Efficient Formation of ALD-based Al-doped ZnMgO Alloys for Electron Transport Layer of Quantum Dot Light-Emitting Diodes

Colloidal quantum dots (QDs) are promising materials for next-generation self-emissive displays. The rapid development of colloidal quantum dot-based light-emitting diodes (QD-LEDs) is paving the way for QD-LEDs to become a core technology for future displays. Currently, ZnO is primarily used as the electron transport layer (ETL) material in QD-LEDs due to its high electron mobility and barrier-free electron transport properties. Therefore, it is essential to produce high-quality ZnO to maximize the performance of QD-LEDs. High-quality ZnO can be fabricated using the atomic layer deposition (ALD) process. ALD enables atomic-level processing through self-limiting reactions of each precursor via sequential adsorption. This method easily produces highly uniform and defect-free thin films and is suitable for display processes due to its capability to operate under vacuum conditions. However, high-quality ALD ZnO films experience charge imbalance due to their high mobility, which hinders optimal performance. To address this issue, a new approach considering charge transport and balance is needed.<br/><br/>While solution-processed alloyed ZnO nanoparticles, which are currently widely used, have improved QD-LED performance by suppressing excessive electron injection through alloying with various compositions, precise control of alloying remains a challenge due to differences in precursor reactivity, and introducing ternary and quaternary oxide components increases the complexity of synthesis conditions. However, to mitigate the high mobility of ALD ZnO, we employed an alternate deposition method (supercycle) by alternating Al₂O₃ and MgO layers between ZnO cycles to form Al-doped ZnMgO. This confirmed that simple composition control over a wide doping range is possible merely by adjusting the cycle ratio of Al₂O₃ and MgO. Al-doped ZnMgO with simultaneous addition of Al and Mg resulted in reduced hole mobility and increased conduction band without significantly decreasing conductivity. This effectively suppressed excessive electron injection without reducing electron injection efficiency, preventing QD negative charging at the ETL/QD junction and enhancing the photoluminescence quantum yield (PLQY). QD-LED devices using ALD Al-doped ZnMgO as the ETL achieved an external quantum efficiency (EQE) of 15.7%, compared to less than 6.5% for QD-LED devices using ALD ZnO, and increased the device lifetime by seven times. This study reports on ALD-based QD-LED devices that systematically introduce a series of doping elements into ALD ZnO and adjust the electrical properties and band gap of the ETL to overcome solubility limits of each element.