Physically Controlled Nucleation for Tunable Quantum Dots and Interface Defect Modification in Light-Emitting Diodes

Cadmium selenide (CdSe)-based colloidal quantum dots (QDs) exhibit unique properties such as tunable colors, narrow emission, high photoluminescence efficiency, and high stability, making them one of the most promising candidates for next-generation displays. CdSe-based QD light-emitting diodes (QLEDs) have attracted more and more attention mainly due to their advantages including high electroluminescence brightness, low turn-on voltage, and ultrathin device structures. However, there are still many challenges, mainly including the lattice defects aroused by lattice strain during the QD growth process and the surface defects caused by ligand desorption. CdSe-based QLEDs with high photoelectronic performance were finally achieved in our work just by optimizing the synthesis processes and further reducing defects in QD shells. There was a great decrease in the defect density of the QD shell indirectly according to their testing results in the fluorescence lifetime and single-carrier devices. After the reaction system was diluted with a solvent in the hot-injection method, there was some blue shift in the QD emission observed from 595 to 562 nm. Then, with the ZnSe shell further coated onto the QD, the size of the effective emission center was reduced to some extent, and further, a blue shift in the emission was obtained with the wavelength down to 533 nm. Finally, on the outside of the as-synthesized QDs, the ZnS shell was used to passivate and further protect the ZnSe layer, which greatly increased the average fluorescence lifetime of the CdSe-based QDs from 22.94 to 36.41 ns. Additionally, a layer of lithium fluoride (LiF) with optimized thickness was further deposited onto the QD emitting layer to prevent ligand desorption from ethanol solvent cleaning. Therefore, the CdSe-based QD fluorescence efficiency was greatly improved and the maximum external quantum efficiency (EQE) of 8.06% for our CdSe-based QLEDs was achieved with a LiF layer of 3 nm.