 24000 poly-TPD layer was seriously dissolved by toluene which was used for the dispersion of QDs, resulting in substantial thickness reduction during the spin-coating of QD layer. The emission, external quantum efficiency, and driving characteristics of the QD-LEDs were investigated. A maximum external quantum efficiency of 2.1% was obtained in the QD-LED with Mw = 84000 poly-TPD layer.