Bandgap tunable Zn1−xMgxO thin films as electron transport layers for high performance quantum dot light-emitting diodes

Abstract

Since the introduction of inorganic ZnO nanoparticles as an electron transport layer (ETL) material, the device performance of colloidal quantum dot light-emitting diodes (QLEDs) has been rapidly improved. Although there have been rapid advances in luminance, efficiency and lifetime, device performance is still limited by balanced electron/hole injection into the quantum dot layer. In this study, solution-processed Zn_1−xMg_xO (ZMO) films with tunable bandgaps were designed as an ETL for high performance QLEDs. It was found that the band gap of ZMO broadened as the energy level varied with increasing Mg concentration. As a consequence, the energy barrier between the Al cathode and ZMO ETL was tuned to balance the electron/hole injection towards a better device performance. The luminance intensity increased from 20 229 to 36 685 cd m⁻², while the current efficiencies increased from 3.74 to 13.73 cd A⁻¹, and the power efficiencies increased from 4.65 to 12.94 lm W⁻¹ with a 0.05% Mg doping ratio. Particularly, the optimized device based on ZMO exhibited an enhanced external quantum efficiency (EQE) of 9.46%, which was 3.67-fold compared with that of pure ZnO nanoparticles (2.58%). The proposed approach provides a new method for the design and fabrication of high performance QLEDs by incorporating multielement semiconductors with variable bandgaps as an ETL.