High‐Performance Blue Quantum‐Dot Light‐Emitting Diodes by Alleviating Electron Trapping

Abstract

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...

Ultrastable and High-Efficiency Deep Red QLEDs through Giant Continuously Graded Colloidal Quantum Dots with Shell Engineering

“…Efforts have been made to address the unbalanced charge injection issue: these include the selection of appropriate hole transport layers with deep energy levels to match the valence band maximum (VBM) of the CdSe-based QDs and enhance hole injection, − doping of the electron transport layer of ZnO nanocrystals with Mg 2+ ions to reduce electron mobility and increase the electron injection barrier, − and the insertion of an ultrathin insulating layer to impede the electron injection. − However, the increased electron injection barrier requires higher driving voltages to get the same luminance, meanwhile generating more heat that deteriorates the device. ,,, Tailoring the structure of the QDs offers another efficient way to achieve balanced charge injection, which is dominated by the semiconductor shell materials. − A CdS shell has been widely used for its wide band gap which offers excellent surface passivation, leading to a near-unity photoluminescence quantum yield (PLQY) of CdSe/CdS QDs. − Unfortunately, the low conduction band minimum (CBM) of CdS favors electron injection, while its deep VBM impedes hole injection, causing heavily unbalanced charge injection. ZnS and ZnSe (having a wider band gap than CdS) differ from each other in terms of CBM and VBM levels. ,,− The shallower VBM of ZnSe compared to that of ZnS can enhance hole injection without impeding electron injection.…”

All-inorganic lead halide perovskite nanocrystals applied in advanced display devices

“…[33][34][35][36] After years of great efforts, CdS, ZnS and InP based colloidal QDs become the mainstream luminous materials of QD-LEDs, which show over 20% external quantum efficiency (EQE) and an extremely long lifetime of a million hours for red, green and blue (RGB) devices. [37][38][39][40][41][42] But, the color gamut (Rec. 2020) display device based on colloidal QDs is always below 90% due to their inhomogeneous size and imperfect material quality.…”

Modified Zinc Magnesium Oxide for Optimal Charge‐Injection Balance in InP Quantum Dot Light‐Emitting Diodes

“…Further, there are several possible loss mechanisms in QLEDs. Interfacial trap states in charge transporting layers (CTLs), [19][20][21] defect sites on QDs, [22][23][24][25] and charge injection imbalance [7,26] can reduce QLED performance. Non-radiative Auger recombination is considered the primary loss mechanism, which is caused by excess charge injection to QDs.…”