A Layer-by-Layer Growth Strategy for Large-Size InP/ZnSe/ZnS Core–Shell Quantum Dots Enabling High-Efficiency Light-Emitting Diodes

Shell is of great significance to the enhancement in the photoluminescence quantum yield (PLQY) and stability of core–shell-type quantum dots (QDs). InP/ZnS core–shell QDs without intrinsic toxicity have shown huge potential as a replacement for the widely used cadmium-containing QDs; however, it is still challenging to control the growth of InP-based core–shell QDs due to the lattice mismatch between the InP core and ZnS shell. Here, we report on the synthesis of ∼15-nm-size InP/ZnSe/ZnS QDs with a thick ZnS outer shell by a layer-by-layer shell growth strategy. The ZnS shell was prepared by a circularly gradient temperature rise and long reaction procedure in each step, which not only ensures relatively low precursor concentration preventing the anisotropic growth of QDs but also allows the low-reactivity source to be decomposed sufficiently to achieve layer-by-layer growth of a thick ZnS shell. The resulting QDs show the highest PLQY of 73%, narrow emission line width of up to 40 nm, wide spectrum tunability, and excellent stability. Furthermore, the thick ZnS shell also effectively suppresses nonradiative Förster resonant energy transfer and Auger recombination within QDs. As a result, these enable our quantum dot light-emitting diodes (QLEDs) to achieve a record external quantum efficiency of 6.6% in heavy-metal-free red QLEDs.