Li and Mg Co-Doped Zinc Oxide Electron Transporting Layer for Highly Efficient Quantum Dot Light-Emitting Diodes

Zinc-oxide (ZnO) is widely used as an n-type electron transporting layer (ETL) for quantum dot (QD) light-emitting diode (QLED) because various metal doping can be possible and ZnO nanoparticle can be processed at low temperatures. We report here a Li- and Mg-doped ZnO, MLZO, which is used for ETL of highly efficient and long lifetime QLEDs. Co-doping, Mg and Li, in ZnO increases its band gap and electrical resistivity and thus can enhance charge balance in emission layer (EML). It is found also that the O–H concentration at the oxide surface decreases and exciton decay time of QDs on the metal oxide increases by co-doping in ZnO. The inverted green QLEDs with MLZO ETL exhibits the maximum current efficiency (CEmax) of 69.1 cd/A, power efficiency (PEmax) of 73.8 lm/W, and external quantum efficiency (EQEmax) of 18.4%. This is at least two times higher compared with the efficiencies of the QLEDs with Mg-doped ZnO ETL. The optimum Li and Mg concentrations are found to be 10% each. The deep-red, red, light-blue, and deep-blue QLEDs with MLZO ETLs exhibit the CEmax of 6.0, 22.3, 1.9, and 0.5 cd/A, respectively. The MLZO introduced here can be widely used as ETL of highly efficient QLEDs.