Performance Improvement of Quantum-Dot Light-Emitting Diodes Using Double-Layered Electron Transport Layer

Quantum-dot light-emitting diodes (QLEDs) are emerging as a next-generation display due to the excellent light-emitting properties of quantum dots (QDs). In QLEDs, ZnO or ZnMgO nanoparticles (NPs) are widely used as the materials for an electron transport layer (ETL) due to their high electron mobility and similar conduction band minimum (CBM) level with that of QD emissive layer (EML). However, these characteristics of ZnO ETL induce the spontaneous injection of excess electrons to the EML, causing a charge imbalance with holes. To solve this issue, in this study, ZnS/ZnMgO double-layered ETL is introduced into the inverted InP-based red QLEDs. Compared to previously reported organic interlayers to block excess electrons, ZnS ETL has chemical robustness and good processability. The synthesized ZnS NPs have a wider bandgap, with their CBM upshifted by 0.7 eV compared with that of ZnMgO NPs. This high CBM becomes an effective energy barrier, which can suppress the injection of excess electrons and improve the charge balance. Consequently, our red-emitting InP QLEDs with ZnS/ZnMgO double-layered ETL exhibit 1. 30-fold higher external quantum efficiency (EQE) and 1. 37-fold higher current efficiency (CE) compared to those of pristine devices. We believe that ZnS/ZnMgO double-layered ETL structure is a promising alternative to improve device efficiencies by controlling the charge balance.