ZnMgO Nanoparticles via Ultrasonic-Assisted Synthesis for Electron Transport Layer in InP-Based QD-LEDs

Colloidal quantum dot-based quantum dot light-emitting diodes (QD-LEDs) have garnered attention as next-generation displays due to their high color purity, exceptional brightness, and simple solution-processing. Recently, the incorporation of Mg-doped ZnO nanoparticles (ZnMgO or ZMO) as the electron transport layer (ETL) in QD-LED structures has led to significant performance improvements by reducing excessive electron mobility compared to holes and suppressing surface defects. ZMO nanoparticles are synthesized through a solution process; however, traditional heating methods result in a very low Mg doping ratio compared to the amount of Mg precursor added. To increase the Mg doping ratio in ZMO, ultrasonic-assisted synthesis was introduced. When ultrasonic waves are applied to the ZMO precursor solution, cavitation bubbles are generated and collapse, momentarily producing high heat and pressure, thereby enhancing the low reactivity of the Mg precursor. Compared to heating-assisted synthesis, ZMO obtained via ultrasonic-assisted synthesis demonstrated relatively higher Mg content, as confirmed by X-ray photoelectron spectroscopy (XPS). Additionally, absorption and transmission electron microscope (TEM) measurements showed that the nanoparticles were relatively more uniform and smaller. When the ultrasonic synthesized ZMO and heating synthesized ZMO were applied as the ETL layer in green-emitting InP-based QD-LED devices, the smaller particle size and higher Mg content of the ultrasonic synthesized ZMO led to a more significant reduction in electron mobility compared to the heating synthesized ZMO. This resulted in maximum external quantum efficiencies (EQE) of 2.06% and 1.50%, respectively, with the ultrasonic synthesized ZMO achieving a 1.37-fold higher device performance.<br/><br/>To further exploit the benefits of ultrasonic-assisted synthesis in enhancing the reactivity of the Mg precursor, Mg precursor was added to pre-synthesized ZnO, followed by ultrasonic treatment, resulting in Mg-coated ZnO (ZnO/Mg). When ZnO/Mg was used as the ETL in environmentally friendly QD-LED devices with red and green-emitting InP-based quantum dots, the maximum EQE achieved were 11.35% and 2.51%, respectively, surpassing devices using both ZnO and ZMO. The brightness lifetime measurement of devices incorporating ZnO/Mg showed that it took approximately 43 hours to reach 50% brightness at 2,200 cd/m², demonstrating higher stability compared to devices using ZnO and ZMO (1.69-fold increase compared to ZnO). The enhanced stability of devices with ZnO/Mg is attributed to the formation of a stable ETL layer under electrical stress, which was confirmed by the sustained electrical stability of electron-only devices incorporating ZnO/Mg over time.