ZnO Nanoparticles for Quantum-Dot-Based Light-Emitting Diodes

Zinc oxide (ZnO) nanoparticles (NPs) are widely used as electron-transport layers in quantum dots (QDs) light-emitting diodes (QLEDs). In this work, we show that the size of the NPs can be tuned with the sol–gel synthesis temperature while keeping a constant mass yield. As the NP size decreases, the surface defect density reduces and the band gap broadens. In return, it prevents exciton quenching at the ZnO NPs/emitting QDs (core–shell CdSe@ZnS) interface. Moreover, as the conductivity of the ZnO NP films decreases, the electron–hole balance in QLEDs improves. When the synthesis temperature decreases from 60 to 0 °C, the diameter of the NPs shrinks from 5.4 to 2.8 nm. The optical band gap broadens from 3.44 to 3.66 eV and the energy of the minimum of the conduction band increases from −3.81 to −3.64 eV below the Fermi level. Consequently, the radiative decay rate of a CdSe@ZnS QDs layer coated on ZnO NP films increases from 11.62 to 13.75 ns. The smaller NPs exhibit a faceted surface with a lower density of defects. Under UV illumination, the intensity of the band to band emission from the ZnO NPs increases while the emission from defects decreases as the NPs diameter becomes smaller. The conductivity of the ZnO film decreases by more than 1 order of magnitude, and the current efficiency of QLEDs increases from 35.8 to 50.8 cd/A.