Surface Engineering of SnO2 Electron Transport Layers for Efficient Quantum Dot Light Emitting Diodes
Authors: Kim Yong Il , Jung Woon Ho , Lee Hyeonjun , Kwon Yong Woo , Lim Jaehoon 
Published: December 2024

Quantum dot light-emitting diodes (QLEDs) have rapidly emerged as a frontrunner in the next generation of display technologies, offering significant advantages over traditional display technologies. Their appeal primarily lies in their ability to emit light efficiently while covering a wide color gamut, which is crucial for achieving high-quality display performance. One of the critical components in the architecture of QLEDs is the electron transport layer (ETL). This layer plays a significant role in transporting electrons from the electrode to the quantum dots while also blocking holes, which ensures efficient recombination of electrons and holes within the quantum dots. Metal oxide-based ETLs, particularly those made from Zn(Mg)O, have been widely used due to their favorable electronic properties, which significantly enhance the overall efficiency of QLED devices. Zn(Mg)O, with its excellent electron mobility and appropriate energy levels, has helped in achieving high external quantum efficiencies (EQE) in QLEDs. However, the performance of Zn(Mg)O-based QLEDs are highly sensitive to the adsorption of oxygen, water, thiol, and acidic compounds from the environment. Uncontrolled enhancement of device efficiency due to altered surface chemistry (so-called positive aging), causes complication for designing driving circuits, hampering the development of commercial devices.
One material that has garnered considerable attention as a potential alternative to Zn(Mg)O is tin oxide (SnO2). SnO2 has garnered attention as an alternative ETL material in QLEDs due to a wide band gap, high electron mobility, high transmittance in visible range and chemical stability, as recently reported for superior shelf stability compared to Zn(Mg)O-based ETLs. Recent studies have reported that SnO2-based QLEDs offer superior shelf stability compared to Zn(Mg)O-based QLEDs, which could potentially make them more suitable for reproducibility. However, despite these promising properties, SnO2-based QLEDs have so far exhibited lower performance compared to their Zn(Mg)O-based counterparts. This performance gap suggests that the electrical properties of SnO2 in the context of QLEDs are not yet fully understood and require further investigation.
Herein, we synthesized SnO2 nanoparticles (NPs) using a hydrothermal method, which is known for producing high-crystalline nanomaterials. The SnO2 NPs were characterized using a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), and optical spectroscopy. These techniques confirmed that the synthesized SnO2 NPs exhibited high crystallinity, a uniform size distribution, and a wide band gap exceeding 4.1 eV. The high crystallinity and uniform size are particularly important as they contribute to the consistent electronic properties of the ETL, which in turn affects the overall performance of the QLED. In an effort to further improve the performance of SnO2-based QLEDs, we applied an acid treatment to the SnO2 nanoparticles (NPs). The purpose of this treatment was not only to clean the surface of SnO2 NPs by removing organic ligands but also to passivate electron traps associated with surface hydroxide groups (–OH) on the SnO2 NPs. Excessive organic ligands can degrade the electrical properties, while surface hydroxides are known to act as traps for electrons, which can reduce the efficiency of the electron transport layer (ETL) by preventing electrons from reaching the quantum dots. By reducing the ratio of –OH to O2
⁻ (IOH/IO), as confirmed by X-ray photoelectron spectroscopy (XPS), we were able to successfully passivate these traps, thereby enhancing the electron transport properties of the SnO2 ETL. As a result of this acid treatment, the QLEDs incorporating acid-treated SnO2 demonstrated improved electrical conductivity, lower turn-on voltage, and increased luminance efficiency. Acid-treated SnO2-based QLEDs exhibit an EQE of up to 14.6%. This is a significant improvement and, to the best of our knowledge, represents the highest efficiency reported for QLEDs employing a single ETL layer. From these results, we confirmed the potential of using SnO2 as a substitute for Zn(Mg)O as an ETL material.