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

Abstract

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.

Keywords: balanced charge carriers; efficient quantum dot light-emitting diodes; high photoluminescence quantum yields; hole injection springboard; outermost CdZnS shell.

Figure 1

a) A typical type‐II core/shell structure of a QD and an energy diagram in a conventional QLED; b) the design strategy used in this work, in which the outermost cadmium‐doped zinc sulfide (CdZnS) shell is used as a springboard to make hole injection easier (from the core to the outermost shell, the layers with yellow, blue, and red color represent the core, ZnS shell, and outermost CdZnS shell, respectively). LUMO = the lowest unoccupied molecular orbital, CB = conduction band, HIL = hole injection layer, HOMO = the highest occupied molecular orbital.

Figure 2

a,b) Transmission electron microscope (TEM) images of R3 with different scale values; c) the corresponding core/shell structure of R3; d) high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐TEM) image of R3; e−h) energy dispersive spectroscopy elemental maps of Cd, Zn, Se, and S.

Figure 3

a) The UV–visible absorption spectra for the red QD solutions; b) the PL spectra of the red QD solutions and films; c) the PL decay curves; d) the PLQY values for the solutions and films of the red QDs; e) the images of red QD solutions under a 365 nm UV light.

Figure 4

a−e) The VB energy levels of the core/shell structured red QDs (R1−R5).

Figure 5

a) Device structure of D3; b,c) cross‐sectional TEM images of D3; d) EDS compositional mapping images of the corresponding section in (c).

Figure 6

a) EL spectra of red QLEDs; b) the image of D3; c) J−V−L curves; d) CE, e) PE, and f) EQE−L curves of the devices.

Figure 7

The performance of the green and blue QLEDs fabricated by the CdZnS shell based QDs: a) EL spectra; b) J−V−L curves; c) CE/PE−L curves; d) EQE−L curves; e,f) the images of working green and blue QLEDs.