Cadmium-Doped Zinc Sulfide Shell as a Hole Injection Springboard for Red, Green, and Blue Quantum Dot Light-Emitting Diodes

A new strategy is developed in which cadmium-doped zinc sulfide (CdZnS) is used as the outermost shell to synthesize red, green, and blue (RGB) quantum dots (QDs) with the core/shell structures of CdZnSe/ZnSe/ZnS/CdZnS, CdZnSe/ZnSe/ZnSeS/CdZnS, and CdZnSe/ZnSeS/ZnS/CdZnS, respectively. Firstly, the inner ZnS and ZnSe shells confine the excitons inside the cores of QDs and provide a better lattice matching with respect to the outermost shell, which ensures high photoluminescence quantum yields of QDs. Secondly, the CdZnS shell affords its QDs with shallow valence bands (VBs). Therefore, the CdZnS shell could be used as a springboard, which decreases the energy barrier for hole injection from polymers to QDs to be below 1.0 eV. It makes the holes to be easily injected into the QD EMLs and enables a balanced recombination of charge carriers in quantum dot light-emitting diodes (QLEDs). Thirdly, the RGB QLEDs made by these new QDs exhibit peak external quantum efficiencies (EQEs) of 20.2%, 19.2%, and 8.4%, respectively. In addition, the QLEDs exhibit unexpected luminance values at low applied voltages and therefore high power efficiencies. From these results, it is evident that CdZnS could act as an excellent shell and hole injection springboard to afford high performance QLEDs.