Tailored ZnO Functional Nanomaterials for Solution‐Processed Quantum‐Dot Light‐Emitting Diodes

Abstract

Recent improvements in efficiency and luminance of quantum-dot light-emitting diodes (QLEDs) promise a versatile technology for next-generation lighting and display applications. This is accomplished due to the advances in colloidal quantum-dot (CQD) synthetic methods together with proper engineering of the charge balance in these devices. The exciton quenching mechanisms occurring at the interface between the QD emissive layer and the zinc oxide (ZnO) electron transport layer (ETL) are one of the important parts of the charge transport path, affecting efficiency and long-term stability. Herein, a comprehensive overview of the advances in the engineering of ZnO-based ETLs, in terms of device efficiency and operational stability, is attempted. It is specifically highlighted that significant improvements can be achieved using various ZnO ETL defect passivation methods. This review also describes the key requirements for high-performance QLEDs from the ETL engineering aspect and catalyzes for further interdisciplinary explorations to realize reliable devices for practical applications.

Efficient InP Green Quantum‐Dot Light‐Emitting Diodes Based on Organic Electron Transport Layer

“…As shown in Figure a, when the QDs directly in contact with the ZnMgO, the PL intensity is significantly reduced, which is a common phenomenon and is attributed to the exciton quenching caused by the defects of ZnMgO. [ 15 ] By replacing the ZnMgO with PO‐T2T, the PL intensity is remarkably increased. These results are further verified by time‐resolved (TR) PL.…”

“…Due to the matched conduction band minimums (CBM) of ZnO and QDs, electron transfer between QDs and ZnO could occur spontaneously, which could turn the QDs into the dark state. [ 15,16 ] In addition, the ZnO NPs contain many hydroxyl groups and oxygen vacancies, which serve as the trap states that quench the excitons. [ 17,18 ] 3) Positive aging.…”

“…[ 19–21 ] Such a positive aging phenomenon could persist for several days, making it difficult to predict the degradation behavior of devices. [ 15 ] 4) Agglomeration. The ZnO NPs are easily aggregated in the solution, [ 19 ] making it difficult for storage, processing, and handling.…”

Synergistic Regulation of Hole and Electron Transport Layers for Efficient Injection Balance in Deep Blue Quantum Dot Light-Emitting Diodes

“…Quantum dot light-emitting diodes (QLEDs) attract considerable attention because of the remarkable properties of the quantum dots (QDs) such as tunable emission spectra, narrow full width at half-maximum (fwhm) of emission peaks, and high photoluminescence quantum yields. − However, the obvious carrier injection difference between the hole transport layer (HTL) and electron transport layer (ETL) in deep blue QLEDs results in severe unbalanced charge injection compared to red and green QLEDs, which becomes the key challenge for better device efficiency and lifetime. − The reason for the phenomenon is that the electron mobility of ZnO nanoparticles (NPs), which are commonly used as ETLs, is significantly higher than the hole mobility of most organic hole transport materials for HTLs. , Moreover, the poor hole injection from the HTL also comes from the energy level mismatch at HTL/QD interfaces due to the deep valence band (VB) level of QDs. , While at the ETL/QD interface, electrons are easily injected into the emitting layer (EML) due to the minor conduction band (CB) gap between ZnO NPs and the QDs. The inequivalence leads to excess electron injection into the EML, which negatively charges the QDs and intrigues exciton quenching through nonradiative recombination and even electron shuttling into the HTL side. , …”

“…8−11 The reason for the phenomenon is that the electron mobility of ZnO nanoparticles (NPs), which are commonly used as ETLs, is significantly higher than the hole mobility of most organic hole transport materials for HTLs. 12,13 Moreover, the poor hole injection from the HTL also comes from the energy level mismatch at HTL/QD interfaces due to the deep valence band (VB) level of QDs. 14,15 While at the ETL/QD interface, electrons are easily injected into the emitting layer (EML) due to the minor conduction band (CB) gap between ZnO NPs and the QDs.…”