Modified Zinc Magnesium Oxide for Optimal Charge‐Injection Balance in InP Quantum Dot Light‐Emitting Diodes

Abstract

An efficient radiative recombination process in the emission layer is required to develop efficient QLEDs. Further, there are several possible loss mechanisms in QLEDs. Interfacial trap states in charge transporting layers (CTLs), [19][20][21] defect sites on QDs, [22][23][24][25] and charge injection imbalance [7,26] can reduce QLED performance. Non-radiative Auger recombination is considered the primary loss mechanism, which is caused by excess charge injection to QDs. [23,[27][28][29] Efficient and balanced charge injection into the QD emission layer is needed to achieve radiative recombination without severe loss mechanisms. The energy level distribution between the QD and adjacent CTL is critical because larger energy gaps significantly impede the charge transport between layers. QDs produced for light-emission applications have energy bands that are located at deeper sites (valence band level approximately 6.5 eV) compared to those of commercially available CTLs. [23] Hole injection from a hole transport layer (HTL) to an emission layer (EML) has been widely reported to be inefficient owing to energetic misalignment. [30][31][32] Enhancing hole injection into emissive QDs was proposed as the key parameter for balancing charge injection; specifically, this was performed via HTL doping to achieve hole mobility or a double HTL structure for gradient hole injection. [33,34] Balanced charge injection can be achieved by inhibiting electron injection from the electron transport layer (ETL) to the EML. The insertion of an electron injection inhibitor such as poly(methyl methacrylate) (PMMA) at the ETL/EML interface has been shown to suppress the excess electron injection to the QD layer. [7] The ETL can be doped to achieve a lower electron mobility. [35] Zinc oxide (ZnO) nanoparticles (NPs) have been commonly used as an ETL. [4,9,36,37] NPs have facile synthesis processes and stable electrochemical characteristics that are favorable features for electrical applications in optoelectronic devices. [5,38] The conduction band minimum (CBM) of ZnO NPs is similar to that of QDs, which leads to excessive injection of electrons into the QD layer. [26] The excess electron injection can induce nonradiative (Auger) recombination in the emission layer that reduces the device performance and operational stability of QLEDs. [26,39] Mg doping of ZnO has been widely reported to lower the electron mobility in ZnO NP and to suppress the electron injection through the ZnO NP ETL. Notably, Mg-doped ZnO (ZMO) exhibited a decrease in the mobility when the doping concentration of Mg was varied. [40] An Balanced charge injection into the emissive layer is a prerequisite for achieving highly efficient quantum dot light-emitting diodes (QLEDs). The similar energy distribution of charge transport layers and indium phosphide (InP) quantum dots (QDs) facilitates excess electron injection to the InP QD layer. In this study, magnesium-doped ZnO nanoparticles (ZMO NPs) are modified to suppress the electron injection to the InP QD layer. Particula...

Zn-Doped SnO2 NPs as Electron Transport Layer in Green CdSe/ZnS Quantum Dot Light-Emitting Diodes

“…A typical QLED usually has a sandwich structure, which is mainly composed of an electron transport layer (ETL), a QD emitting layer (EML), a hole transport layer (HTL), and a hole-injection layer (HIL). The electron transport layer is mainly categorized into the organic electron transport materials (e.g., 2,2′,2″-(l,3,5-benzenetriyl)-tris­( l -phenyl-l-H-benzimidazole) (TPBi) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)) and inorganic metal oxide nanoparticles (NPs) (e.g., SnO 2 NPs, , ZnO NPs, − and TiO 2 NPs − ). The QLEDs based on metal oxide NPs show an excellent electroluminescence (EL) performance.…”

“…The QLEDs based on metal oxide NPs show an excellent electroluminescence (EL) performance. It is noteworthy that ZnO NPs are widely used because of the advantages of high electron mobility, high transmittance, and tunable energy levels. , Until now, the red, green, and blue Cd-based QLEDs with ZnO ETL all reached external quantum efficiencies (EQEs) over 20%. − However, due to the reaction with acidic encapsulating resins, the ZnO-based QLEDs show positive aging, − which may cause stability disruption.…”