Quantum dot (QD) light-emitting diodes (LEDs) are ideal for large-panel displays because of their excellent efficiency, colour purity, reliability and cost-effective fabrication1-4. Intensive efforts have produced red-, green- and blue-emitting QD-LEDs with efficiencies of 20.5 per cent4, 21.0 per cent5 and 19.8 per cent6, respectively, but it is still desirable to improve the operating stability of the devices and to replace their toxic cadmium composition with a more environmentally benign alternative. The performance of indium phosphide (InP)-based materials and devices has remained far behind those of their Cd-containing counterparts. Here we present a synthetic method of preparing a uniform InP core and a highly symmetrical core/shell QD with a quantum yield of approximately 100 per cent. In particular, we add hydrofluoric acid to etch out the oxidative InP core surface during the growth of the initial ZnSe shell and then we enable high-temperature ZnSe growth at 340 degrees Celsius. The engineered shell thickness suppresses energy transfer and Auger recombination in order to maintain high luminescence efficiency, and the initial surface ligand is replaced with a shorter one for better charge injection. The optimized InP/ZnSe/ZnS QD-LEDs showed a theoretical maximum external quantum efficiency of 21.4 per cent, a maximum brightness of 100,000 candelas per square metre and an extremely long lifetime of a million hours at 100 candelas per square metre, representing a performance comparable to that of state-of-the-art Cd-containing QD-LEDs. These as-prepared InP-based QD-LEDs could soon be usable in commercial displays.

Highly efficient and stable InP/ZnSe/ZnS quantum dot light-emitting diodes

Quantum dots (QDs) are being highlighted in display applications for their excellent optical properties, including tunable bandgaps, narrow emission bandwidth, and high efficiency. However, issues with their stability must be overcome to achieve the next level of development. QDs are utilized in display applications for their photoluminescence (PL) and electroluminescence. The PL characteristics of QDs are applied to display or lighting applications in the form of color-conversion QD films, and the electroluminescence of QDs is utilized in quantum dot light-emitting diodes (QLEDs). Studies on the stability of QDs and QD devices in display applications are reviewed herein. QDs can be degraded by oxygen, water, thermal heating, and UV exposure. Various approaches have been developed to protect QDs from degradation by controlling the composition of their shells and ligands. Phosphorescent QDs have been protected by bulky ligands, physical incorporation in polymer matrices, and covalent bonding with polymer matrices. The stability of electroluminescent QLEDs can be enhanced by using inorganic charge transport layers and by improving charge balance. As understanding of the degradation mechanisms of QDs increases and more stable QDs and display devices are developed, QDs are expected to play critical roles in advanced display applications.

Stability of Quantum Dots, Quantum Dot Films, and Quantum Dot Light-Emitting Diodes for Display Applications

We introduce stoichiometry control within both core and shell regions of InP/ZnSe/ZnS core/shell/shell quantum dots (QDs) to advance their properties drastically, approaching those of state-of-the-art CdSe-based QDs. The resulting QDs possess near-unity photoluminescence quantum yield, monoexponential decay dynamics, narrow line width, and nonblinking at a single-dot level. Quantum-dot light-emitting diodes (QLEDs) with the InP/ZnSe/ZnS core/shell/shell QDs as emitters exhibit a peak external quantum efficiency of 12.2% and a maximum brightness of >10 000 cd m–2, greatly exceeding those of the Cd/Pb-free QLEDs reported in literature. These results pave the way toward Cd/Pb-free QDs as outstanding optical and optoelectronic materials.

Stoichiometry-Controlled InP-Based Quantum Dots: Synthesis, Photoluminescence, and Electroluminescence.

Over 30% External Quantum Efficiency Light-Emitting Diodes by Engineering Quantum Dot-Assisted Energy Level Match for Hole Transport Layer

Colloidal quantum dots are promising emitters for quantum-dot-based light-emitting-diodes. Though quantum dots have been synthesized with efficient, stable, and high colour-purity photoluminescence, inheriting their superior luminescent properties in light-emitting-diodes remains challenging. This is commonly attributed to unbalanced charge injection and/or interfacial exciton quenching in the devices. Here, a general but previously overlooked degradation channel in light-emitting-diodes, i.e., operando electrochemical reactions of surface ligands with injected charge carriers, is identified. We develop a strategy of applying electrochemically-inert ligands to quantum dots with excellent luminescent properties to bridge their photoluminescence-electroluminescence gap. This material-design principle is general for boosting electroluminescence efficiency and lifetime of the light-emitting-diodes, resulting in record-long operational lifetimes for both red-emitting light-emitting-diodes (T95 > 3800 h at 1000 cd m−2) and blue-emitting light-emitting-diodes (T50 > 10,000 h at 100 cd m−2). Our study provides a critical guideline for the quantum dots to be used in optoelectronic and electronic devices. New design principles for bridging the photoluminescence and electroluminescence of colloidal quantum dots are needed. In this work, the authors report electrochemically-inert ligands as a general material-design strategy for realizing high-performance LEDs based on quantum dots.

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Electrochemically-stable ligands bridge the photoluminescence-electroluminescence gap of quantum dots.

For the state-of-the-art quantum dot light-emitting diodes, while the ZnO nanoparticle layers can provide effective electron injections into quantum dots layers, the hole transporting materials usually cannot guarantee sufficient hole injection owing to the deep valence band of quantum dots. Developing proper hole transporting materials to match energy levels with quantum dots remains a great challenge to further improve the device efficiency and operation lifetime. Here we demonstrate high-performance quantum dot light-emitting diodes with much extended operation lifetime using quantum dots with tailored energy band structures that are favorable for hole injections. These devices show a T95 operation lifetime of more than 2300 h with an initial brightness of 1000 cd m−2, and an equivalent T50 lifetime at 100 cd m−2 of more than 2,200,000 h, which meets the industrial requirement for display applications. The commercialization of light-emitting diodes based on emissive quantum dots (e.g. QLEDs) is hindered by their inherent poor operational lifetime. Using an intelligent energy-level design strategy, Qian et al. demonstrate QLEDs with operational lifetime that meets industrial display standards.

/pdf/highly-stable-qleds-with-improved-hole-injection-via-quantum-39kj3rhmeo.pdf

Highly stable QLEDs with improved hole injection via quantum dot structure tailoring

The low efficiency and fast degradation of devices from ink-jet printing process hinders the application of quantum dot light emitting diodes on next generation displays. Passivating the trap states caused by both anion and cation under-coordinated sites on the quantum dot surface with proper ligands for ink-jet printing processing reminds a problem. Here we show, by adapting the idea of dual ionic passivation of quantum dots, ink-jet printed quantum dot light emitting diodes with an external quantum efficiency over 16% and half lifetime of more than 1,721,000 hours were reported for the first time. The liquid phase exchange of ligands fulfills the requirements of ink-jet printing processing for possible mass production. And the performance from ink-jet printed quantum dot light emitting diodes truly opens the gate of quantum dot light emitting diode application for industry. Designing efficient and scalable quantum dot LEDs meeting industrial requirements remains a challenge. Here, the authors, by leveraging the liquid phase exchange of d-MX2 ligands, present printed quantum dot LEDs with external quantum efficiency over 16% and half lifetime of more than 1,721,000 hours.

https://scholars.cityu.edu.hk/files/56623312/s41467_020_15481_9.pdf

High efficiency and stability of ink-jet printed quantum dot light emitting diodes

A solution-processed molybdenum oxide (MoOx) as the hole injection layer (HIL) by doctor-blade coating was developed to improve the efficiency and lifetime of red-emitting quantum-dot light-emitting diodes (QD-LEDs). It has been demonstrated that by adding isopropyl alcohol into the MoOx precursor during the doctor-blade coating process, the morphology, composition, and the surface electronic structure of the MoOx HIL could be tailored. A high-quality MoOx film with optimized charge injection was obtained, based on which all-solution-processed highly efficient red-emitting QD-LEDs were realized by using a low-cost doctor-blade coating technique under ambient conditions. The red QD-LEDs exhibited the maximum current efficiency and external quantum efficiency of 16 cd/A and 15.1%, respectively. Moreover, the lifetime of red devices initializing at 100 cd/m2 was 3236 h under ambient conditions, which is about twice as long as those with a conventional poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) H...

Improving Charge Injection via a Blade-Coating Molybdenum Oxide Layer: Toward High-Performance Large-Area Quantum-Dot Light-Emitting Diodes.

Although organic metal halide perovskite light‐emitting diodes (PeLEDs) have been developed rapidly in recent years, their commercialization and further applications are still hindered by ion migration‐induced inferior operational stability. Although the formation of a quasi‐2D structure is a well‐known solution that can efficiently suppress ion migration, the operation lifetime improvement is still limited. In this study, long‐chain phenylbutanammonium is used to replace the commonly used phenethylammonium as the L‐site cations. With large steric hindrance, ion migration inside the PeLEDs is effectively suppressed. Consequently, the PeLED with near‐infrared emission at 763 nm exhibited an operational lifetime (T90) of up to 5.3 h, which is 53‐fold higher than that of the control device. This study demonstrates the feasibility of significant stability improvement using a simple cation engineering strategy for PeLEDs.

Improving the Operational Stability of Near‐Infrared Quasi‐2D Perovskite Light‐Emitting Diodes by Cation Engineering

As an alternative method of the electron transport layer (ETL) of Zinc Oxide nanoparticles, an in-situ solution process was developed to prepare ZnO thin films for manufacturing perovskite light-emitting diodes (PeLEDs). After optimizing the processing conditions, the band energy and electron mobility are adjusted appropriately, which shows advantages to balance the carrier injection in PeLEDs. The near-infrared PeLEDs with ZnO film prepared by in-situ method demonstrated external quantum efficiency of 22%, which is compatible with their counterpart using ZnO nanoparticles. Our report provides a feasible strategy for the low-cost fabrication of PeLEDs with high performance. 

Chaoyu Xiang

Papers

Highly stable QLEDs with improved hole injection via quantum dot structure tailoring

High efficiency and stability of ink-jet printed quantum dot light emitting diodes

Improving Charge Injection via a Blade-Coating Molybdenum Oxide Layer: Toward High-Performance Large-Area Quantum-Dot Light-Emitting Diodes.

Improving the Operational Stability of Near‐Infrared Quasi‐2D Perovskite Light‐Emitting Diodes by Cation Engineering

In-situ Solution Processed Zinc Oxide as Electron Transport Layer for High-performance Perovskite Light-emitting diodes