Recent Strategies in Electron Transport Layers Based on Metal Oxide Nanoparticles for Quantum Dot Light Emitting Diodes

Quantum dot light emitting diodes (QLEDs) have attracted considerable attention as next-generation display devices because of their high color purity, wide color gamut, and high luminous efficiency. Their performance is governed not only by the quantum dot emissive layer but also by the electron transport layer (ETL), which controls electron injection and transport, hole blocking, and interfacial recombination. In particular, metal oxide nanoparticles are widely used as ETL materials because of their high electron mobility, favorable energy level alignment with quantum dots, and compatibility with low temperature solution processing. QLEDs with ETLs based on metal oxide nanoparticles, however, suffer from excessive electron injection and surface defects such as oxygen vacancies and hydroxyl groups, which allow charge to accumulate at the interface and quench excitons, causing efficiency roll-off and limiting operational stability. Here, we review strategies developed to address these limitations, categorized into materials engineering (doping and surface treatment) and device structure engineering (interlayer insertion and double-layer ETLs).