Blue-emitting heavy-metal free QDs simultaneously exhibiting photoluminescence quantum yield close to unity and narrow emission line widths are essential for next-generation electroluminescence displays, yet their synthesis is highly challenging. Herein, we develop the synthesis of blue-emitting QDs by growing a thin shell of ZnS on ZnSe cores with their size larger than bulk Bohr diameter. The bulk-like size of ZnSe cores enables the emission to locate in the blue region with a narrow emission width close to its intrinsic peak width. The obtained bulk-like ZnSe/ZnS core/shell QDs display high quantum yield of 95% and extremely narrow emission width of ∼9.6 nm. Moreover, the bulk-like size of ZnSe cores reduces the energy level difference between QDs and adjacent layers in LEDs and improves charge transport. The LEDs fabricated with these high-quality QDs show bright pure blue emission with an external quantum efficiency of 12.2% and a relatively long operating lifetime.

of high-quality QDs, QLEDs have gained huge progress and the performance is comparable with that of state-of-the-art organic light-emitting diodes. Especially, the red and green QLEDs have delivered the external quantum efficiency (EQE) over 20% and operating lifetime up to a few millions of hours. [13,14] Currently, blue QLEDs become the short slab for the QLED-based display technology.The introduction of wide bandgap shells (such as ZnS), which help to confine the charges/excitons to the cores and to passivate various defects, pave the way to high photoluminescence (PL) quantum yields (QYs), and stability for blue QDs. [3,[15][16][17] However, the other side of the coin is that the wide bandgap shells increase the holeinjection barrier inevitably, which is particularly pronounced in blue QLEDs due to the inherently lower valance band (VB) of blue QDs. Therefore, how to improve hole injection or charge balance becomes a core research area for further development of QLEDs. Efficient blue QLEDs with EQE over 16% were achieved by introducing poly(methyl methacrylate) (PMMA) or tert-butyldimethylsilyl chloride-modified poly(p-phenylene benzobisoxazole) (TBS-PBO) to impede the electron injection and improve the charge-injection/transfer balance. [18,19] CNPr-TFB hole transporting polymer has been developed by Wu and co-workers, which exhibits a superior hole conductivity and much more stabilized highest-occupied molecular orbital (HOMO) in comparison with TFB. Therefore, much more holes are delivered into the QD emissive layers when it was used as hole-transport layer (HTL). [20] Currently, Se used throughout ZnCdSe/ZnSe QDs and/or cadmium-doped zinc sulfide (CdZnS) as the outermost shell was proposed, which reduced the hole-injection barrier and enhanced hole injection. Then, the resulted blue QLED achieved outstanding luminance up to 62 600 cd m −2 . However, the EQE of this blue QLED was only 8.5% and/or 8.4%. [14,21] Even though these are optimizing strategies, the performance of blue QLEDs still lags behind that of red and green ones. The fundamental and essential issues in the devices need to be uncovered and resolved. To achieve efficient blue QLEDs, HTLs with low HOMO energy levels are commonly employed to achieve balanced charge injection. To date, poly-N-vinylcarbazole (PVK) is the most popular HTL for the blue QLEDs. [15,19,22] Currently, blue quantum-dot light-emitting diodes (QLEDs) remain the bottleneck limiting the development of QLED-based applications. To achieve high-performance blue QLEDs, poly-N-vinylcarbazole (PVK) is usually employed as the hole-transport layer (HTL) to reduce the hole injection barrier. However, fabrication of efficient blue QLEDs with PVK HTL remains challenging and empirical/accidental. Here, it is demonstrated that PVK layer can trap electrons and hence resulting in low device efficiency. This is why the performance of blue QLEDs is highly dependent on the PVK batch received from the manufacturers. As an interlayer, ZnSe/ZnS quantum dots (QDs) are inserted between PVK and bl...