Cd-Free Quantum-Dot Light-Emitting Diodes through Nanometer-Thick ZnS/ZnMgO Double-Electron Transport Layers

Quantum-dot light-emitting diodes (QLEDs) are gaining popularity as next-generation display devices due to their outstanding electroluminescent properties. However, conventional metal oxide-based electron transport layers (ETLs) used in QLEDs often suffer from charge imbalance, primarily due to intrinsic oxygen vacancies affecting their electrical properties, which degrade performance. To address this challenge, we introduce ZnS nanoparticles possessing a wider bandgap than ZnMgO nanoparticles, excellent chemical robustness and processability, to form a double-layer ETL of ZnS/ZnMgO in an inverted InP-based QLED. Owing to the higher energy levels and lower electron mobility of ZnS compared to ZnMgO, excess electron injection can be effectively controlled, resulting in improved charge balance in the devices. As a result, the InP-based QLED with a ZnS/ZnMgO double-layer ETL shows a 1.3-fold higher external quantum efficiency and a 1.84-fold longer lifetime than the QLED without a ZnS layer. Given the facile synthesis and processability of the ZnS nanoparticles introduced here, they may be applicable to a variety of optoelectronic devices, including QLEDs, for effective control of electron injection.