Solution-processed quantum-dot light-emitting diodes combining ultrahigh operational stability, shelf stability, and luminance

Quantum-dot light-emitting diodes (QLEDs) offer high performance and low manufacturing costs, making them promising for display technology. However, challenges remain in achieving sufficient device stability, particularly concerning operational and shelf stability. Shelf stability is significantly affected by a phenomenon known as the "positive aging effect," where the performance of the QLED improves unpredictably during storage. This effect is linked to solution-processed ZnO nanoparticles (NPs) used in the electron-transporting layer (ETL). While the mechanism of positive aging remains a subject of ongoing research, with explanations ranging from oxygen vacancies to chemical reactions between ZnO and encapsulation resin to surface stabilization involving HO-ZnO, simply isolating ZnO from ambient reactants doesn't solve the problem. Passivating ZnO NP surfaces with capping ligands has been explored but has yielded unsatisfactory results at the device level due to the abundance of active surface sites and insufficient surface coverage. Alternatively, blocking the exciton quenching channel between the QDs and the ZnO-based ETL during device fabrication offers another route to shelf-stability if the electrical conductivity change of the ZnO-based ETL occurs after a negligible storage period. Previous studies have shown that using ultrasmall ZnO NPs or SnO₂ NPs individually in the ETL can improve shelf stability, but these approaches either limit the current density and maximum luminance or limit the electron mobility. This study proposes a novel solution using a combination of SnO₂ and ZnO in a bilayer ETL design to address these limitations and achieve superior performance.

Several research groups have investigated the positive aging effect in QLEDs. Chen et al. attributed the phenomenon to oxygen vacancies in ZnO, while Acharya et al. highlighted the chemical reaction between ZnO and the encapsulation resin. More recently, Zhang et al. proposed a more general explanation based on surface stabilization in the form of HO-ZnO. Despite these insights, simply isolating ZnO from moisture and other H⁺ sources hasn't fully solved the shelf-stability problem, as it negatively affects the electroluminescence efficiencies and operational lifetimes. Efforts to passivate ZnO NPs with various surface capping ligands have shown partial success in characterization, but not at the device level. Previous work using ultrasmall ZnO NPs or SnO₂ NPs as ETLs showed some improvement in shelf stability, but the former limited luminance while the latter limited electron mobility.

The researchers synthesized ZnO nanoparticles and capped them with 3-mercapto-1-hexanol (MHL) via ligand exchange. The effectiveness of the capping was verified using Fourier Transform Infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), which showed the presence of S-Zn bonds and a reduction in carboxylate groups. Time-resolved photoluminescence (TRPL) measurements indicated that while MHL capping improved the PL decay kinetics, the quenching effect of ZnO was not entirely eliminated, suggesting incomplete surface coverage. To further address the exciton quenching, a SnO₂ nanoparticle buffer layer was introduced between the QDs and the ZnO layer. Ultraviolet photoelectron spectroscopy (UPS) measurements profiled the energy level alignment across the QD/SnO₂/ZnO stack. TRPL measurements confirmed that the SnO₂ buffer layer effectively protected the QDs from ZnO-induced exciton quenching. QLED devices were fabricated with the conventional structure, using the SnO₂/s-ZnO bilayer ETL. The devices' performance was characterized by measuring their current-voltage-luminance (L-J-V) curves, electroluminescence (EL) spectra, and external quantum efficiency (EQE). Operational lifetimes were measured under constant current density. The shelf stability of the devices was assessed by monitoring the threshold voltage (Vth), maximum EQE (EQEmax), and EQE at 1000 cd m⁻² over time. Bias-dependent electro-absorption (EA) measurements were used to determine the flat-band voltage of the emissive layer (VFB_EML) and the devices were also tested under photovoltaic mode to measure their EQEpy spectra.

The study demonstrated that the bilayer ETL design, incorporating a SnO₂ buffer layer, significantly improved the performance and stability of the QLEDs. The QLEDs with the SnO₂/s-ZnO bilayer ETL exhibited a maximum luminance exceeding 100,000 cd m⁻², a T₉₅ operational lifetime of 6200 h at 1000 cd m⁻² (extrapolating to 390,000 h at 100 cd m⁻²), and excellent shelf stability. The SnO₂ buffer layer effectively suppressed ZnO-induced exciton quenching, leading to a substantial improvement in the operational lifetime compared to devices using only ZnO or s-ZnO as the ETL. The TRPL measurements confirmed that the SnO₂ layer effectively prevented exciton quenching, as indicated by a significant increase in the PL lifetime (τ₁) of QDs deposited on SnO₂/s-ZnO compared to those deposited on s-ZnO alone. UPS analysis revealed the energy level alignment and suggested efficient electron injection. The addition of SnO₂ also reduced the leakage current. Bias-dependent electro-absorption (EA) measurements showed that the SnO₂/s-ZnO bilayer ETL maintained a stable energy level alignment across the QD-ETL heterojunction, unlike the s-ZnO ETL which experienced a significant shift in the flat-band voltage (VFB_EML) during shelf storage. Finally, measurements of EQEpy under photovoltaic mode confirmed that the SnO₂ buffer layer effectively suppressed the photoinduced electron transfer from QDs to the ETL.

The findings of this study successfully address the long-standing challenge of shelf-stability in solution-processed QLEDs. The bilayer ETL design combining SnO₂ and ZnO offers a highly effective strategy to overcome the limitations of using either material alone. The significantly enhanced operational lifetime and the complete elimination of the positive aging effect demonstrate the effectiveness of the SnO₂ buffer layer in preventing exciton quenching. The superior performance metrics (high luminance, long operational lifetime, and exceptional shelf stability) position these QLEDs as highly competitive contenders for next-generation display technologies. The results provide compelling evidence that the exciton quenching is primarily driven by photoinduced electron transfer, and the SnO₂ buffer layer effectively suppresses this process. The observed improvements are not merely a result of passivation but are fundamentally linked to the altered interfacial charge transfer dynamics.

This work successfully demonstrates a novel approach to fabricate highly stable and high-performance QLEDs by employing a SnO₂/s-ZnO bilayer ETL. This design effectively overcomes the challenges associated with ZnO-induced exciton quenching and positive aging, achieving exceptional shelf stability and operational lifetime without compromising luminance. This breakthrough is attributed to the synergistic effect of MHL-capped ZnO NPs and the SnO₂ buffer layer, which together suppress photoinduced electron transfer and maintain charge balance. Future research could focus on exploring other buffer materials or optimizing the thickness and composition of the bilayer ETL to further enhance device performance and stability.

While this study demonstrates significant improvements in QLED stability and performance, there are some limitations. The surface coverage of MHL on ZnO NPs might not be entirely uniform, potentially affecting the overall stability. Further investigation into the long-term stability (beyond the 7 days of shelf storage tested) would provide a more comprehensive understanding. The study focused on red QLEDs; investigating the applicability of this bilayer ETL design to other color QLEDs would be valuable. The fabrication method might not be easily scalable for mass production.