Colloidal quantum dots (CQDs), known for their bandgap tunability via quantum size effect, have shown promise as excellent semiconducting materials for optoelectronic devices like quantum dot light emitting diodes (QLEDs), photodetectors, and photovoltaics. Their solution processing capability makes them ideal candidates for inkjet printing (IJP) manufacturing, a drop-on-demand technology offering mask-free patterning and low material consumption, potentially reducing manufacturing costs for next-generation QLED displays. However, the low efficiency and rapid degradation of CQD emission during QLED operation have been major obstacles. Non-radiative recombination, strongly influenced by trap state energy and density, is a key factor. While core/shell structures have improved efficiency and lifetime, reaching EQEs over 20% and lifetimes exceeding 2,000,000 h in some cases, ligand engineering remains crucial. Surface trap states, even with thick shells, create mid-gap states that act as non-radiative recombination centers. Previous attempts to eliminate these traps using various ligands have yielded limited success, and there has been a lack of effective strategies for passivating both cationic and anionic traps simultaneously. Furthermore, solid-phase ligand exchange is unsuitable for large-scale production, and existing methods for liquid-phase exchange struggle to achieve complete dual-ion passivation without compromising optoelectronic properties. This research proposes a novel approach to address these limitations.

The literature extensively covers the challenges and advancements in QLED technology. Studies have demonstrated the effectiveness of core/shell structures in enhancing QLED performance (refs 1-3, 19), highlighting the crucial role of bandgap engineering. Research on ligand effects has shown that surface states significantly influence CQD emission (refs 20-23), with various ligands attempted to mitigate trap states (refs 19, 24, 25). However, a unified approach to passivate both cationic and anionic traps simultaneously has been lacking. The incompatibility of solid-phase ligand exchange with large-scale production has been a significant bottleneck (refs 9, 19, 25), leading to explorations of mixed ligand passivation (refs 11, 12) which, however, has not fully resolved the issue of both sufficient passivation and good emitting properties. The use of inkjet printing for QLED fabrication has also been explored (refs 13-18), but achieving both high efficiency and stability remains a challenge. This research builds upon this existing body of knowledge, aiming to provide a solution to the outstanding problems related to efficiency and stability in IJP QLEDs.

The study employed a combination of computational modeling, material synthesis, characterization, and device fabrication techniques. Density functional theory (DFT) simulations were used to study the trap states on the ZnSe (100) surface of CQDs. The simulations explored different types of under-coordinated cations and anions, identifying the origin of mid-gap trap states and evaluating the effectiveness of different ligands in passivating these states. CdSe core and ZnSe shell QDs were synthesized using previously reported methods, followed by purification to obtain QDs (P-QDs) with negligible ligands. Solution ligand exchange was then performed to produce cation-passivated CQDs (OA-QDs), anion-passivated CQDs (ZnCl2-QDs), and dual-ion-passivated CQDs (Zn(OA)2-QDs). These were characterized using NMR, TEM, XPS, and FTIR to confirm ligand exchange and assess the level of surface passivation. High-resolution differential scanning calorimetry (DSC) was employed to measure the binding energy of different ligands to the QDs. The dual-ion-passivated CQDs were then incorporated into state-of-the-art QLED architectures via inkjet printing, with device performance characterized through current-voltage-luminance measurements, single-carrier device studies, electroluminescence spectroscopy, and lifetime testing. Finally, the device degradation mechanism was investigated using capacitance-voltage (CV) measurements and thermal admittance spectroscopy (TAS) to analyze trap density and energy levels. The experimental methods for quantum dot synthesis, ligand exchange, and QLED fabrication are detailed in the Materials and Methods section of the original paper.

DFT simulations revealed that both under-coordinated Se (anionic) and Zn (cationic) sites on the ZnSe shell surface contribute to mid-gap trap states, with Se-2C sites being particularly impactful. The proposed Zn(OA)2 ligand demonstrated superior ability to passivate both cationic and anionic traps, with a significantly higher binding energy (-4.1 eV) compared to CH3COOH (-2.56 eV) and CH3SH (-2.86 eV), as confirmed by DFT calculations and DSC measurements. NMR, TEM, and XPS analyses verified the successful ligand exchange and dual-ion passivation by Zn(OA)2, showing a greater surface anion and cation passivation compared to OA and ZnCl2. QLED devices fabricated using Zn(OA)2-QDs exhibited superior performance compared to devices using OA-QDs and ZnCl2-QDs. The Zn(OA)2-QDs based devices achieved an external quantum efficiency (EQE) of 16.6%, surpassing previous reports for IJP QLEDs. Single carrier device analysis showed a more balanced charge injection with Zn(OA)2-QDs. The lifetime tests showed that Zn(OA)2-QDs devices had a significantly longer operational lifetime (LT95@1000 nits of 1833 h) than devices with other ligands, with an estimated LT50 of 1,721,000 hours at 100 cd m⁻². CV measurements revealed negligible capacitance changes in Zn(OA)2-QDs devices after operation, unlike devices with OA or ZnCl2, where significant increases indicated trap formation. TAS analysis further confirmed that Zn(OA)2-QDs devices had exceptionally low trap densities, with only slight changes after aging, while devices with OA or ZnCl2 showed significant increases in trap density after aging.

These findings directly address the long-standing challenges of low efficiency and rapid degradation in inkjet-printed QLEDs. The successful implementation of the dual-ionic passivation strategy using Zn(OA)2 ligands is a major advancement. The superior performance of Zn(OA)2-QDs-based devices, including the high EQE and remarkably long lifetime, demonstrates the effectiveness of this approach. The results highlight the critical role of surface ligand engineering in determining QLED device performance, exceeding existing benchmarks for inkjet printing techniques. The approach could be extended to other CQD materials and device architectures, potentially impacting the development of high-performance, cost-effective QLED displays and other optoelectronic devices.

This study presents a significant advancement in QLED technology by demonstrating high-efficiency and highly stable inkjet-printed quantum dot light-emitting diodes. The key to success lies in the dual-ionic passivation strategy employing Zn(OA)2 ligands, which effectively suppressed mid-gap trap states. The resulting devices achieved an unprecedented combination of high EQE (16.6%) and exceptionally long operational lifetime (1,721,000 h). Future work could explore the optimization of the inkjet printing process, investigate different CQD materials, and explore the application of this technique in other optoelectronic devices.

The study focused on a specific type of CQD (CdSe core/ZnSe shell) and a particular device architecture. While the methodology is potentially applicable to other CQDs, further research is needed to assess its generality. The lifetime measurements were conducted under specific conditions (constant current density, ambient temperature), and the long-term stability under varying conditions should be further investigated. The computational model simplified the complex CQD surface chemistry; therefore, future work should consider more detailed simulations to enhance the understanding of the underlying physical mechanisms involved in the ligand passivation and device degradation.