Enhancing Stability and Efficiency in Tandem Quantum-Dot LEDs through IZO-Based Interconnecting Layers

Quantum-dot light-emitting diodes (QLEDs) is one of the most promising technologies for next-generation displays that can offer superior color accuracy, high brightness, and efficiency. They leverage quantum dots nanoscale semiconductor particles to emit light when exposed to an electric field. Their ability to produce highly saturated colors and achieve external quantum efficiencies (EQE) of over 20% made them a good alternative to organic light-emitting diodes (OLEDs) in displays for televisions, smartphones, and other electronic devices. However, QLEDs still face significant challenges, especially in terms of long-term operational stability and power efficiency, which limit their wide use in commercial products. One approach that has been tried to overcome these limitations is through tandem QLED structures which stack multiple quantum-dot layers separated by interconnecting layers (ICLs) and theoretically can double the brightness, EQE, and current efficiency compared to single-layer devices while maintaining the same power efficiency. This is achieved by enabling one injected electron to generate two excitons, effectively doubling the light output. Furthermore, tandem structures have the potential to extend the operational lifespan of the devices. However, the practical implementation of tandem QLEDs has been fraught with difficulties. Existing designs often suffer from high driving voltages due to inefficient charge transport across the interconnecting layers, leading to accelerated device degradation and reduced stability. Additionally, light extraction in tandem devices is complicated by the multilayer structure, which causes interference effects that reduce overall efficiency. To this account, a recent paper in Nano Letters, conducted by PhD candidate Cuixia Yuan, Zinan Chen, Fengshou Tian, and led by Professor Shuming Chen from the Department of Electrical and Electronic Engineering at Southern University of Science and Technology, reported improved electrical and optical performance of these devices by developing a more efficient interconnecting layer. The team focused on incorporating an indium-zinc oxide (IZO) bridging layer in the interconnecting structure to reduce the electron injection barrier, and by this lowered the driving voltage. They also optimized the optical design of the device to maximize light extraction by adjusting the cavity length and employing a top-emitting structure.

First, the authors examined the electron injection properties of different ICL materials and compared the current-voltage (I-V) characteristics of several ICL configurations, including one with a basic MoO₃/ZnMgO structure. This baseline structure showed poor performance, with a high electron injection barrier of 0.61 eV which contributed to the observed high driving voltage and low current density. To improve charge injection, the researchers introduced a 2 nm IZO bridging layer into the ICL. This adjustment dramatically reduced the electron injection barrier from 0.61 eV to 0.50 eV. This change resulted in a significant improvement in the current density which means more efficient electron transport across the ICL. Further experimentation showed that introducing a conductive ZnMgO (c-ZnMgO) layer which enhances electron tunneling led to even better performance. The optimized IZO/c-ZnMgO-based ICL showed a reduction in turn-on voltage from 7.0 V to 3.8 V, a marked improvement over the earlier designs. According to the authors, the reduction in driving voltage was important because it contributed to minimizing the electrical stress on the devices and resulted in stability and operational lifetime improvement.

The researchers also focused on optimizing the optical performance of the tandem QLEDs. They investigated the light extraction efficiency (LEE) by adjusting the thickness of the ZnMgO layer and the IZO-t electrode in the device’s cavity. By performing theoretical simulations and using finite-difference time-domain techniques to model electromagnetic field profiles, they identified the optimal thicknesses for these layers. These adjustments allowed the devices to achieve constructive interference, which significantly boosted the light output. The impressive record-high EQE of 49.01% for red tandem QLEDs demonstrates the device’s potential to surpass current display technologies in terms of brightness and color purity, essential features for high-definition displays in televisions, smartphones, and other electronic devices. Additionally, this increase in EQE was a direct result of the optimized cavity design, which maximized the light extraction from both the bottom and top emitting layers in the tandem structure.

Further testing of the tandem QLEDs focused on their long-term operational stability. The researchers measured the devices’ T95 lifetime, which indicates the time it takes for the brightness to reduce to 95% of its initial value. The optimized tandem QLEDs, incorporating the IZO and c-ZnMgO layers, demonstrated exceptional stability, with a T95 lifetime of over 50,000 hours at an initial luminance of 1000 cd/m². This was a substantial improvement over previously reported tandem QLEDs and marked the longest recorded lifetime for such devices. The enhanced stability was attributed to the efficient charge injection and balanced charge transport provided by the IZO-based interconnecting layer. Additionally, the lower driving voltage reduced the degradation rate, further contributing to the device’s longevity. The researchers also fabricated tandem QLEDs using an ultrathin aluminum-based ICL to evaluate its performance against the IZO-based structure and found that the aluminum-based ICL showed comparable electrical performance but reduced stability which may be due to the oxidative properties of MoO₃ that can readily reacts with metals like aluminum, forming Al₂O₃, and impairing the ICL’s electron injection ability. In contrast, the IZO-based ICL resisted oxidation more effectively, providing superior long-term stability.

In conclusion, Professor Shuming Chen and colleagues successfully developed an IZO-based ICL and optimized the tandem device structure, addressing two of the primary challenges facing QLEDs: high driving voltage and poor operational stability. The introduction of a conductive ZnMgO layer and a carefully engineered IZO bridging layer successfully reduced the electron injection barrier, significantly lowering the driving voltage. This improvement enhances the overall electrical performance, reducing the energy demand of the device while mitigating stress, which often leads to device degradation.

Professor Shuming Chen’s research team has long focused on tandem QLEDs, having achieved significant milestones in their earlier work (ACS Nano 2018, 12, 697−704; Adv. Funct. Mater. 2017, 27, 1700610). The recent breakthrough in Nano Letters, builds upon the extensive knowledge and experience they have accumulated in this field. We believe this breakthrough opens the door for the integration of tandem QLEDs into consumer electronics, promising more reliable and longer-lasting displays with better energy efficiency. Additionally, the advancements in charge injection and light extraction in this work provided a framework for further improvements in the efficiency and longevity of other optoelectronic devices including photovoltaic cells and light sensors, which also rely on efficient charge transport and light management. We also believe the barrier-free interconnecting layer approach reported in the study could be adapted for other multi-layered devices, enhancing their performance and enabling new possibilities in fields like flexible electronics and transparent displays. However, this work does present some shortcomings, such as the need for multiple processing methods for tandem QLEDs, including spin coating and vacuum evaporation. These issues highlight areas for future improvement. Moving forward, the team is committed to developing new tandem structures, like all-solution-processed tandem QLEDs, which are better suited for mass production. They also intend to delve deeper into the working mechanisms of tandem QLEDs, focusing on electron generation and injection in the ICL, as well as the charge balance within the tandem QLED.