High‐Performance, Solution‐Processed, and Insulating‐Layer‐Free Light‐Emitting Diodes Based on Colloidal Quantum Dots

Abstract

Quantum-dot light-emitting diodes (QLEDs) may combine superior properties of colloidal quantum dots (QDs) and advantages of solution-based fabrication techniques to realize high-performance, large-area, and low-cost electroluminescence devices. In the state-of-the-art red QLED, an ultrathin insulating layer inserted between the QD layer and the oxide electron-transporting layer (ETL) is crucial for both optimizing charge balance and preserving the QDs' emissive properties. However, this key insulating layer demands very accurate and precise control over thicknesses at sub-10 nm level, causing substantial difficulties for industrial production. Here, it is reported that interfacial exciton quenching and charge balance can be independently controlled and optimized, leading to devices with efficiency and lifetime comparable to those of state-of-the-art devices. Suppressing exciton quenching at the ETL-QD interface, which is identified as being obligatory for high-performance devices, is achieved by adopting Zn Mg O nanocrystals, instead of ZnO nanocrystals, as ETLs. Optimizing charge balance is readily addressed by other device engineering approaches, such as controlling the oxide ETL/cathode interface and adjusting the thickness of the oxide ETL. These findings are extended to fabrication of high-efficiency green QLEDs without ultrathin insulating layers. The work may rationalize the design and fabrication of high-performance QLEDs without ultrathin insulating layers, representing a step forward to large-scale production and commercialization.

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

“…However, the peak EQE is still lower than the theoretical value of ≈30% if the effect of light interference between different layers is taken into account in a multilayer hybrid organic LED, and the lifetime of QLEDs is far below the requirement of display or lighting applications, which was only in the range of hundreds of thousands of hours with an initial brightness of 100 cd m −2 . To further maximize EQE, minimize power consumption and improve lifetime, world‐wide researchers currently put significant efforts into many attempts on different types of QDs and electroluminescent (EL) device structures …”

Highly Luminescent Quantum Dots in Remote‐Type Liquid‐Phase Color Converters for White Light‐Emitting Diodes

“…However, they have less than optimal optical properties due to nonradiative recombination by defects . As a result, despite the safety issues, Cd‐based QDs are still attracting much attention and are seen as promising candidate materials for versatile applications in nonbiological environments owing to their outstanding performance with respect to tunable luminescence across the ultraviolet–visible–near‐infrared (UV–vis–NIR) region, broad absorption bands with narrow emission bands, high quantum yields, chemical stability, and photostability . It is important to note that QDs have been passivated with higher bandgap semiconductor layers, such as ZnS and CdS, to increase the quantum yield.…”

Recent progress in the device architecture of white quantum-dot light-emitting diodes

“…These features make QDs very attractive for light-emitting diode (LED) applications. With the great efforts being devoted to this field, QD-based light-emitting diodes (QLEDs) have attained much progress of late, and the performance of CdSebased QLEDs is approaching that of the state-of-theart organic light-emitting diodes (OLEDs) [1,4,7,[14][15][16][17][18]. In particular, QLEDs can offer much higher color purity than OLEDs.…”