Highly Stable Blue QLEDs through Energy Level Control with Air-Stable ZnSeTe/ZnSe/ZnS Quantum Dot

Next-generation high-performance displays will be made using quantum dots (QDs) with adjustable band gaps, their superior optical properties, and ease of processing in solutions. Achieving long operational lifetimes and near-theoretical-maximum external quantum efficiency (EQE) has been realized in red and green InP-based quantum dot light-emitting diodes (QLEDs) [1,2]; however, an alternative solution is necessary to cover the blue region, such as employing a ternary ZnSeTe alloy structure with a band gap around 460 nm to replace cadmium-free blue-emission quantum dots.<br/>While semiconductor quantum dots made of ZnSeTe have recently shown extremely high photoluminescence quantum yield (PL QY) and EQE, the well-known methods of highly toxic hydrofluoric acid (HF) in their synthesis have hindered their commercialization.[3] Additionally, the PL QY in ZnSe/ZnSe/ZnSe/ZnS quantum dots rapidly decreases under ambient conditions despite having double shells due to rapid oxidation. Furthermore, QLEDs made with these QDs have high driving voltage and short operational lifetime, which are difficult to use in practical applications.<br/>We have developed a non-toxic synthetic method that completely eliminates the need for toxic HF in the synthesis of ZnSeTe/ZnSe/ZnS heterostructure quantum dots suitable for QLED applications. Instead of using HF, we introduced excessive metal halides, resulting in significantly improved PL QY. Importantly, we have demonstrated highly stable ZnSeTe/ZnSe/ZnS quantum dots in both the solution and the solid state through a straightforward ligand exchange process, which enhances surface binding via thiolate functional groups.<br/>In QLED devices, we investigated how the Te ratio and the size of ZnSeTe QDs affect the EQE and stability. In addition, to alleviate hole transport layer (HTL) degradation, which has an adverse effect on stability in the inverted structure, the band alignment between EML and HTL was adjusted through ligand exchange in the EML. ZnSeTe QDs, stable in both material and device, make it possible to achieve complete electroluminescent QLEDs.<br/><br/><b>References</b><br/>[1] Won, Y.-H.; Cho, O.; Kim, T.; Chung, D.-Y.; Kim, T.; Chung, H.; Jang, H.; Lee, J.; Kim, D.; Jang, E. Highly Efficient and Stable InP/ZnSe/ZnS Quantum Dot Light-Emitting Diodes. <i>Nature</i> 2019, 575 (7784), 634−638.<br/>[2] Moon, H.; Lee, W.; Kim, J.; Lee, D.; Cha, S.; Shin, S.; Chae, H. Composition-Tailored ZnMgO Nanoparticles for Electron Transport Layers of Highly Efficient and Bright InP-Based Quantum Dot Light Emitting Diodes. <i>Chem. Commun.</i> 2019, 55 (88), 13299−13302.<br/>[3] T. Kim, K. H. Kim, S. M. Choi, H. Jang, H. K. Seo, H. Lee, D. Y. Chung & E. Jang, Efficient and stable blue quantum dot light-emitting diode, <i>Nature </i>2020, <i>586</i>(7829), 385-389.