Key Takeaways & Executive Findings
- •• • The ZnO@ZnMgO core-shell ETL increases the average electron population (<N_e>) in the QD emissive layer from 0.33 (ZnMgO-only) to 0.61 at 4 V, nearly doubling the pre-charged QD fraction and directly enhancing radiative recombination efficiency. • • Green QLEDs fabricated with the ZnO@ZnMgO ETL achieve a peak external quantum efficiency (EQE) of 30.66%, a maximum luminance of 1,615,039.85 cd/m2, and a low turn-on voltage of approximately 2 V, setting new benchmarks for solution-processed QLEDs. • • The T95 operational lifetime exceeds 29,000 hours at an initial luminance of 1,000 cd/m2, representing a significant improvement in stability, which is critical for commercial display applications requiring long-term reliability. • • The core-shell design synergistically combines the high electron conductivity of the ZnO core with the low defect density of the ZnMgO shell, mitigating interfacial quenching and charge trapping, thereby addressing the long-standing bottleneck of insufficient electron injection and interface-induced efficiency loss in QLEDs.
Abstract
Quantum dot light-emitting diodes (QLEDs) are emerging as a leader in next-generation display technology. In principle, the efficiency of QLEDs is highly reliant on the radiative recombination rate of injected electrons and holes in the QD emissive layer. Within a solitary light-emitting cycle, a pre-negative-charged QD bursts into a fleeting sparkle upon encountering a hole, much like a lighted piston within a roaring engine. More pistons bring higher horsepower. The challenge of achieving highly efficient QLED lies in how to increase the number of pre-negatively charged QDs. To address these limitations, we developed a ZnO@ZnMgO core-shell nanoparticle (NP)-based electron transport layer (ETL). This design synergistically combines the high conductivity of ZnO core and the low defect density of the ZnMgO shell. Measured by electron-excited transient absorption, the average electron population (
1. Introduction
Quantum dot light-emitting diodes (QLEDs) have emerged as frontrunners for next-generation displays and solid-state lighting, offering exceptional color purity, tunable emission, and high photoluminescence quantum yields. However, despite reported external quantum efficiencies (EQEs) exceeding 20% for red, green, and blue devices, their commercial viability is hampered by insufficient carrier recombination efficiency, primarily due to inadequate electron injection and interfacial quenching. The electroluminescence process relies on the recombination of injected holes with pre-charged negative quantum dots (QDs); thus, increasing the proportion of charged QDs is pivotal to enhancing radiative recombination, particularly at low driving voltages.
Conventional ZnO nanoparticle (NP) electron transport layers (ETLs) are widely used for their high conductivity and suitable band alignment, yet they suffer from intrinsic defects—such as zinc interstitials and oxygen vacancies—that act as charge traps and luminescence quenchers, degrading device performance and lifetime. Prior mitigation strategies, including insulating interlayers and doping, have shown limited success. This work introduces a ZnO@ZnMgO core-shell NP ETL that synergistically combines the high conductivity of the ZnO core with the low defect density of the ZnMgO shell. This architecture effectively increases the average electron population in the QD emissive layer, as directly measured by transient absorption, leading to record-high EQE, luminance, and operational stability.
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ZHANG Han, LIU Weipeng, WANG Xiaosuo, ZHAO Leilei, WANG Chenyang, ZHANG Xiangtong, SHEN Huaibin (2026). Improving Average Electron Population in Quantum-Dot Emissive Layer via Core-Shell ZnO@ZnMgO Nanoparticles for QLEDs with Efficiency Exceeding 30%. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-4063-2
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Frequently Asked Questions
What is the mechanism by which the ZnO@ZnMgO core-shell ETL increases the average electron population in the QD emissive layer compared to pure ZnMgO?
The ZnO@ZnMgO core-shell structure combines the high electron conductivity of the ZnO core with the low defect density of the ZnMgO shell. This design reduces interfacial charge trapping and quenching, allowing more efficient electron injection into the QD layer. Transient absorption measurements show that at 4 V, the average electron population (<N_e>) increases from 0.33 (ZnMgO-only) to 0.61, indicating a higher fraction of pre-charged QDs available for radiative recombination with holes.
How does the ZnO@ZnMgO ETL affect the operational lifetime of QLEDs under continuous operation?
The ZnO@ZnMgO ETL significantly enhances device stability. The T95 lifetime (time to 95% of initial luminance) exceeds 29,000 hours at an initial luminance of 1,000 cd/m2. This improvement is attributed to reduced interfacial defects and suppressed luminescence quenching, which minimize degradation pathways during prolonged operation.
What are the peak performance metrics achieved with the ZnO@ZnMgO-based green QLEDs, and how do they compare to state-of-the-art devices?
The green QLEDs achieve a peak external quantum efficiency (EQE) of 30.66%, a maximum luminance of 1,615,039.85 cd/m2, and a low turn-on voltage of approximately 2 V. These values represent top-tier performance in the QLED field, surpassing many previously reported devices and demonstrating the effectiveness of the core-shell ETL in enhancing carrier balance and radiative efficiency.
What is the significance of the average electron population (<N_e>) metric, and how is it measured?
The average electron population (<N_e>) quantifies the number of electrons residing in the QD emissive layer under electrical bias, directly correlating with the probability of radiative recombination. It is measured using electron-excited transient absorption spectroscopy, which probes the occupancy of electron states in the QDs. A higher <N_e> indicates more efficient electron injection and a greater fraction of negatively charged QDs, leading to higher electroluminescence efficiency.
What are the potential scalability and manufacturing challenges for integrating ZnO@ZnMgO ETLs into commercial QLED production?
The ZnO@ZnMgO core-shell nanoparticles are synthesized via solution-phase methods, which are compatible with low-cost, large-area fabrication techniques such as spin-coating and roll-to-roll processing. However, achieving uniform shell thickness and composition across large areas may require precise control of reaction conditions. Additionally, the long-term stability of the core-shell structure under ambient conditions and its compatibility with other device layers must be validated for mass production. Nevertheless, the demonstrated performance improvements and the simplicity of the approach suggest strong potential for industrial adoption.
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